Chapter 9. Borderline cases: bone-conduction device versus middle ear implant versus cochlear implant

9.0 Introduction

During a round table discussion on borderline cases (CI2024, in Las Palmas), an overview was presented of Nijmegen studies aiming at borderline areas between conventional hearing aids (behind-the ear device or BTE) and implantable hearing devices: Cochlear implant (CI), middle ear implant (MEI) and implantable bone-conduction device (BCD). 

Four borderline areas are addressed

–Sensorineural hearing loss:

  • Severe/profound hearing loss: when switching from BTE to CI?
  • If the hearing loss is moderate to severe, what is the better option: a BTE or MEI?

–Conductive/mixed hearing loss, 

  • What is the best amplification option? A BTE or BCD or MEI?
  • In case of mixed hearing loss with a severe sensorineural component, when switching from a powerful BCD/MEI to a CI?

The focus is on device benefit in terms of speech recognition in quiet. 

 

9.1 Sensorineural hearing loss

9.1.1 When switching from a BTE to a CI in severe/profound sensorineural hearing loss? 

 

The concept of ‘equivalent hearing loss’ is used, comparing speech recognition score of CI users with speech recognition scores of a large group of BTE users with varying degrees of sensorineural hearing loss (Snik et al., 1997). The speech measure chosen in this overview (and in all Nijmegen studies) is the (unilateral) phoneme recognition score in quiet obtained with monosyllables (MPS), presented at a normal conversational level (65 dB SPL; Snik, 2007, Verhaegen et al., 2008). 

In order to determine the ‘equivalent hearing loss’, data of (53) BTE users with moderate to profound sensorineural hearing loss were gathered; Appendix 9.1 presents the selection criteria and the BTE-fitting procedure. In Figure 1, individual, unilateral MPS scores are presented as a function of the PTA3 (mean hearing loss at 0.5, 1 and 2 kHz); the MPS of the ear with the best hearing (lowest PTA3) is presented. The blue line connects median MPS scores of the BTE users divided into classes, according to their PTA3.  These data will be used for a comparison with CI data.

Figure 1. The individual MPS values as a function of the PTA3 of the BTE users and the median (p50) and p25 MPS data of the group of CI users (dotted lines). MPS stands for Monosyllables Phoneme Scores

 

Table 1 presents MPS scores, obtained one-year after cochlear implantation of 139 successive but selected CI users with postlingual onset. Inclusion criteria and details about the types of CI used are presented in Appendix 9.1. The Table presents the median and mean MPS as well as the p25 MPS (the 25e percentile). The p50 (median) and p25 values are indicated in Figure 1 as dotted lines. The figure shows that the median and the p25 MPS values are ‘equivalent’ to those of BTE users with a PTA3 of 75 dB HL and 85 dB HL (expressed in multiples of 5 dB), respectively. In other words, if a CI candidate has a PTA3 > 85 dB HL, then the chance that his/her MPS with CI is higher than with his/her BTE is >75%. If the PTA3 is 75 dB SPL, the chance level is 50%. This suggests that the p25 value (and thus 85 dB HL value) might be the better criterion. 

 

Table 1. The median, p25 and mean MPS scores and the related word recognition scores (WRS) of the CI users

measure MPS WRS
median (p50) 77% 57%
p25 63% 30%
mean 75% 53%
mean, from a later study* 79% 60%

* more recent data from a Nijmegen study (Huinck et al., 2019), see Appendix 9.1 for details

The last line of the table shows the outcome of a study carried out one decade later, by Huinck et al. (2019). They studied the advantage of relatively early implantation by comparing the MPS of two groups of CI users. Group 1 comprised CI users selected using rather conventional inclusion criteria (speech recognition with BTE less than 30% and/or PTA3 > 85 dB HL). Their data are presented in Table 1, last line and are rather well comparable to those used in the previous analysis.

Huinck et al. used the data of Group1 for a comparison with CI users selected according to more relaxed criteria (MPS with BTE between 30% and 60% and/or PTA3 between 65 dB HL and 85 dB HL). The one-year post-implant MPS of the second group was significantly higher than that of Group 1; viz. 85% versus 79%. The authors suggested that relatively early implantation is beneficial, which was ascribed to less ‘auditory deprivation’. 

This suggests that the less conservative PTA3 > 75 dB HL criterion might be the better choice. 

9.1.2 Moderate to severe sensorineural hearing loss, better results with a MEI or a BTE?

 

Nowadays, regarding MEIs, only the Vibrant Soundbridge is widely used (VSB; Med-El, Innsbruck, Austria). In Nijmegen the VSB has (only) been applied if a BTE was contraindicated (owing to chronic external otitis; Snik and Cremers, 2001). MPS data were gathered and used to answer the question which device is more powerful, a BTE or a VSB. The same straightforward method was performed as in the previous section; Figure 2 presents the MPS as a function of the PTA3 of 30 patients, fitted with a VSB (for details regarding inclusion criteria and devices used, see Appendix 9.1). A best-fit line is presented; the full black line. The blue line in the figure is copied from Figure 1, referring to BTE users. Evidently, the VSB scores are below the BTE scores. E.g., at a PTA3 of 65 dB HL, the MPS with VSB is approx. 25% worse than that obtained with BTE, increasing to 45% at 75 dB HL.

The figure suggests that, if the PTA3 exceeds 50-60 dB HL, a BTE should be fitted for better speech recognition, unless a BTE is contraindicated. Compared with the p50 MPS from CI users, the ‘equivalent hearing loss’ is approx. 60 dB HL, so, if a BTE is contraindicated, cochlear implantation might be considered if the PTA3 exceeds 60 dB HL. 

Although the capacity of the VSB is limited, it should be noted that this device is highly appreciated by patients with moderate sensorineural hearing loss and chronic external otitis. They can use this device all day without pain or itching (Snik and Cremers, 2001). The Med-El Company advocates the present VSB only for patients with sensorineural hearing loss suffering from chronic external otitis.

Figure 2.  The individual MPS of MEI users plotted as a function of the PTA3. For reference purposes, more recent literature data (Maier et al., 2015); mean MPS versus mean PTA3, presented as a red dot (see Appendix 9.1 for details) 

 

9.2 Conductive and mixed hearing loss

 

9.2.1 What is the best option: a BTE or a BCD? 

 

Most often, the cause of a (chronic) conductive/mixed hearing loss is aural atresia/stenosis or a chronic draining ear. In either case, a BTE is contraindicated. However, in other aetiologies like otosclerosis or tympanosclerosis, a BTE might be used. 

Whether a (powerful) BTE or a BCD is more effective, depends on the width of the air-bone gap. Assume a powerful BTE is used with a mean maximum output (MPO) of 112 dB HL and a mean gain of 50 dB. If the air-bone gap is e.g. 55 dB, then the effective MPO is really limited: viz. 112 minus 55 thus 57 dB HL, and the effective gain is -5 dB. Even the less powerful conventional BCDs can provide such a MPO and gain. 

The MPO and gain of a BCD are not affected by the width of the air-bone gap. The mean MPO of more powerful BCDs like the percutaneous BCD (pBCD; see section 9.2.2) is significantly higher than that 57 dB HL; a MPO of 85 dB HL with a mean gain of up to 20 dB is achievable.

Mylanus et al. (1998) and de Wolf et al (2010) compared outcomes (MPS) obtained with powerful BTEs and pBCDs in (Nijmegen) patients with conductive/mixed hearing loss. They found that if the air-bone gap exceeds 30-35 dB the pBCD provided higher gain and therefore better speech scores than the BTE; below 30 dB the BTE provided the best result.

In other words, if a BTE is not contraindicated while the mean air-bone gap exceeds 35 dB, then a powerful (thus percutaneous) BCD might be the better amplification option.

 

9.2.2 If a BCD is indicated, which type of BCD should be used? In other words, can we rank today’s BCDs?

 

Before discussing the capacity of BCDs, an overview of the different types that are  available. 

  1. The non-surgical, conventional BCD, which is kept in place by a softband, steal headband or adhesive coupling. Such devices are referred to as transcutaneous passive BCDs, e.g., the Baha Softband (Cochlear BAS, Goteborg, Sweden), Ponto Softband (Oticon Medical, Askim, Sweden) and the Adhear (Med-EL, Innsbruck, Austria)
  2. BCD with magnetic coupling, requiring a surgically placed subcutaneous magnet for coupling purposes. This device is also referred to as a transcutaneous passive device, e.g., the Sophono device (Medtronic, Minneapolis, USA) and the Baha Attract (Cochlear BAS; see Figure 3) 
  3. Another transcutaneous approach consists of an actuator (the vibrator) implanted subcutaneously in the mastoid, driven by an externally worn audio-processor. This type of BCD is referred to as a transcutaneous active BCD. Available devices nowadays: the Bonebridge (Med-E; see Figure 3), the Osia (Cochlear BAS) and the Sentio (Oticon Medical)

Figure 3. Different types of BCDs

 

  1. The percutaneous BCD (pBCD) with a skin penetrating implant, connecting the BCD to the skull without attenuation caused by the damping tissue. Available devices: the Baha (Cochlear BAS) and the Ponto (Oticon Medical; see Figure 3). It should be noted that (only) for the pBCDs more powerful audio-processors are available besides the standard processors.

 

The different types of BCDs are not equivalents. In contrast to BTEs, all BCDs (and MEIs) have a limited capacity. E.g., the loudest sound that BCDs can produce (the MPO or maximal power output) varies between 50 dB HL and 85 dB HL, while men can tolerate sounds up to 105-115 dB HL.

The MPO of BCDs and also that of the VSB have been studied. Table 2 gives a literature overview. The table shows that data from different studies are rather consistent. It should be noted that in contrast to the other BCDs and VSB, the MPO of pBCD can easily be derived from the available data sheets, following the procedure introduced by Carlsson and Hakansson (1997). 

The Table doesn’t provide the MPO of the passive transcutaneous devices. Their mean MPO lays between 50 dB HL and 60 dB HL (Van Barneveld et al., 2018); included in the Table are only the more powerful options.

What are the consequences of a low MPO? Figure 4 presents the audiogram of a patient with mixed hearing loss. Bone conduction thresholds are shown as well as loudness discomfort levels. The patient was fitted with a passive tBCD with magnetic coupling as the bone-conduction thresholds laid well within the application range of that device. Measured MPO values are also displayed. The mean bone conduction threshold is 32 dB HL and the mean MPO of the tBCD is 55 dB HL. Consequently the ‘auditory window’ with this BCD, is just 23 dB (55 minus 32 dB HL). 

Figure 4. Audiogram of a patient with mixed hearing loss. As a BCD is applied, only the bone conduction thresholds are relevant. MPO values are also displayed 

Figure 5.  On the right side, the (simplified) audiogram with (mean) MPO levels of different types of BCDs. Now, on the left, the level of every-day sound sources, expressed in dB HL. Arrows indicate  ‘projections’ of the (grey) speech area into the patient’s auditory window. tBCD: transcutaneous passive BCD; pBCD: percutaneous BCD; BCD-SP: pBCD with powerful sound processor.

 

Figure 5, left side, presents the outside world with all kinds of sounds, expressed in dB HL. The grey area represents the speech area (normal conversational speech at 65 dB SPL). Evidently, projecting properly relevant sounds (not only speech at normal conversational level, but also with raised voice or someone talking at some distance or the patient’s own voice and the other sounds) in the narrow ‘auditory window’ of 23 dB is not easily possible. 

Now, the right figure also shows the MPO of more powerful BCD solutions (values taken from Table 2). With a standard pBCD, the auditory window becomes 37 dB and if a more powerful processor is used, like the Baha 5 SP, then the window becomes 55 dB, the maximum achievable window with todays BCDs. With the VSB, this window is 50 dB. Thus, from an audiological point of view, pBCD is the best option followed by the VSB device. As Figure 5 shows, the speech area is approx. 35 dB wide. This value is used to define application ranges for different types of BCD and the VSB. We use the following assumption: for a given patient, a specific BCD can be used only if the ‘auditory window’ is at least 35 dB. Simply means that the highest acceptable PTA4bc (average bone-conduction threshold at 0.5, 1, 2 and 4 kHz) is the mean MPO value minus 35 dB. Table 3 gives an overview; it should be noted that, at date, the MPO of the relatively new transcutaneous active BCDs (Osia en Sentio) is not known yet (not published). 

Evidently, the different types of BCDs and the VSB are not equivalents; if the PTA4bc of a transcutaneous passive BCD user exceeds 20-25 dB HL, then an update to a percutaneous device might significantly improve the patient’s performance. Another option is to apply a VSB. For Bonebridge users with a PTA4bc exceeding 30 dB HL, a pBCD or VSB are the next options. 

 

9.2.3 In severe mixed hearing loss, when to switch to a CI?

 

Following the same procedures as used before, Figure 6 presents the MPS versus PTA3 (bone-conduction) of 26 patients with mixed hearing loss with moderate to severe sensorineural component using the most powerful BCD at that time, the Baha Cordelle (Verhaegen et al., 2009). For selection criteria etc. see Appendix 9.1. The PTA3 for bone conduction varied between 30 dB HL and 75 dB HL. As the figure shows, when using the CI-p50 (77%), the ‘equivalent hearing loss’ is 60 dB HL. 

Figure 6. Individual MPS values as a function of PTA3 if 26 users of the most powerful pBCD

 

Verhaegen et al. (2009) also presented the data of the first five Nijmegen patients switching from a Baha Cordelle to a CI. Two of them suffered from advanced otosclerosis and external otitis and three from chronic middle ear problems. The MPS of these five patients improved significantly from 10% (range 0 to 30%) with BCD to 75% (range 60% to 90%) with the CI. 

Regarding advanced otosclerosis, performance with a CI might be worse than in patients with normal cochlear anatomy (Rotteveel et al., 2010). Poor performance is caused by anomalies of the cochlea leading to problems with the insertion of the electrode array. This might lead to a partial insertion. Because of poor bone quality, facial nerve stimulation might occur. Then, certain electrodes have to be switched off (Rotteveel et al., 2010). Therefore, the MPS scores of CI users with otosclerosis are worse than those of patients with a normal bony structure; the p50 and p25 MPS values for CI users with otosclerosis are 55% and 40% respectively (Rotteveel et al., 2010), thus more than 20% below those of CI users with anatomically normal cochleae (Figure 1). Using Figure 6 the otosclerosis-specific p50 and p25 (not shown) result in ‘equivalent hearing loss’ values of 70 dB HL and 75 dB HL, respectively.

Cochlear implantation in the other group of patients with severe mixed hearing loss, suffering from chronic middle ear problems, is also challenging. Nevertheless, cochlear implantation in such patients remains of utmost importance, as it is a ‘last resort’ solution. 

 

9.2.4 Which acoustic implantable device to use for patients with conductive/mixed hearing loss?

 

Previous sections clearly suggest that the pBCD is the best option from an audiological point of view. That device has some other specific advantages as well, but also a disadvantage. The advantages:

  1. Of course application of a pBCD requires surgery, however minimal invasive surgery, in contrast to all the other implantable options. Implantations have even been performed in outpatient clinics 
  2. pBCD is the only implantable device that can be updated when hearing deteriorates. Besides the standard sound-processor, power and superpower processors are available
  3. The distortion of MRI images is limited in contrast to most other implantable BCDs and VSB. Explantation of MEIs and active tBCDs because of MRI have been reported 

Disadvantage of pBCDs are infections around the percutaneous implant and implant loss owing to poor fixation in the skull bone (poor osseo-integration).

Elaborations.

Regarding point 1: Minimal invasive surgery lowers the burden for the patient and the costs of the intervention. At the other end, placing a MEI in patients with conductive/mixed hearing loss might be challenging, especially in patients with congenital anomalies (e.g., aural atresia) and in patients with chronic otitis media. In the latter case, subtotal petrosectomy is a required first surgical step (Verhaert et al., 2013).

Regarding point 2: Longevity is an important issue, especially when treating elderly people. This is illustrated in Table 4, which shows the effectiveness of several types of BCDs and the VSB in relation to age of patients with mixed hearing loss. Few data were found on the deterioration of the sensorineural component in patients with mixed hearing loss. Therefore, as an approximation, we used age-related hearing thresholds taken from the ISO standard (ISO 7029, 2017), with an additional cochlear loss of 20 dB HL owing to cochlear damage caused by middle ear infections and/or medical treatments in the past (group ISO 7029+; see Appendix 9.1 for details). We used these data for illustrating the effect of aging on the effectiveness of several devices.  

 

Table 4. Effective use of devices in progressive (cochlear) hearing loss in patients with cochlear damage caused by chronic otitis media in the past (ISO 7029+). 

  

device PTAbc max,

dB HL

ISO 7029+

years

Passive tBCD* 25 up to 50 
Active tBCD** 30 up to 60
VSB 45 up to 80
pBCD 50 up to ±85

* with magnetic coupling; ** the Bonebridge

 

Table 4 shows that when using a passive tBCD with magnetic coupling, the PTAbc max for applying that device (25 dB HL; taken from Table 3) is passed at age 50 years. Evidently, the pBCD and VSB are the best solutions as they might be used effectively life long.

Disadvantages of percutaneous implantation are skin problems around the implant and implant loss, leading to revision surgeries. Through the years, the number of revisions has dropped, owing to more than 35 years of experience with percutaneous implants. New implantation procedures have been introduced as well as optimized implants. Table 5, first line, presents Nijmegen data. Systematically, all minor and major complications have been documented for each patient, starting in 1988. Patients were seen at least 5 times during the first years after implantation and at least twice a year, later on. Apart from planned visits, additional visits took place at the patient’s initiative. Dun et al. (2012) published data on medical complications of 903 successive adult patients, implanted from the start in 1988 up to 2008. Table 5 shows that one revision surgery has been performed per 30 years of follow-up (column 5). Wazen et al. (2008) published a similar study. They started later, in 1998. That might be the reason why their result is better than that of Dun et al. More recently, prospective multicentre studies have been published using a specific new, improved percutaneous implant and/or implant procedure (Nelissen et al., 2014; Kruyt et al., 2018; see Table 5). Indeed, further progress has been made, as relatively less revision surgeries were needed. 

It should be noted that the follow-up period of the latter two studies is relatively short, 3 years. However, as has been shown by Dun et al., most complications occur in the first year post implantation. 

The last columns of the table presents data related to skin problems around the percutaneous implant (percentage of visits with some skin reaction). Severity of skin reaction was expressed in Holgers grades 1 to 4. Grade 2, red and moist tissue, requires local treatment; grades 3 and 4, more severe conditions, might require revision surgery (Wazen et al., 2011).  Indeed, skin problems remain common; however, the percentage of serious skin problems requiring some type of intervention is decreasing (Holgers grades 2 to 4; last column). 

 

Table 5.  Number of revisions related to the total follow-up period and the percentage of soft tissue reactions

Author, year n revisions Summed follow-up relatively All skin reactions3 Holgers grade > 2
Dun, 2012 9031 1452 4.267 yrs 1 in 30 yrs 15% 4,3%
Wazen, 2008 218 23 990 yrs 1 in 43 yrs 4
Nelissen, 2014 52 2 156 yrs 1 in 78 yrs 23,1% 1,8%
Kruyt, 2018 39 2 117 yrs 1 in 58 yrs 15,5% 1,5%

1only the adults; 2revisions because of soft tissue problems plus lost implants; 3Holgers grades 1 to 4; 4no Holgers grades reported 

 

Similar systematic studies on complications with the other types of BCDs and MEIs are lacking. 

It is suggested, based on the data of Table 5, that one revision surgery in 40 years might be considered as the standard for all implantable devices.

 

9.3. Concluding remarks

 

Fortunately, nowadays several amplification options are available for deaf and hearing-impaired patients for whom conventional amplification (BTE) is no longer effective or contra-indicated (CI, BCD, MEI). Thanks to the investigators and the companies that put these devices into the market! 

Regarding the future; a main concern is the costs of these interventions; each update of any implantable device comes with a higher price. Cost is a major issue; already in 1998, Daniels & Sabin concluded: ’no country in the world can afford all of the medical care that providers can render to populations’ (Daniels and Sabin, 1998). In other words, there is a great need for better affordable hearing devices including implants, see also the ‘World report on hearing’ (WHO, 2021).

 

Appendix 9.1 

 

Section 9.1.1. Regarding the 53 BTE users: the inclusion criteria were: age between 18 and 70 years, with flat or mildly sloping audiogram (difference in threshold between 0.5 kHz and 4 kHz less than 40 dB HL) with hearing thresholds < 110 dB HL up to 4000 Hz (to exclude patients with dead regions), using BTEs for at least 3 years. They were all fitted with state-of-the-art digital hearing devices using the NAL-NL fitting rule in the Nijmegen clinic, using one and the same validation protocol (Snik, 2007; Verhaegen et al., 2008).  

Selection criteria for the 139 CI users: postlingual deafness with normal cochlear anatomy. Age: between 18 and 70 years. Devices used were the Cochlear CI (N22, N24, Freedom) and Advanced Bionics CI (C1, 90K), almost equally divided. The patients were implanted between 1990 and 2009. Same evaluation protocol was used as for the BTE users. Evidently, the included data are relatively old data, however, comparable to that for a group of 52 CI users implanted in Nijmegen between 2010 and 2016, using similar conservative inclusion criteria. Their mean MPS was 79% (Huinck et al., 2019), close to MPS of reference CI group (75%).

   

Section 9.1.2. Regarding the 30 VSB users: inclusion criteria were: age 18-70 years, with flat or mildly sloping audiogram (difference in threshold between 0,5 kHz and 4 kHz less than 40 dB HL) and with hearing threshold < 80 dB HL up to 4 kHz, using their VSB for at least 1 year. The patients were fitted between 1998 and 2008 (unilaterally) with the AP303 or AP404 speech processor. For the fitting, the NAL rule was applied. The evaluation protocol was the same as used to test the BTE users (Snik, 2007; Verhaegen et al., 2008). 

Evidently, the included data are relatively old data. Maier et al. (2015) presented data of a more recently implanted group (their group 1, n=34). They were fitted with the Amade speech processor, using the NAL fitting rule. Individual data were not presented: in Figure 2, the average MPS is presented at the average pre-intervention PTA3 (red symbol). This average MPS is in agreement with the Nijmegen data (data-point on the best fit line) suggesting that the MPS values used might still be relevant. Note that Maier et al. reported the mean WRS using monosyllables; that score was expressed as a MPS, using a look-up table. 

 

Section 9.2.3. The study group comprised 26 patients, using the most powerful BCD at that time, the Baha Cordelle (Verhaegen et al., 2008). The inclusion criteria were: age 18-70 years, with flat or mildly sloping bone-conduction audiogram and measurable bone conduction thresholds up to 2 kHz. They were all using a pBCD for more than a year. The mean MPO of the Baha Cordelle (0.5 to 4 kHz) is 82 dB HL, 1 to 4 dB below that of more recent powerful pBCDs, as presented in Table 2. 

 

Section 9.2.4. Deterioration of hearing loss in aging patients with chronic middle ear problems. 

For subjects with pure sensorineural hearing loss, ISO 7029 describes the deterioration of hearing sensitivity owing to aging, however, only for otologically normal subjects (ISO, 2017). In case of chronic middle ear problems, progression in cochlear hearing loss might follow the ISO 7029 norm, however, with an additional cochlear loss owing to either the infections or medical interventions in the past. We assessed that additional cochlear loss by comparing the bone-conduction thresholds of patients between 50 and 70 years, using a BCD, with the age-specific ISO 7029 thresholds. The assumption is that cochlear damage caused by the middle ear problems is reduced significantly by using a BCD instead of a BTE (e.g., Macnamara et al., 1996). 

Three studies with relevant data were identified. Table A9.1 shows that the ‘additional sensorineural hearing loss’ is approx. 20 dB (note: the ISO thresholds used were the average for those of men and women).

 

Table A9.1. Bone conduction thresholds compared to age-specific ISO 7029 norms of patients with mixed hearing loss using BCDs; the additional cochlear loss.

Study; first

author

n (ears) 0.5 kHz

dB

1 kHz 

dB

2 kHz

dB

4 kHz

dB

mean 

dB

Bosman, 2013 18 15  20 25 20  20 
Flynn, 2012 20 15 20  20  10  17 
Pfiffner, 2011*  44 19

*their group C; no frequency specific data available

 

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Wazen JJ, Young DL, Farrugia MC, Chandrasekhar SS, Ghossaini SN, Borik J, Soneru C, Spitzer JB. Successes and complications of the Baha system. Otol Neurotol. 2008 Dec;29(8):1115-9. 

 

Wazen JJ, Wycherly B, Daugherty J. Complications of bone-anchored hearing devices. Adv Otorhinolaryngol. 2011;71:63-72. 

 

WHO 2021. World report on hearing. Geneva: World Health Organization; 2021. Licence: CC BY-NC-SA 3.0 IGO.

 

Zwartenkot JW, Snik AF, Mylanus EA, Mulder JJ. Amplification options for patients with mixed hearing loss. Otol Neurotol. 2014 Feb;35(2):221-6.

 

Chapter 10. A rather technical update, March 2021

Are today’s implantable hearing devices better than conventional devices for patients with conductive or mixed hearing loss?

Introduction

In March 2014, we published a paper in ENT & Audiology news, with the same title. The present overview should be considered as an update. Since 2014, several new implantable hearing devices have been introduced, existing device types have been further developed and, unfortunately, some devices have been withdrawn from the market. Below, an overview is given of devices, which are available early 2021, with the focus on their effectiveness; only acoustical implantable devices are considered. The central question is: what are the choices for a patient with a particular hearing loss? In this overview, a division is made between patients with conductive/mixed hearing loss (part 1) and those with sensorineural hearing loss (part 2). This overview can be considered as a summary of the website snikimplants.nl. In the present text, regularly, a reference is given to a specific (sub)chapter of the website for more detailed information; these references are colored red.

Part 1; patients with conductive and mixed hearing loss

Patients with conductive or mixed hearing loss become candidates for amplification if reconstructive surgery is not an option or might lead to poor post-surgical hearing thresholds (see e.g. Nadaraja et al., 2013). The amplification options available nowadays are: the conventional acoustic behind-the-ear devices (BTE) or in-the-ear devices, conventional (non-surgical) bone-conduction devices (BCDs), (semi-)implantable BCDs and active middle ear implants with their actuator coupled to one of the cochlear windows. This overview aims primarily at comparing the audiological capacity (effectiveness) of non-surgical and implantable BCDs, mainly based on literature reviews.

1.1 The limitations of air-conduction devices in conductive and mixed hearing loss

BTEs are effective hearing devices for sensorineural hearing loss, however, less effective, if the patient has an additional attenuation (air-bone gap) on top of the sensorineural hearing loss. Obviously, if the gain of a BTE, measured on an ear simulator, is 50 dB and the maximum output is 115 dB SPL (representative values), and if the hearing impaired patient has sensorineural hearing loss with additionally an air-bone gap of 45 dB, then the gain reduces to just 5 dB (50-45) and the maximum output to just 70 dB SPL (115-45). Wardenga et al. (2020), studying patients with otosclerosis or tympanosclerosis (no contra-indication for using an air-conduction device) indeed showed that the width of the air-bone gap is an important success factor for a BTE fitting.

Fitting a conventional BCD is not necessarily more effective than a BTE, as the bone-conduction route is far less effective than the air-conduction route. Amongst others, Skoda-Türk and Welleschik (1981) showed that the bone-conduction route is even approximately 50 dB less effective. In other words, the first 50 dB produced by the amplifier of such a traditional BCD are ‘lost’, making this solution in principle as (in)effective as a BTE for the patient with a 50 dB air-bone gap. So, both options are not effective; however, if the air-bone gap is limited, below 50 dB HL, then the BTE might be the better choice.

In the case of a malformed, atretic ear or chronically inflamed middle ear, BTEs cannot or should not be used, and thus, BCDs are the next option.

1.2 Conventional, transcutaneous BCDs

Figure 1 shows a conventional BCD, which comprises a sound processor (in a BTE housing) and, separately, the actuator (bone vibrator). The actuator is pressed against the skin, behind the ear, by means of the steel spring headband. This traditional set-up has been updated during the last decades. Instead of the steel headband to keep the actuator in place, a so-called softband has been introduced, specifically developed for young children (Baha Softband; Verhagen et al., 2008). Nowadays, the BCD manufacturers have such a solution available.

figure 2.2

Figure 1, a traditional, conventional bone-conduction device; source Internet

More recently, a BCD with a magnetic coupling (needing surgery; see section 1.2.1) and a BCD with an adhesive coupling, the Adhear device, were introduced (e.g. Favoreel et al. 2020; Osborne et al. 2020).

Below, a comparison is made of these devices, based on a systematic review of published papers (period: 2005-early 2021). Papers presenting audiological data, comparing two different types of the above-introduced BCDs, were included. The search string included device names, and ‘bone-conduction’ and ‘hearing thresholds’. The following eight papers were identified and included in this overview: Verhagen et al., (2008); Christensen et al. (2010); Denoyelle et al. (2015); Powell et al. (2015); Giannantonio et al. (2018); Skarzynski et al. (2019); Favoreel et al. (2020) and Osborne et al. (2020).

The hypothesis is that the new coupling options are approximately as effective as the traditional coupling with the steel headband.

Verhagen et al. (2008) and Christensen et al. (2010) compared the Baha Softband with traditional BCDs with steel headband. Combining these 2 studies (in total n=22) a small difference in aided thresholds was seen of 2.3 dB in favor of the softband. Favoreel et al. (2020), Osborne et al. (2020) and Skarzynski et al. (2019) compared the Adhear with the softband. Combining the data from these 3 studies (n=37), a minor difference was found of 1.6 dB, in favor of the Adhear. So, the ‘traditional’ transcutaneous BCD has been updated and, as such, is still on the market. A major advantage of these devices is that surgery (implantation) is not involved. 

1.2.1 Conventional BCDs, transcutaneous solutions that require surgery

In 2011, another type of coupling was introduced; the magnetic coupling (Sophono device; e.g. Denoyelle et al., 2015). In contrast to coupling the actuator with a headband or with an adhesive, surgery is involved. One magnet is placed subcutaneously, connected to the skull, behind the pinna, and the other magnet is attached to the actuator of the BCD. Some years later, the Baha Attract was released with the same type of coupling (e.g. Powell et al., 2015). Denoyelle et al. (2015) compared the (pre-surgery worn) Baha Softband and the Sophono device. No differences in aided thresholds were found. Aided thresholds obtained with the Baha Attract and Sophono were compared in two other studies (Powell et al. 2015; Giannantonio et al. 2018). After pooling their data (n=32) a difference of 3 dB was found in favor of the Baha Attract.

Next, the mean aided threshold (0.5, 1, 2, 4 kHz) was calculated for all subjects from any of the studies, using one of the non-implantable transcutaneous solutions (in total n=109) and, separately, those using a semi-implantable transcutaneous device with magnetic coupling (n=79). The calculated mean aided threshold was 28 dB HL (s.d. of 3 dB HL, range 20-33 dB HL) and 29 dB HL (s.d. of 2 dB HL, range 25-33 dB HL), respectively. Therefore, as hypothesized, in terms of aided thresholds, all these transcutaneous BCDs seem to be equally effective. So, the type of coupling doesn’t play a part. It should be noted that the reported aided thresholds, in the range of 25-30 dB HL, are relatively poor, especially for young children, who need to develop speech and language (see section 1.6).

Apart from effectiveness, other factors play a role in the selection of a particular BCD, like ease of device use, costs, cosmetics and whether or not the device is comfortable to use. If implanted, additional factors that should be considered as well are: the stability and safety of the implanted part of the BCD, extra costs and MRI compatibility.

The softband is easier to use and better accepted than a steel headband (Verhagen et al., 2008); in contrast to the softband, skin problems occurred more frequently with headband, owing to the rather high local pressure to keep the actuator in place and effective, with the steel headband (Snik et al., 2005). With the adhesive BCDs, problems with the skin have been reported as well. Neumann et al. (2019) reported that 2 of the 12 subjects using the Adhear device discontinued using their device owing to persistent irritation of the skin, while other studies reported no skin problems with the adhesive BCD (Favoreel et al., 2020). Concerning the BCDs with the magnetic coupling; reviewing the literature, Bezdjian et al. (2017) reported that skin problems occurred in 29% of subjects using the Sophono device; in 3.5% of the subjects these problems were severe. Dimitriadis et al. (2017) reported minor skin problems in 4 % of their Baha Attract users and serious skin problems in 3 %.

In short, skin problems are not uncommon and might occur with all these transcutaneous devices; however, the percentage of serious problems leading to non-use or even revision surgery is low.

Regarding children, it should be noted that the youngest age at which the Sophono and Baha Attract magnets can be implanted is 5 years, according to the manufacturers. So, for young children (< 5 years), only the non-surgical options remain.

1.3 BCDs with percutaneous coupling

Two more types of BCD are available that bypass the skin and subcutaneous layers. These layers attenuate the vibrations produced by any transcutaneous BCDs. These two types of BCD comprise a BCD with percutaneous coupling to the skull and a BCD with its actuator implanted, under the skin, connected by a transmission link to an externally-worn sound processor (see below, section 1.3.1).

In the late eighties a percutaneous coupling for BCDs was developed as an alternative for the transcutaneous coupling. Hakansson et al. (1984) showed that their newly developed percutaneous coupling was approx. 15 dB more effective than the transcutaneous coupling, because the vibration-attenuating skin and subcutaneous layers were bypassed. Nowadays, there is a choice between the percutaneous Cochlear Baha devices and Oticon Ponto devices. Apart from the standard sound processor, more powerful BCD processors are available, what is of importance for longevity of the treatment (Chapter 4.2). Regarding aided thresholds obtained with the first generations Baha compared to those of the conventional BCDs, a systematically difference of 6-10 dB was found in favor of the Baha (Cremers et al., 1992; Snik et al., 2004). Recently, a comparison was made between more homogenous groups, viz. children with conductive loss. Aided thresholds were more than 10 dB better with Baha (Chapter 7.2.1).

Another approach to compare BCDs, instead of studying aided thresholds, is to compare maximum output levels (MPO) of different types of BCDs (Chapter 2.3). The MPO, or the loudest sound that a BCD can produce, is a device-characteristic, objective measure. In comparison BTEs, the MPO of BCDs is rather limited; the MPO of BTEs might be 120 dB SPL (average at 0.5, 1, 2 and 4 kHz, which equals approx. 115 dB HL). Transcutaneous BCDs have a MPO, expressed in dB HL, just around 55 dB HL (e.g. Sopohono device; Chapter 2.3). The Baha Attract is an exception, whenever a power processor (BP110) is used instead of the standard processor; the measured MPO was 64 dB HL.

The MPO of standard percutaneous BCD processors is approx. 67 dB HL, while the MPO of the more powerful processors is significantly higher, up to 80-85 dB HL (Chapter 2.3). All these MPO values are limited, restricting the patient’s dynamic range of hearing with the device switched on. The patient’s dynamic range of hearing is, by definition, the difference in dB HL between the hearing (bone-conduction) thresholds and the MPO of the fitted BCD. Studying the MPO of all the different types of BCD leads to similar conclusion regarding effectiveness as that drawn from studying aided thresholds. That was expected because, to deal with that limited MPO which causes distortions of loud sounds, a lower gain setting than usual might be chosen by the subject. Doing so, on the one hand distortions will be less audible, while on the other hand aided thresholds will be worse. Indeed, Cremers et al. (1992), comparing conventional BCDs with headband, with the percutaneous Baha, showed that the aided thresholds were approx. 10 dB better with percutaneous Baha, while harmonic distortion, measured in the sound field, was significantly less with Baha, ascribed to the higher MPO.

Percutaneous BCSs are the most powerful type of BCD on the market (early 2021).

Problems with the skin around the percutaneous implant are not uncommon. A recent study showed that adverse skin reactions, needing treatment, occurred in 15% of the patients (Langerkvist et al., 2020). Complication leading to revision surgery also occur; the longer the follow-up, the higher the chance. To related revisions with follow-up, the revision ratio was introduced (Chapter 5.1.2), being the accumulated follow-up periods of all subjects in the study divided by the number of revision surgeries in that group. That ratio proved to vary between studies and surgical procedures. The traditional surgical approach includes thinning of the skin around the percutaneous implant; in this case, in adults, the ratio was 1 revision in 43/77 years of follow-up (2 studies). Recently, other approaches have been introduced, mainly to simplify the surgical procedure (no thinning of the skin layers). Then the ratio is somewhat less: 1 revision in 30/55 years (also 2 studies). These ratios seem to be acceptable. Complication rates are higher in children; see section 1.6.

1.3.1 Transcutaneous device with the actuator (vibrator) implanted, connected to the skull

In 2001, another concept was introduced; the actuator of the BCD was totally implanted, rigidly connected to the skull. That actuator is connected to the externally sound processor by a transmission link. The first device on the market was the Bonebridge device. Huber et al. (2013) studying BCD devices in cadaver heads, measuring vibration of the cochlear promontory, concluded that the Bonebridge device was as effective as the standard Baha device (BP100). In agreement with that, Gerdes et al. (2016), comparing two groups of patients, one group using the Bonebridge and the other the percutaneous Baha, reported that aided threshold and several speech recognition scores were identical. Comparing the MPOs of the two devices (the standard Baha and the Bonebridge), a difference of 4 dB was seen in favor of the Baha (Chapter 2.3).

As said before, all these MPO values are limited, restricting the patient’s dynamic range of hearing with the device. To deal with that, compression amplification is often used. In principle, compression amplification is advocated for patients with a limited dynamic hearing range caused by physiological factors. Compression can also be used to minimize the consequences of a limited dynamic range caused by a low MPO of the fitted device. However, the better option, instead of using compression, is to apply a more powerful BCD. Recently, another transcutaneous device with the actuator connected to the skull was introduced, the Osia device (Cochlear Company). This rather new device will be discussed soon on the website Snikimplants.nl.

1.4 Active middle ear implants 

Another option instead of a BCD is to use an active middle ear implant with its actuator coupled to one of the cochlear windows; nowadays, the only available option is the Vibrant Soundbridge device (Chapter 2.3). Regarding the audiological outcomes: the MPO is approx. 85 dB HL (Chapter 2.3) thus in the same range as the most powerful percutaneous BCD, the Baha 5 SP. Regarding aided thresholds, using data from a literature search (including a.o. 14 studies with VSB and 18 studies with percutaneous BCD, in patients with mixed hearing loss (Chapter 3, last paragraphs), a spread in outcomes is seen, however, not a structural difference between these two types of devices. This suggests that fitting either a percutaneous BCD or a VSB leads to similar aided thresholds. Both devices worked well in patients with mixed hearing loss with a sensorineural hearing loss component of up to 50-55 dB HL.

The main theoretical advantage of a VSB is that in contrast to a BCD, only the cochlea of the implanted ear is stimulated. With a BCD, owing to the low attenuation of the bone-conducted vibrations through the skull bone, the contralateral cochlea is stimulated as well, although to a lesser degree, referred to as ‘cross’ stimulation. The limited research looking into ‘cross’ stimulation shows no clear advantage of the VSB (regarding directional hearing, see Agterberg et al., 2014); however, no firm conclusion can be drawn yet.

Implantation of the VSB in an atretic ear might be challenging owing to the abnormal anatomy (Mancheno et al., 2017). In chronic inflamed middle ear, first of all, the inflammation should be dealt with, e.g. by a subtotal petrosectomy with obliteration of the middle ear with abdominal fat. Then the VSB can be applied effectively (Chapter 5.2). So, in contrast to BCD, VSB implantation in conductive/mixed hearing loss is more demanding. Regarding complications, post-VSB implantation, again, the relation between reported revision surgeries and the accumulated follow-up time was determined (5 studies). The ratio varied from one revision in 15 years up to one in 28 years (Chapter 5.1.2), thus worse than those found for percutaneous BCDs (see above section 1.3). It should be noted that most of the included studies presented combined data of the VSB applied in conductive/mixed hearing loss and in sensorineural hearing loss.

1.5. Conclusion

For optimal treatment for a particular patient with conductive or mixed hearing loss, choosing the best BCD, BTE or the VSB, is a challenge. Given the effectiveness and the limitations of the different options, counseling is of utmost importance. Transcutaneous BCDs are less vulnerable to skin reactions compared to percutaneous BCDs. Furthermore, the complications with percutaneous implant might require revision surgery. However, the percutaneous BCD is by far the most powerful BCD solution, thus leading to better hearing. As the processor of percutaneous devices can be changed for more powerful processors, the percutaneous BCD might be used life long, in contrast to the other types of BCDs (Chapter 4.2). For children choosing the best option is even more challenging; see next section. Compared to BCD implantation, VSB implantation is more complicated and complications requiring revision surgery are relatively high. On the other hand, as for percutaneous BCDs, longevity is not a major issue.

1.6 Application of BCDs in children

For hearing impaired children, counseling is even more important. For young children, good hearing is essential: the better the hearing the higher the chance that the child will develop normally. According to Northern and Downs (1991), indeed, children need normal hearing (15 dB HL or less) to ensure normal development of speech and language. This statement is in agreement with anecdotal data presented by Verhagen et al. (2008). Therefore, when counseling the parents of a child with a hearing loss, sufficiently powerful amplification options should be advocated.

In case of a congenital conductive hearing loss, whenever the tympanic membrane is at least partly visible (typically the air-bone gap will be near 40 dB HL), BTEs might be an effective solution. A transcutaneous BCD with headband or softband should be used in all the other cases. Nowadays, the adhesive coupling is also an option. As the aided sound field thresholds (mean data of 28-29 dB HL; see above) are significantly above the target of 15 dB HL, replacement of the transcutaneous BCD with the more powerful percutaneous BCD or Bonebridge, should remain on the agenda and, meanwhile, speech and language development should be monitored (Verhagen et al., 2008). The use of percutaneous BCDs involves implantation of a skin penetrating titanium coupling. Concerning the youngest age at which such implantation is feasible, thickness of the skull plays an important role: 3 mm or more is preferred (Snik et al., 2005). This implies that the child must be approximately 4 years old. Even then, loss of implants is a problem. Recently, a literature review was published aiming at complications and implant loss in children (comprising 952 percutaneous implants; Kruyt et al., 2020). Implant loss occurred in 14% and revision surgery was needed in 17% of the implants. However, implant loss with more recently introduced new percutaneous implants with a wider diameter, seemed to be better than that of older types, viz. 8,2% versus 25,5%. Although these figures are significantly worse than the reported complication rates with transcutaneous devices, the higher effectiveness of percutaneous devices should be taken into account while counseling. However, once more, it should be noted that, on the average, aided thresholds in children with conductive hearing loss, using percutaneous BCDs, are more than 10 dB HL better than those in children using conventional transcutaneous BCDs (see review, Chapter 7.2.1).

The VSB with the transducer coupled to one of the cochlear windows has been applied in children as young as 2 months (Mandala et al., 2011). Since then, little has been published. Only one paper was found (with n>5); Leinung et al. (2017) presented results obtained in 13 young children with aural atresia with an average age of 2.5 years. While the mean bone conduction threshold (at 0.5, 1, 2, 4 kHz) of the whole group was 8 dB HL, the mean aided threshold was just 40 dB HL, which is rather poor. In order to advocate VSB implantation in toddlers and young children, more evidence is needed. Studying elderly children, in total 60 children from 4 studies, a better mean aided threshold was found, viz. 31 dB HL, which still is approx. 10 dB worse than that of percutaneous BCDs (Chapter 7.2.1). That figure might improve with growing surgical experience.

In summary, regarding children, choices have to be made, based on the child’s development, audiological aspects (effectiveness) and the drawbacks of the devices. Good hearing is of utmost importance and, as long as the rehabilitation is not optimal in audiological terms, monitoring the child’s progress remains of prime importance.

It should be noted that application of bilateral BCDs in children with bilateral conductive hearing loss, congenital or acquired, is of utmost importance (Chapter 6.2); the vast majority of the children accepts two devices. Papers on bilateral application of the VSB in children are scares. Application of a BCD in unilateral congenital conductive hearing loss, however, is still debatable; profit is limited and non-use is unacceptably high, even with a percutaneous BCD (Chapter 6.2). Within four years after implantation about 40% of the children stopped using the BCD and 25% used it occasionally. Similar long-term data on the use of VSB in such children is lacking.

However, binaural hearing scores in patients with unilateral acquired conductive hearing loss using a percutaneous BCD were much better than those found in congenital cases (adult data; Agterberg et al., 2011). This suggests that previous experience with bilateral input is an important success factor.

Part 2; Acoustic implants for patients with sensorineural hearing loss

The BTE is the standard treatment for patients with mild to severe sensorineural hearing loss; whenever the hearing loss is severe/profound, cochlear implantation is the treatment of choice.

An alternative amplification option for patients with moderate/severe sensorineural hearing loss is an active middle ear implant. Nowadays, two semi-implantable devices, the Vibrant Soundbridge (VSB; Chapter 2) and the Maxum device (Ototronix, USA), and one fully implantable device, the Esteem device (Envoy Medical, St. Paul, MN, USA) are on the market. In 2014, two reviews of the literature were published comparing these devices amongst others, with BTEs; it was reported that no structural audiological benefit was found compared to BTEs (Chapter 8.1). As the implantable devices are expensive and involve surgery, the cost-utility ratio of these devices is unfavorable compared to BTEs. Since the two reviews of the literature were published, in 2014, to our knowledge, just two more clinical trials with audiological data were published; one regarding the Maxum device and one with the Esteem device, as published by company-independent research groups. In contrast, since 2014, 11 papers were identified with audiological data obtained in patients with sensorineural hearing loss using the VSB.

Although these middle ear implants have, on the average, no convincing audiological advantages compared to BTEs, they might be of major benefit for patients who cannot use a BTE (e.g. patients that don’t tolerate ear molds owing to chronic external otitis; see e.g. VSB fact sheet, Med-El, 2015)Edfelt et al. (2014) even showed that for such patients the application of the VSB device is cost effective.

Nijmegen, March, 10th 2021, Ad Snik, Martijn Agterberg

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Blog 2019-1: Percutaneous bone conductor or (active) middle ear implant to treat mixed hearing loss? No clear winner yet

Introduction

To rehabilitate patients with mixed hearing loss, implantable devices like the Vibrant Soundbridge middle ear implant (VSB) or percutaneous bone conductors (BCDs) have been applied. Such implantable devices become the next option whenever conventional hearing devices (e.g. behind-the-ear device) or reconstructive surgery are not possible or probably not effective (e.g. in case of aural atresia or chronic draining ears). Using recent publications, describing clinical trails, a comparison is made between audiological outcomes obtained with either power BCDs and VSBs. Studies in patients with severe mixed hearing loss were considered only if the mean sensorineural hearing loss component was at least 35 dB HL (arbitrary choice).

Medline was used to find relevant studies, published between 2016 and early 2019. Of the 30 detected studies, 25 studies were excluded (see Materials and Methods at the end of this blog for details) and five studies remained:

Bush et al. (2016) published data 58 patients with mixed hearing loss using the VSB with its actuator coupled to either the round window (RW) or the oval window (OW); Lee et al., (2017) and Zahnert et al. (2019) also published such VSB data, on 19 and 24 patients, respectively. Bosman et al. (2018) published data on the application of a recently introduced percutaneous power BCD, viz. the Baha 5SP, applied in 10 patients; Bosman et al. (2019) published also data on another percutaneous power BCD, the Ponto 3SP device, applied in 18 patients.

The Methods and Materials section is added at the end of this blog.

Results

Figure 1 presents the frequency-specific gain values related to the (bone-conduction) threshold; it should be noted that BCDs as well as VSBs with actuator coupled to the cochlea, directly stimulate the cochlea, bypassing the impaired middle ear (and thus the air-bone gap). Therefore, to assess the effectiveness of the device fitting, only the ‘effective gain’ is of importance, defined as the cochlear (bone-conduction) thresholds minus the aided thresholds (e.g. Busch et al., 2016; Snik, 2018).

The figure presents the ‘effective gain’ divided by the cochlear hearing loss thus the bone-conduction threshold, referred to as the gain-threshold (GT) ratio, plotted as a function of frequency (0.5 kHz to 4 kHz). If GT=0, it means that the aided threshold and the cochlear thresholds coincide, indicating no ‘compensation’ of the cochlear hearing loss. Regarding the VSB, the subfigure at the right shows the calculated results using Zahnert et al. (2019), Lee et al. (2017) and Busch et al. (2016), separately. As these curves show a similar trend, data were averaged after weighting for the number of patients per study. The mean VSB data in the left subfigure show the highest GT ratio at 2 kHz with roll-off in higher and lower frequencies. For the Baha 5SP, a broader frequency response is seen comparable to that of the Ponto 3SP; obviously, the latter device is less powerful (lower GT values).

The target GT ratio, according to the well-validated NAL-fitting procedure, is also indicated (Dillon, 2012; see also Materials an Method section at the end of this manuscript). Systematic differences are seen between the measured GT ratios and that NAL-target GT ratio, especially in the high and low frequencies. At 2 kHz the measured values are closest to the NAL-GT target. 

12

Figure 1. The gain-threshold (GT) ratio presented as a function of frequency. Results of the included studies are shown (individual data of the 3 VSB studies, presented in right subfigure, were averaged). According to the NAL-RP fitting rule, the target GT ratio is approx. 0.45 (indicated in the figure) 

The NAL fitting rule is well established for sensorineural hearing loss (Dillon, 2012). However, compared to conventional behind-the-ear devices, BCDs and middle ear implants have a limited dynamic range caused by a relatively low MPO (maximal power output). Therefore, a modified, pragmatic version of the NAL fitting rule has been introduced for these implantable devices (Snik et al., 2019; Materials and Methods section), meant primarily for validation purposes. Figure 2 shows target-aided

3

Figure 2. The difference between prescribed aided thresholds and measured aided thresholds as a function of frequency. The modified NAL-procedure was used. Pooled VSB data are presented, and, separately, the data of the two percutaneous BCD studies

thresholds (obtained with this pragmatic validation rule) minus the measured aided thresholds. A value of 0 means that the result is adequate according to that rule; negative values indicate insufficient gain. Weighted means are presented of the pooled VSB studies and, separately, the results of the Ponto 3SP study and Baha 5SP study. In agreement with Figure 1, most obvious discrepancies are found at 0.5 kHz and 4 kHz, especially for the VSB device, in the order of 15-20 dB. The Baha 5SP seems to be the most adequate device with a rather flat curve around 0; according to Figure 2, the Ponto 3SP device lags behind by approx. 10 dB, what might be ascribed to the difference in MPO of the two BCDs. The MPO of the Baha 5SP is approximately 9 dB higher than that of the Ponto 3SP (Schwartz, 2018). The MPO of the VSB and the Baha 5SP are rather comparable (Chapter 2 in Snik, 2018).

Apart from aided thresholds, all the five studies presented aided speech recognition scores in quiet, however, measurement conditions and the tests used were divers. Nevertheless, in all the studies monosyllabic words were used. Bush et al. (2016) and Zahnert et al. (2019) presented word scores (WRS) obtained at a presentation level of 65 dB SPL. Bosman et al. (2018; 2019) reported phoneme scores (PS) of words presented at 60 dB SPL; which can be recalculated to a word score at 65 dB SPL (applying WRS=PS2.3 and using the slope of the speech performance-intensity curve: 6% per dB). Lee et al. (2017) measured word scores presented at the patient’s most comfortable listening level instead of at a fixed level and, thus, cannot be used for a comparison.

Table 1 shows the outcomes.

Table 1. Speech recognition scores

Study/test VSB, Zahnert VSB, Bush Ponto 3SP Baha 5SP
WRS 65 dB SPL 67% 78%
PS at 60 dB SPL calculated WRS at 65 dB SPL 75%

80%

69%

75%

Note WRS: word recognition score, PS: phoneme score

In summary, for patients with mixed hearing loss, the achieved (effective) gain with either VSB or the percutaneous Baha 5SP show similar results in the mid frequencies (Figures 1 and 2), however, at 4 kHz and especially at 0.5 kHz, best results are found with the Baha 5SP. Regarding speech recognition, results are rather similar (Table 1; the WRS data at 65 dB SPL input). Therefore, from an audiological point of view, differences between devices are limited.

Closing remarks

It is known that the VSB outcomes depend on the efficacy of the actuator coupling to the cochlea (e.g. Busch et al., 2016). New coupling options are being developed (e.g. Lenarz et al., 2018), which might have a positive effect on the frequency response. Another way to deal with the peaked frequency response is to lower the gain of the VSB device in the mid frequencies. However, that is only an option if the implanted device has sufficient reserve.

For the percutaneous BCDs, the coupling of the processor to the skull is more or less standardized and therefore not a major source of variance between subjects.

To choose a device, the performance is highly relevant but also of importance is the status of the middle ear (can a VSB be placed?), invasiveness and complexity of the surgery, risk of medical and technical complications, longevity, cost etc. Regarding the Baha 5SP, with high volume setting, it might be advised to use the behind-the-ear processor away from the ear, with a clip connected to clothing, to deal with feed-back. That might also be considered as a draw back.

Note that the conclusions drawn are valid for the present devices/processors. If more powerful processors or more effective actuators are introduced or if better coupling techniques are developed, conclusions might change.

Appendix.

Materials and methods

Search strategy

This review was carried out in accordance with PRISMA (Preferred Reporting Items for Systematic reviews and Meta-Analyses; Moher et al., 2009). Medline (Pubmed) was used to find relevant studies, published between 2016 and early 2019. Search string was: ((soundbridge OR ponto OR baha) AND (mixed hearing) NOT transcutaneous). Thirty manuscripts were identified; based on the abstracts, 20 manuscripts were excluded because: the focus was on cochlear implantation (1), comprised a review, no original data (5), case reports and how-we-do-it reports (4). Manuscripts dealing with medical/surgical issues (8), technical issues (1) and application in pure sensorineural hearing loss (1) were excluded as well. The full text of the remaining 10 manuscripts was reviewed. One more manuscript was excluded because the mean cochlear hearing loss was below 35 dB HL (Iwasaki et al., 2017), three manuscripts were excluded as the bone-conduction thresholds and/or aided thresholds were not presented frequency specific (essential information for the present analyses; Brkic et al., 2019; Olszewski et al., 2017; Monini et al., 2017) and one manuscript with just 5 relevant patients (Gregoire et al., 2018). Five studies were included; regarding VSB: Bush et al. (2016); Zahnert et al. (2019) and Lee et al. (2017). Regarding BCD, two studies were identified: Bosman et al. (2018) and Bosman et al. (2019).

Table 2 gives an overview of some relevant patients’ characteristics of these 5 studies (age and mean cochlear or bone-conduction thresholds). On the average, the patients using BCDs are older than those using the VSB.

Table 2. Some patient characteristics

Study; first author Lee, 2017 Busch, 2016 Zahnert, 2019 Bosman, 2018 Bosman, 2019
n 19 78 24 18 10
Device VSB VSB VSB Ponto 3SP Baha 5SP
Age, years 55±17 57±17 55 (34-75) 64 (22-80) 70 (59-77)
PTAbc dB HL 45 37 40 38 44

Note. n: number of patients; PTAbc: calculated mean bone-conduction threshold at 0.5, 1, 2, 4 kHz. Regarding the age at implantation, mean age is presented with standard deviation or the median age with range between brackets

Gain measurement

BCDs as well as the VSB with its actuator coupled to one of the cochlear windows stimulate the cochlea directly, bypassing the impaired middle ear. Therefore, the air-bone gap doesn’t play any role when fitting these devices and, to evaluate audiological benefit, the ‘effective gain’ is of importance. The ‘effective gain’ is defined as the cochlear (bone-conduction) thresholds minus the aided thresholds (e.g. Busch et al., 2016; Snik, 2018). The ‘effective gain’ assesses which proportion of the cochlear hearing loss is ‘compensated’ by the device. By definition, the often-used ‘functional gain’ equals the ‘effective gain’ plus the air-bone gap, and is mainly dominated by the width of the air-bone gap. In contrast, the ‘effective gain’ is a measure of the quality of the device fitting and can be compared to target gain values as prescribed by generally used, validated prescription procedures.

Given a certain hearing loss, obviously, the required ‘effective gain’ depends on the cochlear hearing thresholds (the higher the hearing loss, the higher the gain should be). Therefore, the ‘effective gain’ divided by the cochlear threshold (the gain/threshold ratio or GT ratio) is calculated; according to the widely used NAL-RP fitting procedure, this ratio should be approx. 0.45 for 1, 2 and 4 kHz, for proper amplification (Dillon, 2012).

However, for mixed hearing loss, that NAL-prescribed target GT ratio is not easily reached, owing to, amongst others, the limited highest sound level that can be produced by BCDs or VSBs (limited output), characterised by the ‘maximum output’ or MPO of these devices. Therefore, a pragmatically adapted NAL procedure has been introduced for these implants, to be used for validation purposes (Snik et al., 2019). This pragmatic procedure was based on 31 published clinical trials, regarding all types of BCDs and active middle ear implants and, consequently, this new procedure is device independent. The assumption is that appropriate (sufficient powerful) implantable devices have been chosen (regarding the MPO) and that amplification is linear. In short, aided thresholds should equal 0.55 times the cochlear threshold (following the NAL-RP fitting procedure) plus a margin of 5 dB with a minimum value of 25 dB HL at 1, 2 and 4 kHz (margins to deal with the limited MPO of the devices; Snik et al., 2019). Regarding 0.5 kHz, following the NAL-RP rule, calculated target aided threshold is 5 dB higher than that at 1 and 2 kHz, thus an additional margin of 5 dB is set at 0.5 kHz.

References

Bosman AJ, Kruyt IJ, Mylanus EAM, Hol MKS, Snik AFM. On the evaluation of a superpower sound processor for bone-anchored hearing. Clin Otolaryngol. 2018;43:450-455. doi: 10.1111/coa.12989.

Bosman AJ, Kruyt IJ, Mylanus EAM, Hol MKS, Snik AFM. Evaluation of an abutment-level superpower sound processor for bone-anchored hearing. Clin Otolaryngol. 2019 Epub ahead of printing. doi: 10.1111/coa.13084.

Brkic FF, Riss D, Auinger A, Zoerner B, Arnoldner C, Baumgartner WD, Gstoettner W, Vyskocil E. Long-Term Outcome of Hearing Rehabilitation With An Active Middle Ear Implant. Laryngoscope. 2019;129:477-481. doi:10.1002/lary.27513.

Busch S, Lenarz T, Maier H. Comparison of Alternative Coupling Methods of the Vibrant Soundbridge Floating Mass Transducer. Audiol Neurootol. 2016;21:347-355. doi: 10.1159/000453354.

Dillon H. Hearing aids, 2012; Thieme Verlag, Stuttgart

Grégoire A, Van Damme JP, Gilain C, Bihin B, Garin P. Our auditory results using the Vibrant Soundbridge on the long process of the incus: 20 years of data. Auris Nasus Larynx. 2018;45:66-72. doi: 10.1016/j.anl.2017.02.007.

Iwasaki S, Usami SI, Takahashi H, Kanda Y, Tono T, Doi K, Kumakawa K, Gyo K, Naito Y, Kanzaki S, Yamanaka N, Kaga K. Round Window Application of an Active Middle Ear Implant: A Comparison With Hearing Aid Usage in Japan. Otol Neurotol. 2017;38:e145-e151. doi: 1097/MAO.0000000000001438.

Lee JM, Jung J, Moon IS, Kim SH, Choi JY. Benefits of active middle ear implants in mixed hearing loss: Stapes versus round window. Laryngoscope. 2017;127:1435-1441. doi: 10.1002/lary.26244.

Lenarz T, Zimmermann D, Maier H, Busch S. Case Report of a New Coupler for Round Window Application of an Active Middle Ear Implant. Otol Neurotol. 2018;39:e1060-e1063. doi: 10.1097/MAO.0000000000001996

Moher D, Liberati A, Tetzlaff J, Altman DG; PRISMA Group. Preferred reporting items for systematic reviews and meta-analyses: the PRISMA statement. PLoS Med 2009;6:e1000097.

Monini S, Bianchi A, Talamonti R, Atturo F, Filippi C, Barbara M. Patient satisfaction after auditory implant surgery: ten-year experience from a single implanting unit center. Acta Otolaryngol. 2017;137:389-397. doi: 10.1080/00016489.2016.1258733.

Olszewski L, Jedrzejczak WW, Piotrowska A, Skarzynski H. Round window stimulation with the Vibrant Soundbridge: Comparison of direct and indirect coupling. Laryngoscope. 2017;127:2843-2849. doi: 10.1002/lary.26536.

Schwartz, J. MPO Available to the patient: A comparison of the Cochlear™ Baha® 5 Power, Cochlear Baha 5 SuperPower and the Oticon Medical Ponto 3 SuperPower. 26-4-2018. http://pronews.cochlearamericas.com/mpo-available-to-the-patient/

Snik AFM. Auditory implants. Where do we stand at present? 2018; http://www.snikimplants.nl, see chapters 2, 3 and 4.

Snik A, Maier H, Hodgetts B, Kompis M, Mertens G, van de Heyning P, Lenarz T, Bosman A. Efficacy of Auditory Implants for Patients With Conductive and Mixed Hearing Loss Depends on Implant Center. Otol Neurotol. 2019;40:430-435. doi: 10.1097/MAO.0000000000002183.

Zahnert T, Mlynski R, Löwenheim H, Beutner D, Hagen R, Ernst A, Zehlicke T, Kühne H, Friese N, Tropitzsch A, Luers JC, Todt I, Hüttenbrink KB. Long-Term Outcomes of Vibroplasty Coupler Implantations to Treat Mixed/Conductive Hearing Loss. Audiol Neurootol. 2019;23:316-325. doi: 10.1159/000495560.

Blog 2018-1: Implantable transcutaneous bone conductors or percutaneous bone conductors; a free choice? Review of the paper by Cedars et al., 2016

The Baha bone-conduction device (BCD) was developed in the eighties for patients with conductive and mixed hearing loss. When this device is percutaneously coupled, the BCD is a powerful hearing solution (see Chapters 2 and 3, this website) and is generally considered as the gold standard. However, in a number of patients, problems occurred with the skin around the percutaneous implant. Although well manageable in adults (Chapter 5, this website) this has lead to revision surgeries, most frequently in children.

Therefore, new transcutaneously coupled BCDs have been developed (Sophono, Attract). These devices are attached with a magnetic coupling instead of a percutaneous one, with one magnet implanted, connected to the skull. The advantage is that the skin remains intact, however, the price that has to be paid is decreased efficiency of approx. 15 dB, owing to damping of the BCD’s vibrations by skin and subcutaneous layers. The first device on the market was the Otomag device, developed by Siegert et al. Since 2009 this device is known as the Sophono device (Medtronic, Boulder, US). The Baha Attract, introduced 4 years later, uses the same type of coupling (Cochlear BAS, Gothenburg, Sweden; see Chapters 2,3).

Cedars et al. (2016) published a well-documented paper entitled ‘Conversion of traditional osseointegrated bone-anchored hearing aids to the Baha Attract in four paediatric patients’ (2016). That paper was discussed before (Snik, 2017). The reason for the conversion was medical problems ascribed to the percutaneous coupling. Their Figure 5 showed that owing to the conversion, the aided thresholds worsened by 10 to 20 dB. That is quite a high price. Firstly, let us consider the aided thresholds of the child (number 4) with bilateral conductive hearing loss. Using the aided thresholds (data from Figure 5, Cedars et al.), the audibility of speech can be assessed as well as the expected word recognition score, see Figure 1, below. The count-the-dots approach was used (Mueller & Killion, 1991) resulting in an audibility index of 0.78 and 0.35, for the original percutaneous application and the new transcutaneous application, respectively. The related word recognition score can be determined using that audibility index, and is 95% when fitted with a BCD and 60% when fitted with the Attract. Adults with good linguistic skills might be able to deal with deterioration from 95% to 60%, but for children, who need to develop speech and language, it is problematic.

Slide1

Figure 1. The aided thresholds obtained with, in red, the percutaneous BCD and in black, the transcutaneous BCD. The grey area presents the speech area, taken from Mueller & Killion (1990). The audibility index can be determined by counting the dots in the grey area (max. is 100) and divide it by 100

BCDs are also applied in patients with single-sided deafness (SSD). If someone is talking at the deaf side, it is poorly understood owing to acoustic head shadow. This is a high frequency phenomenon; attenuation is approx. 5 dB in the low frequencies and around 10-15 dB, up to 3-4 kHz. Above that frequency, increased attenuation is found, exceeding 25 dB HL at 4-6 kHz (see the classical data of Shaw, 1974). To deal with this, amongst others, a CROS-BCD can be applied. The BCD is placed at the deaf side and works as a transcranial CROS device (e.g. Hougaard et al., 2017).
However, with a transcutaneous CROS-BCD, owing to dampening of the vibrations by the skin and subcutaneous layers, especially in the high frequencies, effective CROS stimulation might be low. Cedars et al. presented data of 3 children with SSD (patients 1 to 3; owing to incomplete data only the results of patients 1 and 2 are discussed) who were using a percutaneous CROS-BCD that was replaced by a transcutaneous one. To obtain CROS-aided thresholds, the normal hearing ear was blocked during the measurements.
Figure 2 shows the original CROS-aided thresholds minus the air-conduction thresholds (normal hearing ear) for the transcutaneous (full lines) and the percutaneous CROS device (broken lines), presented as a function of frequency. Within these two patients an obvious difference is seen between the devices. Attenuation is seen especially with the transcutaneous CROS. That attenuation is worse than the attenuation caused by head shadow, using Shaw’s data (the blue line); in other words, when stimuli are presented at the impaired side, unaided thresholds are better than transcutaneous CROS aided thresholds. The Figure indicates that for these two patients, the application of the transcutaneous CROS-BCD is not an effective solution, in contrast to the percutaneous application (although, unfortunately, information above 4 kHz is missing).

Slide2

Figure 2. Attenuation caused by head shadow (in blue, according to Shaw, 1974), the attenuation of the percutaneous CROS-BCD (broken lines) and transcutaneous CROS-BCD (full lines) of the two patients (P1 in red and P3 in black) calculated from Figure 5, Cedars et al. (2016)

Hougaard et al. (2017) reported non-use and relatively low daily use of the Baha Attract in their 13 adult patients with SSD, and suggested that it probably could be ascribed to poor compensation of head shadow. Furthermore, Hougaard et al. also reported aided thresholds of seven more patients with conductive hearing loss using the transcutaneous Baha. They concluded that the aided thresholds were worse compared to published aided thresholds with the percutaneous Baha. This is in agreement with the data of Cedars et al.’s patient 4.

In summary, indeed problems with the skin around a percutaneous implant are recurrent and annoying. The 12 reported clinical visits for patient 4 in Cedar et al. (2016) might obviously be worrying and upsetting for both the child and the parents. However, according to the review above, in audiological terms, a transcutaneous solution is a major step backwards. Furthermore, transcutaneous BCDs are not free of medical complications either (e.g. Cedars et al., 2016).

The search for the best bone-conduction device, especially for children, should be continued; the options are 1) usage of percutaneous BCDs with more skin-friendly implants, better implantation procedures and improved after care, 2) transcutaneous BCDs with significantly more powerful (and feedback-free) processors.

References
E. Cedars, D. Chan, A. Lau, et al., Conversion of traditional osseointegrated bone-anchored hearing aids to the Baha Attract in four pediatric patients, Int. J. Pediatr. Otrhinolaryngol. 91 (2016) 37-42.
G. Mueller, M. Killion, An easy method for calculating the Articulation Index, Hear. J. 43 (9) (1990) 14-17.
E.A.G. Shaw, The external ear, in: W.D. Keidel, W.D. Neff (Eds.), Handbook of Sensory Physiology, vol. V/1, Springer Verlag, New York, 1974, pp. 455-490.
Snik A. Letter to the editor. Int J. Pediatr. Otorhinolaryngol. 2017;93:172.
Hougaard DD, Boldsen SK, Jensen AM, Hansen S, Thomassen PC. A multicentre study on objective and subjective benefits with a transcutaneous bone-anchored hearing aid device: first Nordic results. Eur. Arch. Otorhinolaryngol. 2017;274:3011-3019.
Dimitriadis PA, Carrick S, Ray J. Intermediate outcomes of a transcutaneous bone conduction hearing device in a paediatric population. Int J Pediatr Otorhinolaryngol. 2017;94:59-63.

Appendix.
It should be noted that using aided thresholds to assess the performance (gain) of a device is not straightforward if non-linear amplification is applied (see appendix 2, this website). Whether or not non-linear amplification was used in the reviewed studies was not reported. With regard to the Hougaard et al. data, it seems that amplification was linear, which is concluded from the fact that the reported ‘gain’ derived from free-field tone thresholds (19.8 dB), was comparable to the gain derived from the ‘supra-threshold’ speech reception thresholds (estimated from their Figure 4b), being 20 dB. That is indicative for linear amplification. Such an analysis was not possible with the data of Cedars et al., as supra threshold measures were lacking.

Chapter 8. Sensorineural hearing loss

Part 2. Challenges and limitations of implantable hearing devices (auditory implants) for sensorineural hearing loss 

8. Sensorineural hearing loss 

8.1 Auditory implants for moderate to severe sensorineural hearing loss

To rehabilitate sensorineural hearing loss, conventional air-conduction hearing aids are the first choice (e.g. behind-the-ear devices or BTEs; Figure 2.1, chapter 2). If the sensorineural hearing loss is not too severe, it is possible to make conversational speech audible again with BTEs. However, speech recognition might still be compromised, especially in noisy situations, despite appropriate amplification owing to the associated deprived processing of speech cues in the cochlea (referred to as cochlear distortion; Plomp, 1978). Today’s digital devices might deal with that; algorithms have been developed that might enhance speech sounds relative to noise, like noise reduction algorithms and adaptive directionality of the microphone (Dillon, 2012).

For patient with severe to profound sensorineural hearing loss (>70-80 dB HL), BTEs might no longer be sufficiently effective because proper restoration of the audibility of speech is not possible and/or because of significant cochlear distortions. Then, cochlear implantation becomes the next treatment option (Hoppe et al., 2015; Huinck et al., 2019). The 70-80 dB HL limit is based on speech recognition tests; above that limit, word score is probably significantly better with a cochlear implant than with a BTE.

An alternative amplification option for patients with a hearing loss below 70-80 dB HL is an (acoustic) auditory implant (Verhaegen et al., 2008). High sound fidelity, high output with less feedback problems and no occluding earmolds were claimed advantages of such auditory implants over BTEs (e.g. Goode, 1995). Since the late nineties, several different types of auditory implants have been introduced for patients with sensorineural hearing loss (Snik, 2011). Nowadays (end 2020), only two semi-implantable devices are still on the market, viz. the Vibrant Soundbridge (VSB; Figure 2.6, chapter 2) and the Maxum device (Ototronix, USA; Pelosi et al., 2014). The latter device comprises a permanent magnet connected surgically to the stapes, in the middle ear. The driver is an electro-magnetic coil placed in a deep fitted in-the-ear hearing aid. Early 2020, the competitor of the VSB, the semi-implantable Cochlear MET device, was taken off the market.

In addition to semi-implantable devices, one fully implantable device is on the market, namely the Esteem device (Envoy Medical, St. Paul, MN, USA; Marzo et al., 2014). Another fully implantable device was the Cochlear Carina, which also was withdrawn from the market early 2020. Rather recently, several systematic reviews were published aiming at the claimed advantages of auditory implants over BTEs. Pulcherio et al. (2014) concluded that, comparing the Carina and Esteem devices to BTEs, no structural audiological benefit was found. As these implantable devices are expensive and involve implant surgery and regularly revision surgery (every 5-10 years, to replace the battery), the cost-benefit ratio of these devices is unfavorable compared to BTEs. Another review by Kahue et al. (2014), comparing published audiological outcomes obtained with middle ear implants (including the VSB and Maxum devices) and BTEs, concluded that a surplus value of the middle ear implants could not be established. Since these reviews were published, screening the literature, just one more clinical trial was found with the Maxum device as well as one with regard to the Esteem device, published by company-independent research groups. Barbara et al. (2018b) reported on 3 patients using the Maxum device; compared to BTEs, two patients had equal speech recognition scores and the third patient had a 10% higher word score with the Maxum device. Regarding the Esteem device, Barbara et al (2018a) published an overview of their outcomes. The patient group described in this paper overlapped with that of a previous paper of the same authors, which was included in the above-mentioned systematic reviews by Kahue et al. and Pulcherio et al.

It might be concluded that compared to BTEs, today’s middle ear implants have no convincing audiological advantages, except for patients who don’t tolerate the earmold of BTEs owing to e.g. external otitis (consensus statement, see Magnan et al., 2005). The recent systematic reviews indicate that that statement remains valid. Some manufacturers of middle ear implants refer to that conclusion (e.g. VSB fact sheet, 2015)Edfelt et al. (2014) showed that for such patients the application of the VSB device is (even) cost effective. Note that according to the set-up of the Maxum device, it is not an option for patients with external otitis. No papers could be identified on the Esteem device applied in patients with external otitis. In contrast, the VSB device is widely applied.

In summary:

There is no convincing evidence that today’s auditory implants for patients with sensorineural hearing loss led to better speech recognition than BTEs, however, such devices (except the Maxum device) might (cost-) effectively be used in patients with sensorineural hearing loss who don’t tolerate earmolds.

To help patients with sensorineural hearing loss and chronic external otitis, bone-conduction devices (BCD) have been applied as well; with a BCD, the ear canal remains open. Bone conduction works but it is not really effective (see Chapter 2). Indeed, studies with the most powerful type of BCD, the percutaneous BCD (Baha), applied in such patients showed insufficient amplification (Snik et al., 1995). Stenfelt et al. (2000) studied the use of Baha in patients with an another reason, namely in patients with a predominant high-frequency sensorineural hearing loss, enabling an open-ear fitting. They also concluded that Baha was not powerful enough. No papers were found on the effectiveness of today’s more powerful BCDs (e.g. the percutaneous super power devices) in such patients.

In summary

To help patients with sensorineural hearing loss in need for amplification, bone-conduction devices have insufficient power 

8.2 Capacity of VSB with today’s sound processors

In contrast to the Maxum and Envoy Esteem devices, the VSB has been widely applied. With the more recent sound processors (Amade or Samba for VSB), improvements in sound quality have been achieved compared to the older processors. Concerning speech recognition, Figure 8.2 shows the %-correct score at normal conversational level (65 dB SPL) of 14 patients using these updated VSB processors. Patients with ski-slope audiograms were excluded, following Verhaegen et al., 2008. The drawn lines are taken from that latter study, based on the speech performance of patients with previous-generation sound processors. The figure shows that the new data points are close to the line or somewhat better (line taken from Verhaegen et al.). Busch et al. (2016) reported a similar conclusion. As a reference, Verhaegen et al showed that with BTE, applied in patients with similar hearing loss, the mean speech score was higher, e.g. at 65 dB HL the mean phoneme score was 90% with BTE versus 70% with VSB.

SNHL PS65vsPTA enkel VSB

Figure 8.1. Speech recognition-in-quiet scores (presentation level 65 dB SPL; phoneme scores) as a function of the hearing loss of 14 VSB-Amade/Samba users (squares). The drawn line is the best fit from a previous study (Verhaegen et al., 2008).

A problem of using auditory implants with an open ear canal, like the VSB, might be interference between the vibrations of the middle ear ossicles generated by the implant’s actuator and direct sounds that activate the ossicles in the normal way. It should be noted that owing to the digitally processing of digital audio processors, the amplified sounds are delayed, typically by 4 ms to 10 ms. That implies that the direct and processed sounds are not in phase for most frequencies, leading to a distorted sound. This phenomenon also occurs when fitting digital BTE devices with open earmolds (Groth & Sondergaard (2004) and Stone et al. (2008)). The latter study showed that the (annoying) interference depends on (and can be adjusted by) the gain level of the device, compression settings and on frequency. The lower the processing delay, the better. Studies looking into this interference when using middle ear implants have not been published yet.

8.3 References chapter 8

Barbara M, Filippi C, Covelli E, Volpini L, Monini S. Ten years of active middle ear implantation for sensorineural hearing loss. Acta Otolaryngol. 2018a;138(9):807-814.

Barbara M, Volpini L, Filippi C, Atturo F, Monini S. A new semi-implantable middle ear implant for sensorineural hearing loss: three-years follow-up in a pilot patient’s group. Acta Otolaryngol. 2018b;138(1):31-35.

Busch S, Lenarz T, Maier H. Comparison of Alternative Coupling Methods of the Vibrant Soundbridge Floating Mass Transducer. Audiol Neurootol. 2016;21(6):347-355.

Carlsson PU, Håkansson BE. The bone-anchored hearing aid: reference quantities and functional gain. Ear Hear. 1997;18:34-41

Dillon H. Hearing aids. 2012, Thieme Verlag, Stuttgart, Germany

Edfelt L, Stromback K, Grendin J, et al., Evaluation of cost-utility in middle ear implantation in the ‘Nordic School’; a multicenter study in Sweden and Norway. Acta Otolaryngol. 2014;134:19-25

Goode RI. Current status and future of implantable electromagnetic hearing aids. Otolaryngol Clin N Am. 1995;28:141-146

Groth J, Sondergaard MB. Disturbance caused by varying propagation delays in non-occluding hearing aid fittings. Int J Audiol. 2004;43:594-599

Huinck WJ, Mylanus EAM, Snik AFM. Expanding unilateral cochlear implantation criteria for adults with bilateral acquired severe sensorineural hearing loss. Eur Arch Otorhinolaryngol. 2019;276(5):1313-1320.

Kahue CN, Carlson ML, Daugherty JA, Haynes DS, Glasscock ME 3rd. Middle ear implants for rehabilitation of sensorineural hearing loss: a systematic review of FDA approved devices. Otol Neurotol. 2014;35(7):1228-37.

Marzo SJ, Sappington JM, Shohet JA. The Envoy Esteem implantable hearing system. Otolaryngol Clin North Am. 2014;47(6):941-52.

Magnan J, Manrique M, Dillier N, Snik A, Hausler R. International consensus on middle ear implants. Acta Otolaryngol. 2005;125:920-921

Pelosi S, Carlson ML, Glasscock ME 3rd. Implantable hearing devices: the Ototronix MAXUM system. Otolaryngol Clin North Am. 2014;47(6):953-65.

Plomp R. Auditory handicap of hearing impairment and the limited benefit of hearing aids. J Acoust Soc Am. 1978;63:533-549

Pulcherio JO, Bittencourt AG, Burke PR, Monsanto R, de Brito R, Tsuji RK, Bento RF. Carina® and Esteem®: a systematic review of fully implantable hearing devices. PLoS One. 2014;17;9(10)

Rameh C, Meller R, Lavielle JP, Deveze A, Magnan J. Long-term patient satisfaction with different middle ear hearing implants in sensorineural loss. Otol Neurotol 2010;31:883-892

Snik  A, Implantable hearing devices for conductive and sensorineural hearing impairment. In: F-G Zeng et al. (eds.) Auditory Prosthesis. New Horizons. Springer Handbook of Auditory Research 39, 2011:85-108

Snik AF, Mylanus EA, Cremers CW. Bone-anchored hearing aids in patients with sensorineural hearing loss and persistent otitis externa. Clin Otolaryngol Allied Sci. 1995;20:31-35

Stenfelt S, Håkansson B, Jönsson R, Granström G. A bone-anchored hearing aid for patients with pure sensorineural hearing impairment: a pilot study. Scand Audiol. 2000;29:175-185

Stone MA, Moore BCJ, Meisenbacher K, Derleth RP. Tolerable hearing aid delays. V. Estimation of limits for open canal fittings. Ear Hear. 2008;29:601-617

Verhaegen VJ, Mylanus EA, Cremers CW, Snik AF. Audiological application criteria for implantable hearing aid devices: a clinical experience at the Nijmegen ORL clinic. Laryngoscope. 2008;118(9):1645-1649

VSB fact sheet (VORP 503/Samba). Vibrant soundbridge System. Med-El website, downloads, 2015

 

 

Chapter 7. Challenges in children; critical choices

7.1 Introduction; children should not be treated as young adults

Counselling adults, who according to their audiogram should profit from amplification, is not always successful. Adults might deny their hearing problems or postpone a hearing aid trial. If their attitude is more positive, they might only be interested in hardly visible devices, even if speech recognition with such devices is not optimal.

For children, there is less room to move because the better the hearing the higher the chance that the child will develop normally.  According to Northern and Downs (1991; chapter 1), indeed, children need (sub)normal hearing (15 dB HL or less) to ensure normal development of speech and language. Therefore, when counselling the parents of a child with conductive or mixed hearing loss, sufficiently powerful amplification options should be advocated (section 4.3).

The central issue during (any) device fitting is audibility (Amlani et al., 2002). A popular tool to assess audibility of speech is the Speech Intelligibility Index (SII). SII predicts the proportion of normal speech that is audible to a patient with a specific hearing loss. The SII can range from 0 to 100, zero means that normal speech is not audible and 100 means that the patient is able to hear normal speech completely. The SII is based on the idealized spectrum of conversational speech with a +/- 15 dB range, often referred to as the speech banana. The speech banana can be represented in the audiogram, see Figure 7.1. To calculate the SII, for each frequency band (1/3 octave wide), the proportion of speech in that band that is audible is weighted according to the contribution of that band to speech intelligibility (weighting factors indicated by the density of the dots in the figure). In the simplified model presented in the figure, the speech banana stretches from 20 dB HL up to 50 dB HL, over a frequency range of 250 to 6000 Hz (Mueller and Killion, 1990; Killion and Mueller, 2010). The number of dots (max is 100 dots) is the SII; so, one dot counts for 1% of the speech cues. In the figure, post intervention hearing thresholds are presented of two different interventions. Counting the dots shows that the SII is 90 and 22 for intervention 1 and 2, respectively. In their paper, Killion & Mueller (2010) published the relationship between the SII and word recognition scores (their Figure 3). The SII of 90 and 22 equal word recognition scores of 90% and 20%, respectively. Evidently, treatment 1 is much more effective than treatment 2. The figure further suggests that when the maximum 15-dB-hearing-loss criterion of Northern & Downs is fulfilled, SII is 100, thus optimal.

Especially in children, hearing aid fitting should be bilaterally, as bilateral input leads to improved hearing and might enable spatial hearing (Dun et al., 2013). Consequently, speech recognition over-a-distance improves as well as in noisy places (binaural summation; use of head shadow, see chapter 6). Lately, it has been shown that young deaf children with bilateral devices (cochlear implants) compared to those with a unilateral device developed better cognitive skills. Statistics showed that this improvement could fully be ascribed to better perception of soft sounds and better speech perception in noisy places (de Raeve et al., 2015). The factor in between better hearing and better cognitive development is incidental learning. Most things that children learn are acquired in informal everyday situation.

7 punt 1

Figure 7.1. Audiogram format with speech area according to Mueller and Killion (gray area). Aided thresholds are shown of two interventions, 1 (red line) and 2 (green line). The SII of the two interventions is determined by counting the dots above the hearing threshold (Source: Mueller and Killion, 1990).

Only approximately 1/3 of their knowledge is acquired in structured, more or less noise-free settings (e.g. in class). The remaining 2/3 during playing, watching TV, talking with parents and other persons, etc. (de Raeve et al., 2015). Especially under these latter conditions, the children profit from bilateral devices.

Probably, the observation that children with unilateral hearing loss (second ear normal) might display delayed educational development, has the same cause (Kuppler et al., 2013). This underlines that hearing with two ears is of utmost importance for the development of speech and language in children.

 

7.2 Literature review: why some interventions don’t work

Let us use the speech audibility concept to find the best intervention for a child with congenital conductive or mixed hearing loss. A literature search was carried out with as search terms: children, conductive hearing loss, mixed hearing loss, and (Baha or Ponto or Sophono or softband or Vibrant Soundbridge or surgery or conventional bone conduction or Bonebridge). Next, those papers were selected in which 1. two interventions were compared, 2. with n>10, 3. unilaterally fitted and 4. published after 2006. Most of the included studies addressed children with congenital hearing loss.

7 punt 2

Figure 7.2. Mean aided thresholds versus the sensorineural hearing loss component (SNHLc) of several study groups of children fitted with: percutaneous BCDs (red symbols, 5 studies*; summed 86 children),  transcutaneous BCDs (yellow symbols; 4 studies*; 130 children) and children with atresia repair (blue symbols, 3 studies*; 109 children). The black dots refer to three studies* in which the VSB was used (without a reference device; 54 children).

* see note at the end of this chapter for references.

 

Figure 7.2 shows data obtained in children with conductive or mixed hearing loss. Concerning the percutaneous BCD devices, overall a mean aided threshold of 19 dB HL was found. That is close to the value that is mentioned in the ‘Practise Guidelines’ for fitting percutaneous devices (17.5 dB HL; Roman et al., 2011) and the 15 dB HL, which is the upper limit for a handicapping hearing loss for children according to Northern & Downs (1991). Concerning the transcutaneous BCDs, viz. the combined data of conventional BCDs (three studies, 49 children), BCDs on softband (two studies, 23 children) and the Sophono BCD (one study, 15 children) show a mean of 29 dB HL. Such a value is expected, owing to the 10 to 15 dB poorer effectiveness of transcutaneous stimulation compared to percutaneous stimulation (Hakansson et al., 1984). Only one study was identified concerning the Sophono device with a mean aided hearing threshold of 33 dB HL (Denoyelle et al., 2015). That mean aided threshold was close to that reported in the original paper introducing the Sophono device by Siegert & Kanderske (2013; mean of 27 dB HL; 27 children).

With regard to surgical atresia repair, the mean long-term hearing threshold is 41 dB HL (three studies). That value is in good agreement with data from Nadaraja et al. (2013) who performed a systematic review of the literature. In fact, the outcomes of two interventions displayed in Figure 7.1 are Farnoosh et al.’s data (2014), with intervention 1 referring to percutaneous BCD application and 2 to atresia repair. The pre-intervention audiograms of the two groups were similar.

On the left side of the figure, the SII is presented calculated according to Killion and Mueller (2010), assuming a ‘flat SNHLc’ (not unusual for predominantly conductive hearing loss). It is obvious that audibility is the highest when using percutaneous BCDs; the SII ranges from 85 to 100. Results with the transcutaneous BCDs are lower. Spread is larger and audibility ranges from subnormal to insufficient; (SII ranging from 75 to 50). Children with such a low SII might be prone to developmental delays. The SII after reconstructive surgery is low, namely < 50. Indeed, Evans & Kazahaya (2007) reported that 90% of their operated children still needed some form of amplification after atresia repair surgery.

Concerning the use of the VSB with its actuator coupled to one of the cochlear windows, no studies were found that presented results compared to another amplification device. However, three studies on VSB application in children with conductive loss were identified with at least 10 children involved. The mean aided thresholds of these three studies with VSB users are presented in Figure 7.1 as black dots. These data are in the same range as those of the transcutaneous BCDs what is somewhat disappointing and might be described to the experimental character of these studies. More data are needed (see update 2020, new section 7.2.1). So far, the Bonebridge device has hardly been applied in children (Sprinzl & Wolf-Magehe, 2015) and is not further considered here (see update 2020, new section 7.2.1).

Children with acquired conductive or mixed hearing loss because of chronic inflammation will benefit from amplification in the same way as those with congenital conductive hearing loss. Mostly, for this indication, BTEs are contraindicated for medical reasons.

Figure 7.2 presents data that can be used for counselling parents of children with conductive hearing loss. In short:

 

Reconstructive surgery in case of congenital aural atresia is, on the average, the least effective treatment for children. 

For children with predominantly conductive hearing loss, today’s percutaneous BCDs provide more adequate gain than transcutaneous BCDs, what might enable a better spontaneous development. While fitting transcutaneous devices, focus should be on audibility of speech.

 

7.2.1 Update of Figure 7.2 using data published between 2016 and 2020 

Update of the data presented in figure 7.2 is based on an additional search of the literature; search period: 2016 (end previous search period) until early 2020. For details of the selection process, see note at the end of this chapter. Seven additional studies were identified, describing 11 groups of subjects with different treatments. Data of the latest evaluation moment were used. The table below presents all the data averaged (viz. the mean post-intervention thresholds and the mean bone-conduction thresholds) per type of treatment, including the data of the previous search (taken from Figure 7.2). The table shows that the mean bone-conduction threshold per type of treatment was rather comparable. Furthermore, the mean post-intervention thresholds varied between 20 dB HL and 41 dB HL.

Using all the studies (from the previous and new literature search) that presented frequency-specific post-intervention thresholds, the mean post-intervention threshold was calculated per frequency and per intervention. Next, the count-the-dots procedure was used to determine the SII and the associated word recognition score. The last column of the table presents these word scores and suggests once more that these treatments are not equivalents. As concluded before, especially the outcomes of atresia repair are disappointing.

The data in the table should be considered with some caution: regarding the Sophono device and the VSB, 2 and 4 studies, respectively, were identified, all published before medio 2016.

 

Table 7.1

Treatment

 

studies Subjects,

cumulative

Mean bc

threshold

Mean post intervention threshold with (range) Estimated WRS; % correct
Atresia repair 5 199 11 dBHL 41  (33-47) dBHL 25%
Percut. BCD 6 135 11 20  (15-22) 95
Bonebridge 7 100 10 27  (19-31) 90
Sophono 2 42 16 35  (35/36) 65
Conv. BCD 8 126 11 33  (27-45) 70
VSB 4 60 11 31  (27-40) 75

Percut. stands for percutaneous; conv. for conventional BCD, including BCD on softband; bc for bone conduction; WRS for word recognition score

 

 

 

7.3 Age at intervention and stability

After neonatal hearing screening and a diagnosed bilateral conductive hearing loss few amplification options are available. When the tympanic membrane is visible, BTEs might be used. A transcutaneous BCD with headband or softband can be used in all the other cases. In neonates, mostly a conventional BCD with softband is chosen. It is easier to use and thus better accepted than a steel headband (Hol et al., 2005). The position of stimulating bone-vibrator over the skull can be changed. Aided sound field thresholds with such devices typically lay between 25 dB HL and 30 dB HL (Verhagen et al., 2008; Denoyelle et al., 2015), which seems to be acceptable but only for basal language learning (Northern & Downs, 1991; Verhagen et al., 2008). Therefore, replacement of a BCD on head/softband by a percutaneous BCD or other powerful solutions should remain on the agenda and, meanwhile, speech and language development should be monitored.

The use of percutaneous BCDs involves implantation of a skin penetrating titanium coupling. Concerning the youngest age at which such implantation is feasible, thickness of the skull plays an important role: 3 mm or more is preferred (Snik et al., 2005). This implies that the child must be approximately 4 years old. Nevertheless, the loss of implants in children, in relation to follow-up, is 3 times higher than in adults (e.g. Dun et al., 2010). Children as young as 2 years have been implanted with varying rates of success. McDermott et al. (2009) reported that implant loss was high amongst the under fives (40% of the implanted children lost an implant) and much less in older children. For children over 10 years implant loss was approx. 1%. Other studies didn’t find such a distinct dependence of implant loss on age at implantation (de Wolf et al., 2008). Recently, improved titanium implants have been brought to the market with, in adults, a significantly better stability, see paragraph 5.1.2. However, application of one of these rather new implants in children was not successful (BI300; Den Besten et al., 2015).

The magnet for coupling a Sophono and Baha Attract device can be implanted from age 5 onwards, according to the companies. Reviewing the literature, Bezdjian et al. reported 3 serious adverse reaction in 99 patients using the Sophono device, mainly children (Bezdjian et al., 2017). Dimitriadis et al. (2016;2017) reported as well that complication rate with the Baha Attract was low, when compared to the percutaneous BCD.

VSB with the transducer placed in the round window niche has been applied in children as young as 2 months (Mandala et al., 2011). Since then, little has been published. Only one paper was found (with n>5); Leinung et al. (2017) presented results obtained in 13 young children with aural atresia. Age at implantation varied from 1.3 to 4.2 years with an average of 2.5 years. While the mean bone-conduction threshold (at 0.5, 1, 2, 4 kHz) of the whole group was 8 dB HL, the mean aided threshold was 40 dB HL, which is poor compared to the data presented in Table 7.1. In order to advocate VSB implantation in toddlers and young children, more evidence is needed.

The preferred age at intervention for reconstructive surgery (atresia repair) is 5-6 years and only relatively mild cases should be considered (e.g. Declau et al., 1999). Concerning stability, revision surgery might be necessary in up to one-third of the cases as reported by Farnoosh et al. (2014). These authors compared atresia repair with percutaneous BCD application, amongst other, with respect to medical complications. Percentage of revision surgery after atresia repair was almost 3 times higher than that after BCD implantation. Therefore, it was suggested that, also in terms of medical aftercare, percutaneous BCDs might be a better choice than atresia repair.

 

7.4 Percutaneous or transcutaneous BCDs in children, counselling issues

Parents might choose for transcutaneous BCD instead of percutaneous BCD, because of stability issues, daily care of the skin around the penetrating implant and/or emotional problems to accept titanium implants sticking out, through the skin. Then the amplification options are to use either the Sophono device (Alpha 2 MPO) or the Baha Attract, applicable from age 4 or 5 years. First data showed that, as expected, the Baha Attract is as effective as Baha on a Softband (Kurz et al., 2014). Nowadays, as sound processor, the Baha 5 power is advocated, which is more powerful than the standard Baha. The use of the Baha power will improve the MPO by several dBs. When bilateral transcutaneous devices are used, in predominant conductive hearing loss, aided thresholds are expected between 20 dB HL and 25 dB HL, with corresponding SII values between 100 and 80.  This seems safe but suggests that these children should be followed-up more closely than percutaneous BCD users concerning their speech and language development. It remains of importance to keep a change to a percutaneous BCD on the agenda.

Another suggestion that has been put forward for children with transcutaneous BCDs is the provision of an additional personal FM system. Those systems, coupled to the child’s BCD might enable better speech recognition, however, mainly in structured situations like classrooms. The benefit of personal FM systems is in debate with respect to incidental learning. In other words, it seems to be better to choose the powerful percutaneous amplification options; the gain and MPO might improve by 10 to 15 dB, compared to transcutaneous BCDs.

Using the data presented by Sylvester et al. (2013) it is evident that if the hearing loss is of the mixed type, even when the sensorineural hearing loss component is just 25 dB HL, the Sophono transcutaneous BCD is not effective. Studies using the Baha Attract in such cases are still missing. In mixed cases it is always better to choose for percutaneous BCDs, and if the sensorineural hearing loss component is advanced, an additionally applied personal FM systems might be helpful.

 

*Note. References belonging to the studies summarized in Figure 7.2: Claros & Pujol, 2013; Frenzel et al., 2015; Verhagen et al., 2008; Farnoosh et al., 2014; Ricci et al., 2011; Denoyelle et al., 2015; Bouhabel et al., 2012; Evans & Kazahaya, 2007; Christensen et al., 2010; Mandala et al., 2011. The update of figure 7.2 (early 2020) was based on an additional literature search using Pubmed. The search string comprised the words: children, hearing, thresholds, and either a device name (Baha, Ponto, Bonebridge, Sophono, Vibrant Soundbridge) or atresia, surgery. Studies were excluded if aiming at subjects with just unilateral hearing loss, single-sided deafness or sensorineural hearing loss. The inclusion criteria used were: 1. either the aided PTA4 or the aided thresholds at 0.5, 1, 2, 4 kHz should have been reported, as well as bone-conduction thresholds and 2. group size should > 9. The identified studies comprised: Zhao et al., 2016; Fan et al., 2019, Bravo-Torres et al., 2018, Kulasegarah et al., 2018, Ren et al., 2019, Ahn et al., 2018, Zernotti et al., 2019, Ratuszniak et al., 2019.

Ahn J, Ryu G, Kang M, Cho YS.  Long-term Hearing Outcome of Canaloplasty With Partial Ossicular Replacement in Congenital Aural Atresia. Otol Neurotol. 2018 Jun;39(5):602-608.

Amlani AM, Punch JL, Ching TY. Methods and applications of the audibility index in hearing aid selection and fitting. Trends Amplif. 2002;6(3):81-129.

Bouhabel S, Arcand P, Saliba I. Congenital aural atresia: bone-anchored hearing aid vs. external auditory canal reconstruction. Int J Pediatr Otorhinolaryngol. 2012;76(2):272-7.

Bravo-Torres S, Der-Mussa C, Fuentes-López E. Active transcutaneous bone conduction implant: audiological results in paediatric patients with bilateral microtia associated with external auditory canal atresia. Int J Audiol. 2018 Jan;57(1):53-60.

Christensen L, Smith-Olinde L, Kimberlain J, Richter GT, Dornhoffer JL. Comparison of traditional bon e-conduction hearing AIDS with the Baha system. J Am Acad Audiol. 2010;21(4):267-73.

Colletti L, Mandala M, Colletti V. Long-term outcome of round window Vibrant Soundbridge implantation in extensive ossicular chain defects. Otolaryngol Head Neck Surg. 2015; 149:134-141

Dun CA, Agterberg MJ, Cremers CW, Hol MK, Snik AF. Bilateral bone conduction devices: improved hearing ability in children with bilateral conductive hearing loss. Ear Hear. 2013;34(6):806-8.

Fan X, Yang T, Niu X, Wang Y, Fan Y, Chen X. Long-term Outcomes of Bone Conduction Hearing Implants in Patients with Bilateral Microtia-atresia. Otol Neurotol. 2019 Sep;40(8):998-1005.

McDermott AL, Williams J, Kuo M, Reid A, Proops D. The Birmingham pediatric bone-anchored hearing aid program: a 15-year experience. Otol Neurotol. 2009;30(2):178-83.

Kulasegarah J, Burgess H, Neeff M, Brown CRS. Comparing audiological outcomes between the Bonebridge and bone conduction hearing aid on a hard test band: Our experience in children with atresia and microtia. Int J Pediatr Otorhinolaryngol. 2018 Apr;107:176-182.

Kurz A, Flynn M, Caversaccio M, Kompis M. Speech understanding with a new implant technology: a comparative study with a new non-skin penetrating Baha system. Biomed Res Int. 2014;2014:416205.

Ratuszniak A, Skarzynski PH, Gos E, Skarzynski H. The Bonebridge implant in older children and adolescents. Int J Pediatr Otorhinolaryngol. 2019 Mar;118:97-102.

Ren R, Zhao S, Wang D, Li Y, Ma X, Li Y, Fu X, Chen P, Dou J. Audiological effectiveness of Bonebridge implantation for bilateral congenital malformation of the external and middle ear. Eur Arch Otorhinolaryngol. 2019 Oct;276(10):2755-2762

Verhagen CV, Hol MK, Coppens-Schellekens W, Snik AF, Cremers CW. The Baha Softband. A new treatment for young children with bilateral congenital aural atresia. Int J Pediatr Otorhinolaryngol. 2008;72(10):1455-9.

de Wolf MJ, Hol MK, Huygen PL, Mylanus EA, Cremers CW. Nijmegen results with application of a bone-anchored hearing aid in children: simplified surgical technique. Ann Otol Rhinol Laryngol. 2008;117(11):805-14.

Zhao S, Gong S, Han D, Zhang H, Ma X, Li Y, Chen X, Ren R, Li Y.  Round window application of an active middle ear implant (AMEI) system in congenital oval window atresia. Acta Otolaryngol. 2016;136(1):23-33.

Zernotti ME, Chiaraviglio MM, Mauricio SB, Tabernero PA, Zernotti M, Di Gregorio MF. Audiological outcomes in patients with congenital aural atresia implanted with transcutaneous active bone conduction hearing implant. Int J Pediatr Otorhinolaryngol. 2019 Apr;119:54-58.

Chapter 6. Bilateral application should always be considered

6. Bilateral application should always be considered

6.1 Introduction

Bilateral hearing refers to hearing with two ears. When listening with two ears instead of one, at least four advantages can be distinguished: 1) loudness summation, 2) use of acoustic head shadow to hear better in noisy places, 3) directional hearing and 4) binaural squelch. In normal hearing listeners binaural hearing is obvious, based on accurate processing of the bilateral inputs, leading to a ‘fused’ percept (binaural hearing). That is not necessarily the case for patients using hearing devices; firstly, a short introduction of these four advantages.

  1. Loudness summation refers to improved hearing owing to summation of sound as heard by the two ears: the input perceived by either cochlea is summed, leading to an increase of perceived loudness of approx. 5 dB.
  2. Improvement in speech recognition in noise owing to effective use of head shadow. Let us assume that the speech is coming from the front and noise from the left. The speech is heard equally well by either ear, but the noise is not. At the right ear, the noise is perceived attenuated (by acoustic head shadow) compared to the left ear. Therefore, the right ear will have the better speech to noise ratio. Selective listening with the right ear, thus ignoring the left ear, leads to better speech perception.
  3. Improved localization of sounds, in the horizontal plane. To identify where sounds come from, the two ears have to work together to detect interaural difference cues caused by the different positions of the two ears with respect to the sound source. These interaural differences comprise interaural loudness differences (ILD; owing to acoustic head shadow) and interaural time differences (ITD, difference in arrival times of the sound at the two ears). Head shadow attenuates primarily high frequency sounds ( > 3000 Hz). As a consequence, high-frequency sounds are perceived louder by the ear nearest to the sound source, creating an ILD. Below 1500 Hz, head shadow causes little attenuation. Instead, ITDs (or, related, interaural differences in phase) are the relevant cue to localize a sound source. Sounds arrive earlier at the ear nearest to the sound source, creating the ITD. Owing to the limited distance between the two ears, the ITD varies between 0 ms (sound presented in the front of the listener) to a maximum of 0.7 ms (e.g. sound presented e.g. at the very left, perceived by the two ears).
  4. Binaural squelch refers to central de-masking. Assume a subject is listening to speech coming from the front and noise coming from the left. The speech in the two ears will be nicely in phase (ITD=0). However, the noise is perceived differently by the two ears, not only because of the head shadow, but also because of ITD, caused by the different travelling times of the noise to the left and right ear. The difference in ITDs caused by the speech and by the noise can be used to perceptually separate the speech and noise signals, referred to as binaural squelch.

Bilateral application of BTEs in patients with bilateral sensorineural hearing loss mostly leads to binaural hearing. However, the outcome might be worse than in normal hearing subjects who are listening with two ears (concerning directional hearing, see e.g. Bogaert et al., 2006). Nevertheless, e.g. Boymans et al. (2008) showed an obvious benefit of BTEs in a large group of patients with bilateral sensorineural hearing loss, with regard to directional hearing and the effective use of head shadow.

6.2 Binaural hearing with bone conduction devices

When using the air-conduction route, the two ears can be stimulated independently because they are acoustically well isolated from each other. That is not the case for bone-conduction stimulation. The skull transmits the bone-conducted vibrations quite effectively, with little damping. Therefore, with one BCD, not only the ipsilateral cochlea is stimulated but also the contralateral cochlea. This is referred to as cross hearing. Attenuation of vibrations from one cochlea varies widely (range more than 20 dB), with a median of just 3 to 5 dB; variation between subjects is significant as well as within subjects, between frequencies (Stenfelt, 2012). In other words, with bone-conduction stimulation, the cochleae are poorly acoustically isolated, however, just sufficiently to enable to detection of interaural differences to some extend (Stenfelt, 2005).

Table 6.1 presents data on binaural effects when changing from unilateral to bilateral listening in patients using bone-conduction devices. A summary of published Nijmegen papers is presented on Baha application (bilateral Baha in bilateral conductive/mixed hearing loss and unilateral Baha application in unilateral conductive hearing loss with a second normal hearing ear). Data are added as obtained in a group of normally hearing controls, listening with two versus one ear; with, in the latter case, one ear plugged and muffed (Agterberg et al., 2011). All the patients were evaluated with one and the same protocol, and they were using linear (analogue) Baha bone-conduction devices. Binaural summation and speech in noise were assessed using speech.. Directional hearing was tested with short (1s) narrow-band noise bursts (Agterberg et al., 2011).

Table 6.1, first row, presents the mean result (and standard deviations) of the ‘binaural’ factors as measured in normal hearing controls. The binaural summation score is the difference between speech recognition scores obtained with one ear versus two ears. Head shadow and binaural squelch have been measured in one experiment. The combined score is included in the Table (column 3). Firstly, speech recognition in noise was measured while the subject was listening with one ear (in unilateral cases with their normal ear, in bilateral cases with their left ear only, thus Baha of the right ear turned off). Speech was presented in front of the subject and the noise at the side of the only hearing ear. Secondly, the Baha at the patient’s other ear was activated (in the normal hearing subjects, one ear had been blocked for the first measurement, which is now unblocked). The presented score is the improvement between these two situations. The asterisks indicate values that differ significantly from 0. The next two columns show directional hearing results for low frequency sounds (relying on detection of ITDs) and for high frequencies sounds (relying on the detection of ILDs), respectively. The mean absolute error is shown (0 means perfect localisation, chance level is 800). Concerning the directional hearing results: statistically significance was assessed by comparing the scores obtained in the unilateral listening situation (not shown) and bilateral listening situation. Thus the first row of the table presents norm values for our set-up.

Let us consider bilateral Baha application in bilateral hearing loss. According to the table, row 2, bilateral Baha application does lead to the expected improvement in binaural summation (column 2) of, approximately, 4 dB. This result is in agreement with the literature (Janssen et al., 2012, review). Although not measured, binaural hearing in control subjects is around 5 dB. Obviously, the inevitable cross stimulation of bone-conduction stimulation plays a minor role when considering binaural summation.

Table 6.1

N Binaural summation (dB) Head shadow & squelch (dB) Directional hearing, 0.5kHz (degrees) Directional hearing, 3 kHz (degrees)
Controls 10 n.a. 4.6 ± 1.6* 7 ± 7^ 8 ± 10^
Bilat CHL, bilat Baha, acquired 17 4.0 ± 2.1* 2.5 ± 1.8* 26 ± 8^ 25 ± 8^
Unilat CHL, acquired 13 2.2 ± 1.5* 3.0 ± 1.7* 16 ± 10^ 20 ± 12^
Unilat CHL, congenital 10 1.3 ± 2.4 1.1 ± 1.9 30 ± 13 31 ± 18

Note. N.a.: not available, CHL: conductive hearing loss.

* significantly different from 0 (p<0.05); ^significantly improved compared to the unilateral listening condition (p<0.05)

Considering row 3, Baha application in acquired unilateral conductive hearing loss, a small but significant binaural summation score was found, however, not for the unilateral congenital cases using Baha (row 4). Their score was not significantly different from zero. Column 3 shows the binaural squelch/head shadow data. The bilateral conductive hearing loss patients (row 2) profit significantly as a group, however, profit is approx. half that of the controls (2.5 vs. 4.6 dB). For unilateral acquired conductive hearing loss, the outcome is approx. the same (3.0 dB), while for the congenital cases, no significant benefit was found (rows 3 and 4). Thus, on the average, head shadow doesn’t help the patients with congenital onset to better understand speech in noisy conditions, according to this study.

Columns 4 and 5 comprise the outcome of the directional hearing tests; the mean absolute difference in degrees between the perceived location and the real location of the activated loudspeaker are presented. For normal hearing controls, an error of 7-80 was found, for the bilateral Baha users approximately 250 and for the patients with unilateral acquired hearing loss using Baha, approximately 200. These three groups showed significantly improved scores when changing from unilateral (mean unaided scores 46-660) to bilateral hearing. Probably, disturbing cross hearing is the reason for the difference in score between the controls and bilateral Baha-users. The unilateral congenital cases showed the worst scores, see next paragraph.

6.3 Unilateral congenital conductive hearing loss; elaborated outcomes

The localisation scores of the patients with congenital unilateral conductive hearing loss with their Baha active were largely unchanged, thus close to their unaided scores (340 and 390 for 0.5 and 3 kHz, respectively; thus mean improvements of just 40 and 80). The most likely explanation is that these patients (without the Baha), being unilaterally hearing since birth, use monaural loudness cues and spectral cues much more effectively than the other patients and normal hearing controls (see also Agterberg et al., 2012; Vogt et al. 2020). Vogt et al. even showed that with the percutaneous BCD turned on, these patients remained relying on monaural cues for sound localisation, despite the second input (via the BCD).

In an attempt to understand the rather poor but variable results of the patients with congenital onset of their hearing loss, we decided to study whether or not age at intervention played a role. The hypothesis was that maybe a sensitive period exists; the earlier implantation, the better the binaural results would be. Figure 6.1 shows the individual summed outcome of the three binaural tests (the head shadow test and the two directional hearing scores) after expressing the outcomes into z-scores, using the standard deviations of the normal hearing group. Results were available from 20 patients with unilateral congenital conductive hearing loss using a Baha for at least 3 months (data taken from Kunst et al., 2008). Figure 6.1 shows the summed z-scores of these 20 patients as a function of age at implantation. For reference purposes the same procedure was applied to the data of the patients with acquired unilateral conductive hearing loss (Table 6.1, row 3); a mean summed z-score of just over 10 was found in the latter group. The Figure shows that a value of 10 is achieved by just one of the patients with congenital unilateral hearing loss. Obviously, also negative summed z-scores were found, indicating a worse result with Baha than unaided. A significant effect of age at intervention is not seen, while the spread in results was large.

5 Slide2

Figure 6.1. Binaural advantage based on the use of head shadow and directional hearing. Summed z-scores are presented as a function of age at intervention. Baha users and non-users are indicated by different symbols (data from Nelissen et al., 2015).

Fitting of a (percutaneous) BCD in patients with unilateral congenital conductive hearing loss to enable binaural hearing might result in a poor outcome; age at implantation doesn’t seem to play a role (in the age-range studied, from 4 years into adulthood).

Furthermore, Nelissen et al. studied non-use in this group of 20 patients using Baha, with a follow-up of more than 5 years. Non-use turned out to be rather common, as indicated in Figure 6.1; 65% non-users. An association was found between ‘long-term’ non-use and the ‘short-term’ (one-year post-intervention) summed binaural advantage score. Non-use was not related to age at intervention. It seems that several of these patients were not able to integrate the new input with that of the normal hearing ear, what is needed for a binaural percept. Thus, not surprisingly, the mean reason to stop as indicated by the patients was complaints about interfering sounds/noise when using the Baha device in situations with background sounds. As patients with unilateral acquired conductive hearing loss do benefit from Baha application (summed z-score of 10 on the average), they seem to profit from their previously developed binaural abilities.

Recently a review of the literature was published (Vogt et al. 2021) focussing on post intervention binaural hearing abilities in children with congenital aural atresia. Interventions comprised not only a percutaneous BCD but also all other types of implantable BCDs, active middle ear implants like the VSB as well as atresia repair. Surprisingly, all outcomes were consistently disappointing, irrespective of the type of intervention (concerned binaural summation, use of head shadow and binaural squelch and directional hearing). Vogt et al. discussed factors that might have played a role like age at treatment, whether the treated ear was a ‘lazy’, not fully maturated, ear as well as the role of the remaining asymmetry in hearing thresholds, as was seen generally. It was argued that the major problem might be the remaining asymmetry in hearing, which typically varied between 20 and 35 dB. The asymmetry in hearing might affect negatively the development of binaural hearing and consequently leads to complaints about competing sounds. Children often reduce the gain of the device to deal with that. Low volume means more asymmetry in hearing; a chicken and egg problem. Special training to teach the child to use the bilateral inputs effectively seems to be indicated.

Concerning the binaural advantage of an active middle ear implant in unilateral conductive hearing loss, it should be noted that in contrast to a BCD, cross stimulation with an active middle ear implant device is absent. Two publications were found in which outcomes obtained with the VSB middle ear implant were compared to that with BCDs in patients with congenital unilateral conductive hearing loss (Agterberg et al., 2014; Vogt et al. 2018). Agterberg et al. compared results of patients fitted either with a Baha (n=4) or with a VSB (n=4). All were experienced device users. An anticipated better result with the cross-stimulation-free VSB device in a directional hearing experiment could not be established. Same conclusion was drawn from a comparison between the VSB and an active transcutaneous BCD, the Bonebridge device (Vogt et al., 2018). This suggests that cross hearing might not be a dominant factor, further research is warranted.

Although unilateral congenital hearing loss in children might lead to developmental delays (in speech language development and academic performance), outcomes of today’s treatment options are grossly disappointing (Vogt et al., 2021). Nevertheless, some children experience improved hearing and are happy with that. No conclusive evidence exists regarding the role of age at implantation, degree of asymmetry in hearing after the treatment nor what intervention is most effective.

6.4. Bilateral application of BCDs and AMEI

Bilateral application in bilateral conductive hearing loss works, see Table 6.1, which is in agreement with Janssen et al. (2012) and Zeitooni et al, 2016. The latter study was carried out in normal hearing controls; binaural advantages were compared as obtained with stimuli/speech presented bilaterally over headphones (air-conduction route) and, separately, bilaterally by bone-conduction transducers (bone conduction route). The authors reported profit in both conditions, however the binaural advantage (using speech in noise) was approx. two times better with headphone presentation. This factor of 2 is in fair agreement with the difference as presented in column 3, Table 6.1, when comparing the outcomes of the normal hearing controls and the bilateral Baha users.

A further question is whether or not (adult) patients with bilateral congenital conductive or mixed hearing loss do profit from fitting bilateral BCDs. Bosman et al. (2001) reported that the ‘binaural advantage’ as found in their patients with acquired bilateral conductive hearing loss (summarized in row 2, Table 6.1) and those of 6 congenital cases were grossly the same. This might suggests that early binaural auditory experience is not a prerequisite for effective use of bilateral Bahas in bilateral conductive hearing loss. More recently, Den Besten et al. (2020) published data on a group of 7 children with bilateral congenital conductive hearing loss, all experienced users of bilateral percutaneous BCDs. They found that all children showed improved sound localisation when using two instead of one BCD, however, localisation scores were poor in most of the children. When (just) considering lateralization, a more obvious advantage was seen. The mean absolute error was 370 ± 170, while with unilateral Baha the error was 640 ± 120 (reference value normal hearing children: 100). One additionally child was tested with bilaterally acquired conductive hearing loss. In this case, sound localisation was close to the score of the controls, suggesting that children with previous binaural experience might profit more from bilateral BCDs than congenital cases. However, this is a preliminary conclusion.

Den Besten et al. also presented data on none-use. Almost 85% of their total group of bilaterally implanted children (n=33) used both percutaneous BCD devices all the time.

Although transcutaneous BCDs have been applied bilaterally, no systematic prospective evaluative study has been found in literature (autumn 2020).  As far as we know, studies are also missing on binaural hearing after bilateral application of middle ear implants, like the VSB, in patients with conductive or mixed hearing loss. Wolf-Magele et al. (2016) studied 10 bilateral VSB users, however 6 of them with sensorineural hearing loss, 4 with mixed loss and Koci et al (2016) reported directional hearing measurements in 10 bilateral VSB users; 8 of them had sensorineural hearing loss. Reported results of these mixed groups of VSB users are favourable, but the outcomes are most probably not representative for patients with conductive or mixed hearing loss. So, specific studies are needed.

Note. A special remark has to be made concerning binaural summation in patients with bilateral conductive hearing loss using (two) implants with limited maximum output (MPO; thus all BCDs and MEIs on the market autumn 2020; see chapter 2). Generally, improved hearing has been observed owing to binaural summation, in the order of 4 dB. This is of major importance. During the initial fitting of two BTEs or two CIs in sensorineural hearing loss/deafness, when switching on both ear-individually-fitted devices for the first time, overstimulation might occur owing to binaural summation. Generally, to deal with that, the volume and maximum output of the individual (thus per ear) fitted devices is lowered, and, quite often, the software does this automatically. Evidently, for devices with limited MPO, such a lowering of gain and output is contra-productive and should be avoided.

In bilateral conductive or mixed hearing loss, the application of bilateral BCDs leads to binaural hearing, although not optimal, with somewhat better result in acquired cases than congenital cases. Device compliance is high.

At last, so far, the patients in the bilateral BCD studies had rather symmetric bone-conduction thresholds. It is not clear what might happen in case of asymmetric bone-conduction thresholds. The theoretical problem is that the better ear might be hindered or even over-stimulated by the a BCD device fitted at the side of the worse cochlea. No data are available on this issue. When using active middle ear implants, such a problem will not occur.

6.5. Binaural hearing and adaptive sound processing

Today’s BCDs and VSBs make use of, amongst others, adaptive sound processing like expansion- and/or compression amplification, noise reduction, feedback reduction and adaptive directional microphones. Interaural cues (ILDs and ITDs) might be distorted by independently working digital sound processors. Sound processing times are relatively long (3 to 9? ms) compared to the maximum possible ITD (0.7 ms) and the bilateral devices are not synchronized. All these factors deteriorate interaural cues, resulting potentially in poor horizontal localization (Bogaert et al., 2006; Beck & Sockalingam, 2010). In principle, these advanced sound processing options are not really necessary when fitting patients with good cochlear function (conductive or mixed hearing loss with mild to moderate SNHLc), because the auditory system can still sort out the important information. On the other hand, some of these processing options might be necessary, to deal with limitation of the devices such as feedback, the unnatural position of the microphone, audible microphone noise and the limited MPO.

It is not evident that patients with conductive hearing loss or mixed hearing loss with a minor to moderate sensorineural hearing loss component do profit from advanced adaptive sound processing. Side effects like deteriorated spatial hearing might occur. Conclusive research is lacking.

New references concerning the update January 2021

den Besten CA, Vogt K, Bosman AJ, Snik AFM, Hol MKS, Agterberg MJH. The Merits of Bilateral Application of Bone-Conduction Devices in Children With Bilateral Conductive Hearing Loss. Ear Hear. 2020;41(5):1327-1332.

Vogt K, Wasmann JW, Van Opstal AJ, Snik AFM, Agterberg MJH. Contribution of spectral pinna cues for sound localization in children with congenital unilateral conductive hearing loss after hearing rehabilitation. Hear Res. 2020;385:107847.

Vogt K, Frenzel H, Ausili SA, Hollfelder D, Wollenberg B, Snik AFM, Agterberg MJH. Improved directional hearing of children with congenital unilateral conductive hearing loss implanted with an active bone-conduction implant or an active middle ear implant. Hear Res. 2018;370:238-247.

Vogt K, Desmet J, Janssen A, Agterberg M, Snik A. Unexplained variance in benefit of treatment of congenital unilateral aural atresia. Audiol Neurotol 2021; in press

Zeitooni M, Mäki-Torkko E, Stenfelt S. Binaural Hearing Ability With Bilateral Bone Conduction Stimulation in Subjects With Normal Hearing: Implications for Bone Conduction Hearing Aids. Ear Hear. 2016;37(6):690-702.

 

Chapter 5. Comparison of interventions in certain groups of patients

5. Comparison of interventions in certain groups of patients

5.1. Congenital middle ear and outer ear anomalies

5.1.1. Surgical repair or amplification?

When counselling patients with hearing loss caused by congenital ear anomalies (like aural atresia), firstly, reconstructive surgery should be considered. Congenital ear anomalies might vary from mild (middle ear anomalies) to severe (atresia of the ear canal) with an associated air-bone gap from 40 to 65 dB. Amongst otologists, reconstructive surgery of congenital anomalies is considered as very challenging because of complicated anatomy and risk of complications. In audiometric terms, benefit is often limited and not stable over time. Theoretically, surgery is successful if the air-bone gap is closed, but mostly, post intervention, an obvious air-bone gap remains. To define a successful outcome after atresia repair, a rather broad criterion is used: if the remaining air-bone gap is 30 dB or less, the outcome is considered as sufficient (Nadaraja et al., 2013). In their systematic review (including 35 studies), Nadaraja et al. showed that this criterion is fulfilled in approx. 50 % of the operated patients, when evaluated 12 months after surgery. In 1999, based on a review of the literature and expert opinions, Declau et al. (1999) already concluded that atresia repair was only safe in experienced hands and they suggested that alternative options like percutaneous BCDs should be considered more often. That conclusion has been underlined by Evans & Kazahaya (2007). They reported that the majority (93%) of their operated atresia patients still needed some type of hearing device. Further evidence came from the Nadaraja et al. study. In their systematic review, they compared studies with Baha application and those using atresia repair, and concluded that the results with Baha were significantly better; the mean improvement (over studies) in hearing thresholds was 38 dB for Baha application and only 24 dB for atresia repair.

Apart from aural atresia, minor congenital (middle ear) anomalies might occur; e.g. isolated congenital stapes ankylosis or mobile stapes with ossicular chain anomalies. Thomeer et al. (2010; 2011; 2012) published one-year post-intervention audiometric data; the mean remaining air-bone gap was 15 to 20 dB. One year after surgery, 65 to 75% of the operated ears had a PTA (air conduction) of 30 dB HL or better. However, as stated by Thomeer, hearing thresholds might have been better with hearing devices.

In case hearing aid fitting is considered in minor congenital middle ear anomalies, the first amplification option is a conventional BTE. If the air-bone gap exceeds 35 dB HL, a percutaneous BCD is the better option; the larger the air-bone gap, the higher the profit of a percutaneous Baha/Ponto device over a BTE (see paragraph 2.2).

Ten years ago, as an alternative amplification option, Colletti et al. (2006) introduced the VSB with its actuator (FMT) directly coupled to the round window of the cochlea (see also paragraph 2.3). Amongst others, Frenzel et al. (2009) and Colletti et al. (2010) reported their experience with VSB in aural atresia cases and reported that this amplification option was safe and effective.

Concerning this VSB application, several coupling options for the FMT to the cochlea have been introduced, either connected to the mobile stapes (footplate) or to the round window. If there is a choice, there is a preference for a coupling to the oval window because of higher output (e.g. Hüttenbrink et al., 2008). A long-term evaluation showed that all FMT couplings were stable, with the best effective gain for incus coupling (Bush et al., 2017). 

Transcutaneous BCDs have also been used, but so far little has been published (Dimitriades et al., 2016). A proper comparison with the percutaneous BCD is lacking.

It should be noted that to obtain a good hearing results, the prescription model introduced in paragraph 4.3 might be taken as a starting point.  

5.1.2 MRI compatibility and stability of these implantable devices

An important issue to consider when choosing an amplification option is MRI safety. MRI is a powerful and increasingly popular diagnostic tool. Statistics show that the incidence of MRI scanning increases steadily over the years. In 2010, the incidence was 51 MRI scans per 1000 subjects (for the Netherlands, CBS, 2012).

Doshi et al. (2014) reviewed literature and manufacturer’s information on MRI compatibility of percutaneous and transcutaneous BCDs and middle ear implants. The percutaneous (titanium) implant is MRI safe; in contrast, the middle ear implants are not MRI safe. Early 2015, a new VSB implant was introduced, the VORP 503, which is MRI safe up to 1.5T. The transcutaneous bone-conduction implants are MRI compatible either up to 1.5T (Bonebridge, Baha Attract) or 3T (Sophono implant).

Whether or not a MRI scan harms the implant or actuator is of importance but also the degree of distortion of the MRI scan by the implanted device. The percutaneous implants affect the MRI image only locally (radius of 15 mm) while the transcutaneous implants, comprising powerful magnets, distort the MRI scans more significantly, with a radius of 10 to 15 cm (Doshi et al., 2014).

Systematic studies on long-term implant stability are relatively scarce but not for percutaneous implants. Reported disadvantages of percutaneous implantation are implant loss and skin irritation around the implant , which might lead to revision surgeries. It should be noted that through the years, less complications occurred, owing to more than 35 years of experience with percutaneous implants. New implantation procedures have been introduced as well as optimized implants. Table 5.1, first line, presents Nijmegen data; systematically, all minor and major complications have been documented for each patient, starting in 1988. Patients were seen at least 5 times during the first years after implantation and at least twice a year, later on. Apart from planned visits, additional visits took place at the patient’s initiative. Dun et al. (2012) published data on medical complications of the first successive 903 adult patients, implanted from the start in 1988 up to 2008. Table 5.1 shows that one revision surgery has been performed per 30 years of follow-up (column 5). Wazen et al. (2008) published a similar study. They started later, in 1998. That might be the reason why their result is better than that of Dun et al. More recently, prospective multicentre studies have been published using a specific new, updated percutaneous implant and/or implant procedure (Nelissen et al., 2014; Kruyt et al., 2018; see Table 5.1). Indeed, further progress has been made, as relatively less revision surgeries were needed.

The last columns of the table presents data related to skin reactions around the percutaneous implant (percentage of visits with some skin reaction). Generally, the severity of skin reaction is expressed in Holgers grades 1 to 4. Grade 2, red and moist tissue, requires local treatment; grades 3 and 4, more severe conditions, might require revision surgery (Wazen et al., 2011). As the table shows, indeed, skin problems remain rather common; however, the percentage of serious skin problems, requiring some type of intervention, is decreasing (Holgers grades 2 to 4; see last column).

Table 5.1. Number of revisions related to the total follow-up period and the percentage of soft tissue reactions

Author, year n Summed revisions Summed follow-up relatively All skin reactions3 Holgers grade > 2
Dun, 2012 9031 1452 4.267 yrs 1 in 30 yrs 15% 4,3%
Wazen, 2008 218 23 990 yrs 1 in 43 yrs 4  
Nelissen, 2014 52 2 156 yrs 1 in 78 yrs 23,1% 1,8%
Kruyt, 2018 39 2 117 yrs 1 in 58 yrs 15,5% 1,5%

1only the adults; 2revisions because of soft tissue problems plus lost implants; 3Holgers grades 1 to 4; 4no Holgers grades reported

It should be noted that the follow-up period of the latter two studies is relatively short; 3 years. However, as has been shown by Dun et al., most complications occur in the first years post implantation.

Concerning MEIs, the number of medical complications and explantations have been reported by e.g. Bernardeshi et al. (2011), Colletti et al. (2013), Mlynski (2014) and Brkic et al. (2019). The derived revision-surgery rates were between 1 in 15 years up to 1 in 28 years of follow-up. Growing pains might play a role, especially with regard to surgical techniques.

It is suggested that, based on the data of Table 5.1, one revision surgery per 40 years of follow-up (for whatever reason) might be considered as the standard.

In case of congenital conductive hearing loss, including aural atresia, often, surgical repair will lead to inferior hearing. So far, only the stability of percutaneous BCDs is well documented, at a level that is an obvious challenge for the other amplification systems. Concerning MRI compatibility, percutaneous BCDs are the best option, not only because titanium implants are MRI safe but also because they affect the MRI scans only marginally.

5.2  Chronic otitis media with effusion and suppurative otitis media

In most cases, otitis media with effusion (OME) is a transient disease, affecting mainly children. Most children recover spontaneously or they are surgically treated (grommet insertion). However, in some patients, OME might be (semi) chronic, e.g. in children with Down syndrome and children with cleft palate (Sheenan & Hans, 2006). Then provision of a (temporary) hearing device might be beneficial. The OME-associated (conductive) hearing loss in children typically varies between 15 dB HL and 35 dB HL what suggests that BTEs can be fitted. A BCD on a headband or softband has also been advocated (Ramakrishnan et al., 2006). The advantage of a BCD is that when the hearing loss fluctuates over time (not unlikely for OME), BTEs have to be refitted, in contrast to BCDs. Note, however, that aided thresholds with conventional transcutaneous BCDs typically lay around 25-30 dB HL, thus close to the unaided thresholds (Snik et al., 2008). Indeed, according to the data of Ramakrishnan et al. (2006), who provided children with OME with BCDs on headbands, it was not a great success.

In patients with chronic OME, BTEs are a good amplification option

Chronic suppurative otitis media (CSOM) is a long-standing middle ear inflammation that results in periods of discharge from the ear. The tympanic membrane is perforated. Chronic suppuration can occur with or without cholesteatoma. Treatment of such patients is mainly directed at the inflammation and, eventually, cholesteatoma removal. This disease might lead to substantial conductive or mixed hearing loss. In patients with suppurative otitis media, BTEs with occluding ear moulds are contraindicated because an occluded ear canal might have an adverse effect on ear discharge. Chronic ear discharge may lead to cochlear damage (e.g. Papp et al., 2003).

While BTEs are not a safe choice, percutaneous BCDs can be used and the outcomes are favourable (Snik et al., 2005). Literature has shown that replacing BTEs by percutaneous BCDs had a significant positive effect on the ear discharge (Macnamara et al. 1996; Mylanus et al. 1998; McDermott et al. 2002). The observed reduction of infections resulted in significantly less outpatient visits (Hol et al., 2004), what is an important advantage in terms of cost-benefit as well as in patient’s convenience.

Another amplification option is middle ear implants, like the VSB device, with the actuator coupled to one of the cochlear windows. This device has successfully been implanted after subtotal petrosectomy with obliteration of the ear cavity with abdominal fat (e.g. Linder et al. 2009 and Ihler et al., 2013). Minor technical and medical problems were reported. Ihler et al. reported one revision surgery in their 10 implanted patients. Mean gain at threshold level (or the effective gain; for its definition, see Appendix 2) was comparable to that of the VSB applications in other types of mixed hearing loss (+2 dB and +4 dB). With the Sophono device (transcutaneous BCD) applied in 10 patients after subtotal petrosectomy, a mean effective gain of -13 dB was found (mean PTAbc was 29 dB HL; Magliulo et al., 2015), suggesting a less powerful application.

Concerning MRI compatibility, percutaneous BCDs are the best option (see paragraph 5.1.2), especially for ears that need follow-up with MRI scans (e.g. after cholesteatoma surgery).

To obtain a good hearing result, it is suggested to use the prescription model introduced in paragraph 4.3, at least as a starting point.

For patients with chronic running ears (CSOM) who need amplification, percutaneous BCDs are the first treatment option. Middle ear implants can be used as well, although implant surgery is complicated by the fact that a stable, infection-free middle ear has to be established, first of all.

5.3 Otosclerosis

5.3.1. Introduction

Otosclerosis is a disease that leads to mixed hearing loss. Large variations in hearing loss have been reported and hearing loss might deteriorate over time at different rates (e.g. Rotteveel et al., 2004; Sziklai et al., 2009). The conductive part is caused by stapes fixation. Although some twenty-year-old patients with otosclerosis might already have a SNHLc of 30 dB HL (Iliadou et al., 2006; Topsakal et al., 2006), approximately 90% of the patients suffer from a predominantly conductive hearing loss and a presbyacusis-like deterioration of SNHLc with relatively early onset (Kursten et al., 1994; Aarnisalo et al., 2003; Topsakal et al., 2006). The other 10% suffer from an obvious mixed hearing loss; around 1.5% of these patients develop a profound SNHLc, exceeding 70 dB HL, referred to as ‘advanced otosclerosis’.

The generally accepted first treatment option is to place surgically a stapes prosthesis to restore the ossicular chain (stapedotomy). This is considered as a safe and effective intervention that reduces the air-bone gap in most cases to below 10 dB. At the high frequencies, the effectiveness might be somewhat worse (e.g. Strömbäck et al., 2012).

5.3.2. Amplification options

If, after stapedotomy, amplification is still needed, then the first choice is a BTE. In case of advanced otosclerosis, a powerful BTE is needed (e.g. van Loon et al., 2014; Wardenga et al. 2020). VBS application with the FMT applied in the classical way (connected to the incus, moving in parallel to the stapes prosthesis) is also an option, as has been shown by Dumon et al. (2009) and Venail et al. (2007).

Kontorinis et al. (2011) and Beltrame et al. (2009) also applied the VBS in patients with otosclerosis, however, with its actuator (FMT) coupled to the round window.

Another option is the use of BCDs. In that case, stapes surgery is not a prerequisite. Burell et al. (1996) used the percutaneous BCD and called it: ‘the third option for otosclerosis’. Since then, little has been published on this application.

Another option was to use the Codacs device; this device, specially developed for patients with advanced otosclerosis is taken of the market 2020. However, still included in this analysis as it was the most powerful treatment option ever.

A literature search was performed aimed at published audiometric outcomes, obtained with any of these amplification options (BTE, VSB, BCD or Codacs; summer 2015); little has been published, especially on BTE fittings after stapedotomy. Figure 5.1 gives an overview of the collected data. Concerning BTE, the data were retrieved from several studies in which stapedotomy was not necessarily performed or unsuccessful (Burell et al., 1996; Lenarz et al., 2013; Busch et al., 2013).

5 Slide1

Figure 5.1. The mean aided thresholds presented versus the mean SNHLc of study groups of patients with otosclerosis using: BTEs (red symbols, 3 studies, n=27), Baha HC300 (yellow symbol, 1 study, n=7), VSB with the FMT coupled to the round window (open dots, 2 studies, summed n=7), VSB applied in the classical way (green symbol, 2 studies, summed n=7) and, finally, Codacs device (black round, 3 studies, n=44). Standard deviations (not indicated) varied between 4 and 6 dB in either direction. Target line for aided thresholds is presented as introduced in paragraph 4.3.

Figure 5.1 suggests that application of either the Codacs device (data from Lenarz et al., 2013; Lenarz et al., 2014; Busch et al., 2013) or the VSB with FMT coupled in the classical way (data from Venail et al., 2007; Dumon et al., 2009), are the most effective options because they are closest to the target line. The studies with the FMT coupled to the round window (data from Kontorinis et al., 2011; Beltrame et al., 2009) seem to be somewhat less effective. This suggests that FMT applied in the classical way is a better option, what is in accordance with the experimental findings of Hüttenbrink et al. (2008).

For patients with advanced otosclerosis there is still a debate on what the best option is; stapedotomy plus powerful BTE or cochlear implantation. Van Loon et al. (2014), reviewing the literature and their own data showed that stapedotomy with BTE fitting in patients referred for CI should still be considered; performing stapedotomy and BTE fitting in several of their CI candidates resulted in adequate speech perception. Another recent observation was that patients suffering from advanced otosclerosis and fitted with the Codacs device had speech in quiet scores equal to or better than those of CI users matched with respect to their SNHLc (in the range of 55-75 dB HL). Moreover, speech recognition in noise was obviously better in the Codacs group (Klundt et al., 2015). This suggests that acoustic stimulation might outperform electric stimulation in patients with severe SNHLc.

First treatment option for patients with hearing loss owing to otosclerosis is stapedotomy plus, eventually, the fitting of BTEs. Often, patients with otosclerosis, referred for cochlear implantation, might still benefit from a stapedotomy and the fitting of powerful BTEs. In cases with advanced otosclerosis, speech perception scores with acoustic devices might be better than those obtained with a cochlear implant.

MRI compatibility of the studied devices has been described in paragraph 5.1.3.

Stability. Stability figures have been presented in paragraph 5.1.3, concerning percutaneous BCDs and VSB. Those stability data were not specified according to aetiology. There is no reason to believe that aetiology plays a significant role when looking at BCDs. Concerning the VSB with its transducer coupled to the incus, it is expected that stability factors will not be worse than those published so far for application of the VSB in sensorineural hearing loss. However, Venail et al. (2007) reported a severe deterioration in hearing in one of their five patients who received a middle ear implant after stapedotomy and the patient became a non-user. Stapedotomy in itself is a rather safe procedure; however, severe hearing deterioration might occur in a few per cent of operated ears (e.g. Rotteveel et al., 2004; Kursten et al., 1994).

Chapter 4. Longevity and a new fitting model

4. Longevity and a new fitting model 

4.1 Introduction

An attempt is made to develop a fitting model based on an acceptable partial use of the dynamic range of hearing. Audibility of normal conversational speech, with its 30 dB wide speech range, is an important factor in that model.

A new goal has been formulated: at least 35 dB of the dynamic range of hearing should be accessible with a device; that means that with proper amplification conversational speech is audible (Zwartenkot et al., 2014 and Rheinfeldt et al., 2015). This criterion is indicated in Figure 4.1. The idea is that 35 dB HL is enough to hear conversational speech adequately while the amplifying hearing device makes use of wide-dynamic range compression with slow release times, thus is working as an automatic volume control. This criterion is referred to as the ‘DR>35 dB rule’. A second criterion is now introduced, namely at least 2/3 of the dynamic range of hearing should be audible with a minimum of 35 dB (referred to as the ‘DR 2/3 rule’). This criterion is based on the data of Figure 3.2. Baha patients in that study were fitted with different types of linear devices with volume wheel. Typically, their aided dynamic range of hearing equals about 2/3 of the unaided range, at least up to 30 dB HL. This new criterion is illustrated by the blue line in Figure 4.1.

Table 4.1 presents the MPO data from Table 2.1. The third and fourth columns of this table indicate up to what mean SNHLc value the different devices fulfill the ‘DR>35 dB rule” or ‘the DR 2/3”. These values deviate from those advocated by most manufacturers.

figure 4.1

Figure 4.1. The minimal desired mean MPO versus mean SNHLc. Red line presents the minimal desired MPO if the aided dynamic range of hearing has to be 35 dB at least (DR>35 dB rule). The blue line gives such target values if at least 2/3 of the dynamic range should be audible with a minimum of 35 dB (DR 2/3 rule).

Amongst others, Table 4.1 suggests that the Sophono Alpha 1 device can only be used in conductive hearing loss: indeed Sylvester et al. (2013) came to a similar conclusion, using sound field measurements and speech tests. The table further suggests that percutaneous BCDs (with their most powerful processors) and the VSB device have approximately the same capacity.

Table 4.1

Device Mean   MPO* ‘Max SNHLc’ if the ‘DR 2/3 rule’ is used ‘Max SNHLc’ if the “DR>35 dB rule” is used
Sophono Alpha 1 53 dB HL 5 dB HL 20 dB HL
Bonebridge 64 dB HL 20 dB HL 30 dB HL
Standard Baha/ Ponto 70 dB HL 25 dB HL 35 dB HL
Super power Baha/Ponto 83-88 dB HL 45-50 dB HL 45-50 dB HL
Vibrant Soundbridge 81dB HL 45 dB HL 45 dB HL
Codacs ** 100 dB HL > 65 dB HL > 65 dB HL
Baha Attract/BP110 64 dB HL 20 dB HL 30 dB HL

* taken from Table 2.1
** no longer available

When using the milder criterion (DR > 35 dB rule), the application ranges are broader, see fourth column. As was argued, a ‘working range’ of 35 dB might be just sufficient for proper amplification of speech, eventually in combination with wide-dynamic range compression (Rheinfeldt et al., 2015). Wide dynamic range compression is the standard choice for patients with pure sensorineural hearing loss using BTEs, who have a limited dynamic range of hearing because of physiological conditions (hair-cell loss). When applying BCDs or MEIs, wide-dynamic range compression is an option, although the limited dynamic range is not caused by physiological conditions but technical restrictions of these devices (low MPO). Evidently, the starting point when choosing an amplification option should be selecting a device with a high MPO. Especially in children, powerful devices should be used, see Chapter 5.

It should be noted that the ‘max SNHLc’ values per device type, as presented in the table, are in agreement with maximum SNHLc values as obtained from clinical studies, as presented in Figure 3.3.c. Using the trend-curves indicated in that figure, the maximum SNHLc per device type could be assessed as well, e.g., approx. 45 dB HL for VSB and 15-20 dB HL for the transcutaneous BCDs. Importantly, this agreement (between Figure 3.3.c and Table 4.1 data) validates the MPO-based ‘max SNHLc-values’, as presented in the table.

Concerning the use of hearing devices for conductive and mixed hearing loss, the limited MPO has to be taken into account. Patients compensate for a low MPO by lowering the gain of the device, leading to an unintended decrease in perception of speech. In short, today’s passive transcutaneous BCDs can be used in patients with normal cochlear function or with a limited SNHLc, the active Bonebridge device in patients with SNHLc below 25 dB HL, percutaneous BCDs and VSB in patients with SNHLc up to 50 dB HL. These values are more conservative than those claimed by the manufacturers and might change when more powerful processors are released.

4.2. Longevity

What about longevity? Owing to aging or a progressive (hereditary) cochlear hearing loss, the SNHLc deteriorates over time. Using data on the age-related hearing deterioration enables the estimation of ‘years of effective use’ of a particular device. However, little is known (reported) on age-related hearing deterioration in patients with mixed hearing loss. One paper was identified; Figure 4.2 shows age-related deterioration in hearing of patients with mixed hearing loss, studied by Iliadou et al. (2006). They reported both air- and bone-conduction thresholds for patients suffering form OTSC7. The figure presents the mean SNHLc (0.5, 1, 2 and 4 kHz averaged) as a function of age; see the first two rows. Data up to 60 years were presented; the indicated mean SNHLc at 70 and 80 years are (non linear) extrapolations. The thick red lines indicate whether or not a certain device can (still) be used effectively. E.g., the figure shows that the Baha, with its ‘max SNHLc’ of 50 dB HL, can be used effectively up to approx. 80 years (the ‘max SNHLc’ refers to the maximum sensorineural hearing loss component for successful application, assuming that the ‘dynamic range of hearing’ with the device exceeds 35 dB; see Table 4.1 and corresponding text). The Ponto device and the VSB might be applied successfully up to an age of approx. 75 years.

Probably, the mildest scenario is that of a patient with a stable conductive hearing loss component and, additionally, presbycusis. Using the median age-related hearing deterioration for men (ISO 7029; 2017), Figure 4.3 is obtained. Note that if the patient suffers from mixed hearing loss caused by chronic otitis media, they might have an additional sensorineural hearing loss caused by the infections and/or treatments. Using the data presented by Bosman at al. (2013; elderly Baha users), comparing the reported bone-conduction threshold with the age-appropriate ISO 7029 threshold, showed a difference (additional loss) of 19 dB HL (mean at 0.5, 1, 2, 4 kHz and averaged over patients). Taken this 19 dB into account, the effective use decreases to 50 years for the Bonebridge and 75 years for VSB and the percutaneous BCDs while for such patients with an age of 50+, the passive transcutaneous BCD with magnetic coupling is not an effective option.

Figure 4.2. Effective device use for a patient with progressive hearing loss owing to otosclerosis (OTSC7), based on the ‘DR 2/3 rule’. SNHLc stands for the cochlear hearing loss of patients

Figure 4.3. Effective device use for a patient with stable conductive hearing loss component and a sensorineural component that deteriorates over time owing to presbycusis. The ‘DR 2/3 rule’ was applied

Again, the percutaneous BCD device and the VSB device seem to be the better choice.

When choosing for a particular treatment, longevity is one of the factors that should be considered. The expected degree of hearing deterioration over time should be assessed from the patient’s history.

It should be noted that the conclusion drawn from Table 4.1 and Figures 4.2 and 4.3 are valid for the specified device types. If more powerful audioprocessor are released, conclusions will change.

Longevity is an often ignored but important factor when counseling treatment options, especially when surgery (implantation) is involved.

4.3 Attempt to formulate a prescription procedure

So far, the evaluation of the capacity of various amplification options concerned the MPO as a low MPO restricts the aided dynamic range of hearing of the patient. To deal with that limitation, compression is often used. It should be noted that compression affects the aided thresholds positively, overestimating the gain provided by the device. A second limiting factor might be audible microphone noise; patients with predominant conductive hearing loss might hear device noise, owing to their (sub) normal cochleae. Then expansion can be used to make the noise inaudible (Dillon, 2012), however, expansion affects the aided thresholds negatively, under-estimating the gain provided by the device. The noise level of a device should be clearly indicated on datasheets, which is only the case for percutaneous BCDs. A documented measuring procedure is available. Although problems with the noise floor have been reported with the VSB processors (e.g. Linder et al., 2009), according to the manufacturer, problems with the Amade soundprocessor are minor and expansion is not used. For the Cochlear Codacs device, the noise floor is approximately 40 dB HL standard (Cochlear’s data sheets), which can be lowered to 25 dB HL and should be explicitly taken into account when considering this device for a patient.

To choose the best device for a given patient is important. Note that when considering the application of a middle ear implant, well-masked bone-conduction thresholds are essential to ensure that the cochlea of the to-be-implanted ear is sensitive enough for successful application of the implant. On the other hand, for the application of BCDs, unmasked bone-conduction thresholds of the to-be-treated ear should be considered. Owing to the limited transcranial attenuation of bone vibrations, the BCD will stimulate the cochlea with the best sensitivity, which might be either the ipsilateral or contralateral cochlea (Stenfelt, 2012). Consequently, the fitting should be based on the unmasked bone-conduction thresholds.

Regarding device fitting, it should be realized that either an implantable BCD or a middle-ear implant with its actuator coupled to one of the cochlear windows, directly stimulate the cochlea, bypassing the impaired middle ear. Therefore, the efficacy of the sound processor fitting depends directly on how well the cochlear loss (or SNHLc), as expressed by the bone-conduction thresholds, is compensated. This simply implies that we can build on our knowledge of fitting conventional hearing devices (e.g. BTEs) in pure sensorineural hearing loss. Desired gain and desired output values can be determined by using validated prescription rules developed for sensorineural hearing loss, like the classical half-gain rule (HGR; often used as a rule of thumb) or the more sophisticated NAL and DSL rules (Dillon, 2012).

To develop a practice-based prescription procedure, we studied published data. The 24 selected studies, introduced in Chapter 3, were used once more. For each of these studies, the gain at cochlear level was calculated as a function of frequency. That gain, referred to as ‘effective gain’, is by definition the frequency specific bone-conduction threshold (cochlear threshold) minus the aided threshold (for more information, see Appendix 2.3). To obtain a relative gain value, corrected for the degree of hearing loss, the ‘effective gain’ was divided by the bone-conduction threshold, referred to as the gain ratio. According to the above-mentioned rule of thumb (half-gain-rule), that ratio should be 0.5 (with some minor corrections; Dillon, 2012).

Twenty of the 24 papers provided all the data needed for such calculations and were included. Figure 4.4 presents the gain ratios, presented per device type and the degree of cochlear loss (SHNLc). To deal with the discontinuities seen in Figures 3.2, 3.3 and 3.3.c, the 20 studies were divided into 3 subgroups, according to the mean SNHLc of the participants. Group 1 comprised the studies in patients with a mean SNHLc below 25 dB HL, group 2 those with SNHLc between 25 and 40 dB HL and group 3 those with a SNHLc exceeding 40 dB HL; see Table 4.2. Using Table 4.1 and noise level data, for each subgroup, the amplification options are added, third row.

Table 4.2. Subgroups to deal with the discontinuities observed in Figure 3.2

Group 1 Group 2 Group 3
Mean SNHLc < 25 dB HL 25 to 40 dB HL > 40 dB HL
Figure 3.2 shows: Negative gain Intermediate Positive gain
Amplification options based on Table 4.1 and noise floor data Percutaneous BCD/VSB/Bonebridge Percutaneous BCD/VSB Percutaneous BCD/VSB/Cochlear MET?
Hypothetical target for gain Predominant conduc-tive loss: compensate the air-bone gap! Intermediate ‘Compensate’ the SNHLc as if it was a pure sensorineural loss

Furthermore, using Table 4.1 and noise level data, for each subgroup, the amplification options are listed.

Lines in Figure 4.4 are labeled according to device type.

1

2

3

Figure 4.4. The gain ratio (gain divided by threshold) as a function of frequency. Figure 4.4A presents the data of group 1 (BAHS stands for Baha and Ponto together: 1 study, n=20; VSB: 3 studies, n=34 and Bonebridge: 1 study, n=12), Figure 4.4B for group 2 (BAHS: 6 studies, n=113; VSB: 7 studies, n=97) and Figure 4.4C for group 3 (BAHS: 1 study, n=12; VSB: 3 studies, n=47).

Obviously, Figure 4.4 shows that all the gain ratios are below 0.5, the prescribed value according to the half gain rule. That rule is more or less outdated. More dedicated rules have been developed, like the NAL rule (Dillon, 2012). Typically, the gain ratio using that rule is lower than 0.5.

The figure also shows that the gain ratio is the highest at 2 kHz, irrespective of device type and subgroup. Furthermore, it is evident that inter-group differences are large (compare subfigures A, B and C). Differences between devices within groups are less outspoken.

These data have been used to develop the practice-based prescription procedure: for mixed hearing loss, it is assumed that the cochlear loss should be ’compensated’ as in sensorineural hearing loss. For predominant conductive hearing loss, the air-bone gap should be compensated; thus the gain (and thus the gain ratio) should be (close to) 0. However, negative gain ratios are seen (Figure 4.4.A). As suggested by Dillon (2012) and discussed in Chapter 1, maybe not the whole air-bone gap has to be compensated but only partially. It is suggested, based on our data, that the aided thresholds should be better than 25 dB HL for those patients with (sub) normal cochlear function. More details are found in Snik et al., 2019.

Table 4.3 presents the target aided thresholds and target effective gain, following the principles of the NAL prescription rule and the data presented in Figure 4.4. A margin of 5 dB has been taken into account.

Table 4.3. Target values as a function of SNHLc for 1, 2 and 4 kHz

SNHLc (dB HL)

0

10

20

30

40

50

60

Target aided threshold (dB HL)

<25

<25

<25

<25

<25

<27

<33

Effective gain (aided minus cochlear thresholds dB)

-25

-15

-5

5

15

23

27

Following the NAL rule, gain at 0.5 kHz might be set 10 dB lower than that at the higher frequencies as indicated in the table, resulting in target aided thresholds that are 10 dB higher than those listed in the table (Snik, et al, 2019).

What are the limitations of this proposed procedure? It should be noted that this evaluation is based on today’s hearing implants. Furthermore, the procedure has not (yet) been validated. However, the gain ratios for Codacs device, a very powerful device (3 studies, 43 participants, see paragraph 5.3.2, mean SNHLc = 55 dB HL) were: 0.35, 0.40 and 0.22 for the frequencies 1 to 4 kHz, which exceed the values for the other devices (Figure 4.4), approaching but still below the proposed target values for 1 and 2 kHz.      

This practice-based prescription procedure can be used irrespective of the type of device used. Prerequisite is that a device is chosen with sufficiently high MPO, according to Table 4.1, preferably following the ‘DR 2/3 rule’.

Importantly, note that for percutaneous BCDs, Hodgetts and Scollie (2017) developed a dedicated prescription rule, based on the DSL fitting rule. That procedure is the preferred option, taking, amongst others, the limited MPO of these BCDs into account.