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 discussed 

–Sensorineural hearing loss:

  • Severehearing loss: when to switch from a BTE to a CI?
  • If the hearing loss is moderate to severe, what is the better option: a BTE or MEI?

–Conductive or 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 should a powerful ‘BCD’/MEI be replaced by a CI?

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

Note. In contrast to other chapters, references are listed at the end of this chapter

 

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 outcome 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 deafness. Inclusion criteria and details about the types of CI used are presented in Appendix 9.1. Table 1 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 less strict 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, only the Vibrant Soundbridge MEI is widely used (VSB; Med-El, Innsbruck, Austria). In Nijmegen the VSB has (only) been applied if a BTE is 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 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 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 infected 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 AB-gap (gap between the air- and bone-conduction thresholds). 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 AB-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 AB-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 p’BCDs’ in (Nijmegen) patients with conductive/mixed hearing loss. They found that if the AB-gap exceeds 30-35 dB, the pBCD provided higher gain and, therefore, better speech scores than the BTE. Below that AB-gap, the BTE provided the best result.

In other words, if a BTE is not contraindicated while the mean AB-gap exceeds 35 dB, then a powerful (thus percutaneous) ‘BCD’ might be the better 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 an elasticsoftband, steal headband or adhesive coupling. Such devices are referred to as conventional 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. This device requires a surgically placed subcutaneous magnet for coupling purposes and 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-El; 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 these ‘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 humans can tolerate sounds up to 105-115 dB HL.

The MPO of ‘BCDs’ and also that of the VSB have been measured. 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 present 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-discomfortable-levels (LDLs). 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. The 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 side, 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 relevant sounds properly (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 progressive severe mixed hearing loss, when to replace a ‘BCD’ by  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 otosclerosis-related anomalies of the cochlea often leading to partial 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, but 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 upgraded 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 t’BCDs’ 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. In contrast, 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: Longterm effectiveness is an important issue, especially when treating elderly people, because their hearing loss might significantly deteriorates over time. 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, 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 becoming older 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, what might lead 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 with providing Baha devices, namely 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%

1 only the adults; 2 revision surgeries because of skin 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, we suggested 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 leads to a higher price. Rising cost of medical healthcare is a major issue in many countries; 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, 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. The data used are relatively old, 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 device-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 who were 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. It is likely that the cochlear damage caused by the middle ear problems occurred in the past when the patients were still using a BTE, thus before using a ’BCD’ (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 owing to chronic ear infections.

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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Verhaegen VJ, Mulder JJ, Mylanus EA, Cremers CW, Snik AF. Profound mixed hearing loss: bone-anchored hearing aid system or cochlear implant? Ann Otol Rhinol Laryngol. 2009 Oct;118(10):693-7.

 

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 Sep;118(9):1645-9. 

 

Verhaert N, Mojallal H, Schwab B. Indications and outcome of subtotal petrosectomy for active middle ear implants. Eur Arch Otorhinolaryngol. 2013 Mar;270(4):1243-8.

 

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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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.

VSB fact sheet (VORP 503/Samba). The Vibrant Soundbridge system. Med-El website, downloads; visited June 2015.

Wardenga N, Diedrich V, Waldmann B, Lenarz T, Maier H. Hearing Aid Treatment in Patients with Mixed Hearing Loss. Part I: Expected Benefit and Limitations after Stapes Surgery. Audiol Neurootol. 2020;25:125-132.

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

8.1 Auditory implants for moderate to severe sensorineural hearing loss

To help patients with a 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 with BTEs. However, speech recognition might still be troublesome, especially in noisy situations, despite the amplification. This is caused by the hearing-loss-associated impaired temporal-spectral processing of speech sounds in the cochlea (referred to as cochlear distortion; Plomp, 1978). Today’s digital devices make use of  algorithms that enhance the speech sounds relative to the noise, like noise reduction algorithms and adaptive directionality of the BTE’s microphone (Dillon, 2012).

Note. In contrast to other chapters, references are listed at the end of this chapter

For patients with severe to very severe 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 because of severe cochlear distortion. Then, cochlear implantation becomes the next treatment option (Hoppe et al., 2015; Huinck et al., 2019), as elaborated in the next chapter, section 9.1. An alternative amplification option for patients with hearing loss below 70-80 dB HL is an (acoustic) auditory middle-ear implant (MEI, Verhaegen et al., 2008). High sound fidelity, high output with less feedback problems and the absence of an occluding earmold were claimed advantages of MEIs over BTEs (e.g. Goode, 1995). Since the late nineties, several different types of MEIs have been introduced for patients with sensorineural hearing loss (Snik, 2011). Nowadays (early 2021), two semi-implantable MEIs are 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 with electronics placed in a deep fitted in-the-ear hearingdevice. Early 2020, a competitor of the VSB, the semi-implantable Cochlear MET, was taken off the market.

In addition to semi-implantable MEIs, one fully implantable MEI is on the market, namely the Esteem device (Envoy Medical, St. Paul, MN, USA; Marzo et al., 2014). Another fully implantable MEI was the Cochlear Carina, which also was withdrawn from the market early 2020. Several systematic reviews have been published aiming at the claimed advantages of MEIs over BTEs. Pulcherio et al. (2014) concluded that, comparing the Carina and Esteem devices to BTEs, no structural audiological benefit was found. It should be noted that these implantable devices are 1. expensive and 2. involve (implant) surgery and 3. regularly, need revision surgery (every 5-10 years) to replace the battery; therefore,  the cost-benefit ratio of these devices is highly unfavorable compared to BTEs. Another review by Kahue et al. (2014), comparing audiological outcomes obtained with the  ( VSB and Maxum devices on the one hand and BTEs at the other, concluded that a surplus value of the MEIs could not be established. Since these reviews were published, while screening the more recent literature, just one more study was found with the Maxum as well as one with the Esteem device, published by manufacturer-independent research groups. Barbara et al. (2018b) reported on 3 patients using the Maxum; compared to BTEs, two patients had equal speech recognition scores and the third patient had a 10% higher word score with the Maxum. 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 MEIs have no convincing audiological advantages and are expensive solutions. However, an exception was made for patients who don’t tolerate the earmold of BTEs owing to e.g. chronic external otitis, as formulated by Magnan et al., 2005 (consensus statements).The recent systematic reviews indicate that that statement remains valid. Edfelt et al. (2014) showed that for such patients the application of the VSB is cost effective! Note that according to the set-up of the Maxum, that device is not an option for patients with external otitis. No papers could be identified on the Esteem device, applied in patients with external otitis.

One manufacturer of MEIs  acknowledges using their device only in patients with external otitis (VSB fact sheet, 2015) 

In summary:

There is no evidence that MEIs for patients with sensorineural hearing loss led to better speech recognition than BTEs. However, the VSB might be used (cost-) effectively in patients with sensorineural hearing loss and chronic external otitis.

To help patients with sensorineural hearing loss and chronic external otitis, bone-conduction devices (‘BCDs’) 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 powerful percutaneous ‘BCD’ (Baha), applied in such patients showed that even with maximum amplification the result was insufficient (Snik et al., 1995). Stenfelt et al. (2000) came to a similar conclusion. More recent studies are lacking. In summary

To help patients with sensorineural hearing loss in need for amplification, studies showed that ‘BCDs’ 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 the 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 upgraded 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 (phoneme score) of patients with previous-generation sound processors. The figure shows that the new data points are close to the line or somewhat better. Busch et al. (2016) reported a similar conclusion. As a reference, Verhaegen et al. showed that with a BTE, applied in patients with similar hearing loss, the mean speech score was significantly higher, e.g. at 65 dB HL the mean phoneme score was 90% with BTE versus 70% with VSB (see also the next chapter, section 9.1.2.

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 a MEI with an open ear canal, like the VSB, might be the interference between the MEI processed sound and the direct sounds. It should be noted that owing to the digital processing of 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, which might lead 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 and compression settings.  Studies looking into this interference when using MEIs 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; their 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 (see section 4.3).

The main purpose of (any) device fitting is optimising the audibility of speech (Amlani et al., 2002). A popular tool to assess the audibility of speech is the Speech Intelligibility Index (SII). SII is the proportion of normal speech that is audible to a patient with a specific hearing loss; the SII ranges from 0 to 100, zero means that normal speech is not audible and 100 means that the patient is able to fully hear normal speech. The SII is based on the idealised 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). Next, these outcomes are summed for all 1/3 octave frequencies. In this simplified model, 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) also published a graph with the relationship between the SII and word recognition scores (their Figure 3). Using that graph, the SII of 90 and 22 matches with word recognition scores (WRS) of 95% 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 in noise and might enable spatial hearing (see Chapter 6). Lately, statistics showed that children with bilateral compared to unilateral hearing devices developed better cognitive skills, which was caused by 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 situations.

7 punt 1

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

Only approximately 1/3 of the knowledge of young children is acquired in structured quiet settings (e.g., in school). The remaining 2/3 is acquired during the day while 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 hearing 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

Now, we will use the audibility of speech (thus the SII) 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 at least 10 children involved, 3. with a unilaterally fitted device and 4. published after 2006. Most of the included studies addressed children with congenital conductive hearing loss.

7 punt 2

Figure 7.2. Mean device-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 (however, without a comparison with another 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, evidently applied in pure condictive hearing loss (SNHLc was around 10 dB HL) overall, a mean device-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 young 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) a mean value of 29 dB HL was found. That value was 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 device-aided hearing threshold of 33 dB HL (Denoyelle et al., 2015). That mean aided threshold was close to the reported threshold in the original paper, introducing the Sophono device (Siegert & Kanderske, 2013; reporting a mean value 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. It should be noted that the outcomes of two interventions displayed in Figure 7.1 are Farnoosh et al.’s data (2014), with intervention 1 referring to a 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). It is obvious that SII is the highest when using percutaneous ’BCDs’; ranging from 85 to 100 (corresponding WRS: 93 to 100%). Results with the transcutaneous ’BCDs’ are lower. Spread is larger and the SII ranges from subnormal (75; WRS of 90%) to moderate(50; WRS of 70%). Children with such a low SII might be prone to developmental delays. The SII after reconstructive surgery is particularly low, below 50. Indeed, Evans & Kazahaya (2007) reported that 90% of their operated children still needed some kind of amplification after atresia-repair.

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’ has hardly been applied in children (Sprinzl & Wolf-Magele, 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 children 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 atresia of the ear canal is, on the average, the least effective treatment. 

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.

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 persons 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 (more precisely, the mean threshold at 0.5, 1, 2, 4 kHz of each study was used to calculate the average over studies) per type of treatment, including the data of the previous search (presented in Figure 7.2). Table 7.1 shows that the mean bone-conduction threshold (column 3) per type of treatment is rather comparable. The mean post-intervention thresholds (next column) 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 type of intervention to enable calculating the SII and the associated word recognition score (WRS). The last column of the table presenting the WRS, suggests once more that these treatments are not equivalents. As concluded before, especially, the outcomes of surgical atresia repair are disappointing followed by the passive transcutaneous ‘BCD’s. The results obtained with the ‘Bonebridge’ and percutaneous ‘BCD’ are the best.

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.

 

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 softband can be used in all the other cases. It is easier to use and thus better accepted than a steel headband (Hol et al., 2005). The position of the stimulating bone-vibrator over the skull can easily be changed. The ‘BCD’-aided thresholds with such devices typically lay between 25 dB HL and 30 dB HL (e.g., Verhagen et al., 2008; Denoyelle et al., 2015), which might be acceptable but only for learning speech at a basal level (Northern & Downs, 1991; Verhagen et al., 2008). Therefore, replacement of a ‘BCD’ on head/softband by a percutaneous ‘BCD’ or other more powerful solutions should remain on the agenda and, meanwhile, speech and language development should be monitored.

The use of percutaneous ’BCDs’ implies implantation of a skin-penetrating titanium implant, anchored in the skull, behind the ear. Concerning the youngest age at which such implantation is feasible, the thickness of the skull plays an important role. At least 3 mm is preferred (Snik et al., 2005). This implies that the child must be approximately 4 years old; nevertheless, even then, the loss of these implants is approx. 3 times higher than in older children and adults (e.g., Dun et al., 2010). McDermott et al. (2009) reported that implant loss was high amongst the under fives and much less in older children. Other studies didn’t find such a distinct dependence of implant loss on age at implantation (e.g., de Wolf et al., 2008). More recently, improved titanium implants have been brought to the market with a significantly better stability (see paragraph 5.1.2). 

The subcutaneous magnet, for the coupling of a Sophono or Baha Attract processor, can be implanted from age 5 onwards, according to the manufacturers. Bezdjian et al. reviewed the literature and found 3 serious adverse reactions in 99 patients using the Sophono device, mainly children (Bezdjian et al., 2017). Dimitriadis et al. (2016, 2017) reported that the complication rate with the Baha Attract was low, when compared to the percutaneous ‘BCD’.

VSB with the transducer placed in the ‘round-window’ niche of the cochlea has been applied in children as young as 2 months (Mandala et al., 2011). However, since 2011, little has been published. Only one paper was found (with at least 5 implanted children); Leinung et al. (2017) presented results obtained in 13 young children with atresia of the ear canal. 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 device-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, furthermore, only relatively mild atresia-cases should be considered for surgery (e.g., Declau et al., 1999). Concerning stability, revision surgery might be necessary, 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

When a child is approx. 4 years old, the conventional ‘BCD’ should be ‘upgraded’. Parents might choose for transcutaneous ‘BCD’ instead of percutaneous ‘BCD’, because of stability issues, daily care of the skin around the skin-penetrating-implant and/or emotional problems to accept titanium implants sticking out, through the skin. Then the remaining 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 a sound processor, the Baha 5 power is advocated, which is more powerful than the standard Baha. The use of the Baha power processor will improve the MPO by several dBs. When bilateral transcutaneous ‘BCDs’ are used, in predominant conductive hearing loss, aided thresholds are expected between 25 dB HL and 35 dB HL, with corresponding SII values between 85 and 50 (corresponding WRS: 93% and 70%).  This might be just safe but suggests that these children should be followed-up more closely than bilateral percutaneous ‘BCD’ users concerning their speech and language development. It still remains of importance to keep a change to a percutaneous ‘BCD’ on the agenda.

Another suggestion that has been put forward to further support 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, can only be used effectively in structured situations like classrooms. 

*Note. References belonging to the studies summarised 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 be more than 9. The newly 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.

Chapter 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 three advantages can be distinguished: 1) loudness summation, 2)) directional hearing and 3) binaural squelch. In normal hearing persons binaural hearing is obvious, based on accurate processing of the unilateral inputs, leading to a ‘fused’ percept (binaural hearing). That is not evident for patients using hearing devices; however, firstly, a short introduction of the three advantages.

  1. Loudness summation refers to improved hearing owing to summation of sound as heard by each ear: the input perceived by either cochlea is summed, leading to an increase in loudness of approx. 5 dB.
  2. Localization of sounds, in the horizontal plane. To identify where a sound is coming from, the two ears have to work together. Interaural differences in perceived sounds are detected, caused by the different positions of the two ears with respect to the sound source. These interaural differences concern interaural loudness differences or ILD; owing to acoustic head shadow and interaural time differences or ITD, caused by a difference in arrival times of the sound at the two ears). The head, as an acoustic barrier, attenuates primarily high frequency sounds (> 2000 Hz). As a consequence, high-frequency sounds are being perceived as louder by the ear nearest to the sound source, creating an ILD. Below 1500 Hz, the 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 (sounds from the front) to a maximum of 0.7 ms (by a sound presented at the very right or left).
  3. Binaural squelch refers to central de-masking. Assume someone is listening to speech coming from the front and ambient noise also coming from the front. Then the noise might mask the speech. However, if the noise is coming from the left, then the speech sounds in the two ears will (still) be in phase (ITD=0). However, not the ambient noise (ITD= 0.7 ms, the maximum value). The differences in ITDs enables the perceptually separation (de-masking) of the speech and the ambient noise, referred to as binaural squelch.

Bilateral application of BTEs in patients with bilateral sensorineural hearing loss leads to binaural hearing. However, the binaural advantage might be worse than in normal hearing persons, listening with two ears (see e.g., Bogaert et al., 2006, studying sound localisation). Nevertheless, e.g., Boymans et al. (2008) showed an obvious benefit of bilateral BTEs in a large group of patients with bilateral sensorineural hearing loss, regarding sound localisation and the effective use of head shadow.

6.2 Binaural hearing with ‘BCDs’

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, thus 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-stimulation’. The attenuation in dB of the bone-conducted vibrations from one cochlea to the other varies widely with a median of just 5 dB. The variation between subjects is significant as well as within subjects, between the frequencies (Stenfelt, 2012). In other words, for bone-conduction stimulation, the cochleae are poorly acoustically isolated, however, just sufficiently to enable the detection of interaural differences, although, impaired (Stenfelt, 2005).

Table 6.1 presents data on binaural effects when changing from unilateral to bilateral listening in patients using ‘BCDs’ and, as a reference, normal hearing persons. In the latter case, unilateral hearing refers to hearing with one ear occluded. The first row of the Table presents the mean result (and standard deviations) of the ‘binaural’ outcomes as measured in normal hearing persons (Agterberg et al., 2011) and might be considered as standard values. The other rows present an overview of the outcomes of published Nijmegen studies concerning bilateral ‘BCD’ application in bilateral conductive/mixed hearing loss and unilateral ‘BCD’ application in unilateral conductive hearing loss, thus with a second normal hearing ear. All the patients and the normal hearing persons were evaluated with one and the same protocol. 

Binaural summation and speech in ambient noise were assessed using speech; directional hearing was tested using narrow-band noise with a short duration (1s) presented by one loudspeaker out of seven at a location between + 900 and – 900 azimuth (Agterberg et al., 2011). Concerning the binaural summation score, firstly, speech recognition was measured while the patient was listening with one ear (in unilateral cases with their normal ear; in bilateral cases with their left ear only, thus the ‘BCD’ on the right ear was turned off). To quantify binaural squelch, firstly, speech recognition in noise was measured while the patient was listening unilaterally. Speech was presented in front of the patient and the noise at the side of the only hearing ear. Secondly, the ‘BCD’ near the patient’s other ear was activated (in the normal hearing subjects, one ear had been occluded for the first measurement, which is open again for the second measurement). 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 frequency sounds (relying on the detection of ILDs), respectively. The mean absolute error is shown (0 means perfect localisation). Concerning these results: statistically significance was assessed by comparing the scores obtained in the unilateral listening situation (not shown) and bilateral listening situation. 

Let us consider bilateral ‘BCD’ application in bilateral hearing loss. According to the table, row 2, bilateral ‘BCD’ application does lead to the expected improvement in binaural summation (column 1) of, approximately, 4 dB. This result is in agreement with the literature (Janssen et al., 2012; review). Although not measured, binaural hearing in normal hearing subjects is around 5 dB. Obviously, here, the inevitable ‘cross-stimulation’ during bone-conduction stimulation, plays a minor role.

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 (‘BCD’ application in acquired unilateral conductive hearing loss) a limited but significant binaural summation score was found, however, not for the unilateral congenital cases (row 4). Column 2 shows the binaural squelch/head shadow data. The bilateral conductive hearing loss patients (row 2) profit significantly, however, profit is approx. half that of the normal hearing persons (2.5 vs. 4.6 dB). For unilateral acquired conductive hearing loss, the outcome is approx. the same, while for the congenital cases, no significant benefit was found. 

Columns 3 and 4 comprise the outcome of the sound localisation tests; the mean absolute difference (in degrees) between the perceived location and the real location of the activated loudspeaker are presented. For normal hearing persons, an error of 7-80   was found, for the bilateral ‘BCD’ users approximately 25and for the patients with unilateral acquired hearing loss using ‘BCD’, approximately 200. These three groups showed significantly improved scores when changing from unilateral hearing (mean unaided scores not presented but in the range of 46-660) to bilateral hearing. Probably, disturbing ‘cross-stimulation’ is the reason for the difference in score between the normal hearing persons and the bilateral ‘BCD’-users. Again, 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 ‘BCD’ active were largely unchanged, namely 340 and 390 for 0.5 and 3 kHz without the ‘BCD’ and  300 and 310 for 0.5 and 3 kHz with the ‘BCD’. The most likely explanation is that these patients, being unilaterally hearing since birth, have learned to use some specific monaural cues rather effectively. For normal subjects such cues are less relevant because they are inferior, rather marginal binaural cues (as elaborated by Agterberg et al., 2012 and Vogt et al. 2020). Vogt et al. even showed that with a 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 moderate 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 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 persons. Results of 20 patients who used a ‘BCD’ for at least 3 months were available (Nelissen et al., 2015). 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; a mean summed z-score of just over 10 was found in that group. The Figure shows that a value of 10 is achieved by just one of the patients with congenital unilateral hearing loss. Also negative summed z-scores were found, indicating a worse result with ‘BCD’ than without. A significant effect of age at intervention is not seen, while the spread in results is large.

5 Slide2

Figure 6.1. Binaural advantage based on the use of head shadow and sound localisation. Summed z-scores are presented as a function of age at intervention. Data of the ‘BCD’ users that continued using their device and those who stopped are indicated by different symbols.

Fitting of a (percutaneous) ‘BCD’ in patients with a 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. (2015) reported that with a follow-up > 5 years, 65% of the patients had stopped using their ‘BCD’. An association was found between ‘long-term’ non-use and the ‘short-term’ (one-year post-intervention) summed ‘binaural advantage score’. The 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, which is needed for a binaural percept. Thus, not surprisingly, the main reason to stop, as indicated by the patients, was complaints about interfering sounds/ambient noise when using the ‘BCD’ in situations with background sounds. Patients with unilateral acquired conductive hearing loss do benefit from ‘BCD’ application; obviously, they profit from their previously developed binaural abilities.

More recently, a review of the literature was published (Vogt et al., 2021) focussing on post-intervention binaural hearing abilities in children with congenital atresia of the ear canal. Interventions included not only a percutaneous ‘BCD’ but also all other types of implantable ‘BCDs’, MEIs like the VSB as well as surgical atresia repair. Surprisingly, all outcomes were disappointing, irrespective of the type of intervention (concerning binaural summation and/or binaural squelch and/or directional hearing). Vogt et al. discussed factors that might have played a role namely 1. age at treatment, 2. whether the treated ear was a ‘lazy’ ear, which means an ear with a not fully maturated neural network, and 3.  the role of the remaining asymmetry in hearing thresholds, post-intervention, as was seen generally. It was argued that the main problem might have been the remaining asymmetry in hearing after the interventions, which typically varied between 20 dB and 35 dB. This asymmetry might hinder the development of binaural hearing.

Concerning the binaural advantage of a MEI in unilateral conductive hearing loss, it should be noted that in contrast to a ‘BCD’, ‘cross-stimulation’ with a MEI is absent. Two publications were found in which outcomes obtained with the VSB MEI were compared to those 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 ‘BCD’ (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-stimulation’ is not a major issue for these patients.

Although unilateral congenital hearing loss in children might lead to developmental delays (in speech language development and performance in school), 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 was found regarding the role of age at treatment, degree of asymmetry in hearing after the treatment nor what intervention is the most effective one.

6.4. Bilateral application of ‘BCDs’ and MEI

Bilateral application in bilateral conductive hearing loss works, see Table 6.1, what is also reported by e.g., Janssen et al. (2012; systematic review) and Zeitooni et al, 2016. The latter study was carried out in normal hearing persons; 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 2, Table 6.1, when comparing the outcomes of the normal hearing persons and the bilateral ‘BCD’ 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 (summarised in row 2, Table 6.1) and those of 6 congenital cases were grossly the same. This might suggest that early binaural auditory experience is not a prerequisite for effective use of bilateral ‘BCDs’ 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 reported that all children showed improved sound localisation when using two instead of one ‘BCD’, however, localisation scores were moderate in most of the children. Den Besten et al. also presented data on none-use. Almost 85% of their 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 (early 2021).  As far as we know, studies are also missing on binaural hearing after bilateral application of MEIs, 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 not necessarily 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) devices instead of one. Improved hearing has been observed owing to binaural summation, in the order of 4 dB, see Table 6.1. This is of major importance; as the amplification of ‘BCDs’ is limited owing to the relatively low MPO, see Chapter 4, Figure 4.4 and related text. The ‘gain ratios’ for groups B and C as presented in that figure will improve significantly with the extra 4 dB of gain. For example, at 2 kHz, the NAL-prescribed value of approx. 0.45 is now reached for both devices of group C while for group B a value of 0.38 is found instead of 0.27. 

However, a problem might occur when fitting two devices. When programming two BTEs in bilateral pure sensorineural hearing loss, either BTE is programmed individually. When switching on both BTEs, overstimulation might occur owing to binaural summation. Generally, to deal with that, the volume and maximum output of the two BTEs is lowered; mostly, the software does this automatically. Evidently, for devices with limited MPO like the ‘BCDs’ and MEIs, 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 ‘BCD’s leads to binaural hearing, although not optimal, with somewhat better result in acquired cases than congenital cases. Device compliance is high.

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 a ‘BCD’ device fitted at the side of the worse cochlea. No data are available on this issue. When using MEIs, such a problem might 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/or 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), furthermore the bilateral devices are not synchronized. All these factors might 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

5.1. 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 AB-gap gap (difference between air- and bone-conduction thresholds) from 40 to 65 dB. Amongst otologists, reconstructive surgery of congenital anomalies is considered as very challenging because of the deviating anatomy, and risk of complications. In audiometric terms, the profit is often limited and not stable over time. Theoretically, surgery is successful if the AB-gap is closed, but mostly, post intervention, an obvious AB-gap remains. To define a successful outcome after atresia repair, a rather conservative criterion is used: if the remaining AB gap is 30 dB or less, the outcome is considered as sufficient (Nadaraja et al., 2013). In their systematic review (35 studies were included), Nadaraja et al. showed that this criterion is met in approx. 50 % of the operated patients, as evaluated 12 months after surgery. Already in 1999, based on a review of the literature and expert opinions, Declau et al. (1999) concluded that atresia repair was only safe in experienced hands and they suggested that alternative options like ‘BCDs’ should be considered more often. That conclusion has been underlined by Evans & Kazahaya (2007). They reported that the majority (93%) of their operated patients with an atresia of the ear canal 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 surgical atresia repair. They concluded that the results with Baha were significantly better. More recently, a systematic review by Shannon et al. (2023) came to the same conclusion.

Apart from atresia of the ear canal, minor congenital (middle ear) anomalies might occur; e.g. isolated congenital stapes fixation or a mobile stapes with ossicular chain anomalies. Thomeer et al. (2010; 2011; 2012) published one-year post-intervention audiometric data; the mean remaining AB-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 AB-gap exceeds 30 dB HL, a percutaneous ‘BCD’ is the better option; the larger the AB-gap, the higher the profit of a percutaneous Baha/Ponto device over a BTE (for details, 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) published their results with the VSB applied in patients with atresia of the ear canal 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 incus, if the stapes is mobile, or to the round window. If there is a choice, there is a preference for a coupling to the incus 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 the incus coupling (Bush et al., 2017) (for the definition of ‘effective gain’ see Appendix 2).

Regarding the application of non-percutaneous ‘BCDs’: there is no a priori reason to expect that the application of transcutaneous versus percutaneous ‘BCD’s depends on aethiology.

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 2020, the incidence was 66 MRI scans per 1000 habitants (in the Netherlands).

Doshi et al. (2014) reviewed literature and manufacturer’s information on MRI compatibility of percutaneous and transcutaneous ‘BCDs’ and MEIs. The percutaneous (titanium) implant is MRI safe; in contrast, the MEIs 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). It should be noted that with each upgrade, MRI safety is specified, so these data might not be up to date.   

Maybe more important is the degree of distortion of the MRI scan caused by the implanted device. The percutaneous implants affect the MRI image only locally (radius of 15 mm) while the transcutaneous implants, comprising powerful coupling magnets, distort the MRI scans more significantly, with a radius of 10 to 15 cm (Doshi et al., 2014). In contrast to damage to the implant by a MRI machine, the distortion of the MRI scan might not change when the devices are upgraded. So, the best option regarding distortion of the MRI image is a percutaneous ‘BCD’ and hearing implants (MEIs) with a piezo-electric actuator instead of an electromagnetic one.

Another subject; implant stability. Systematic studies on long-term implant stability are relatively scarce but not for percutaneous ‘BCDs’. Reported disadvantages of percutaneous implantation are implant loss and skin irritation around the percutaneous 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, upgraded 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), Sprinzl et al. (2021). 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, in contrast to the application of ‘BCDs’ and, probably MEIs. So far, only the stability of percutaneous ‘BCDs’ is well documented. The presented number of revision surgery related to the follow-up period is acceptable at a level that is an obvious challenge for the other implant systems. Concerning MRI compatibility, percutaneous ‘BCDs’ are the best option, and MEI if provided with a piezo-electric actuator.

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 which 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. However, more recently, an interesting new approach was launched; the use of commercially available bone-conduction headset with a remote microphone (Holland Brown et al., 2019; 2021, see also the Hear Glue Ear App). The authors state that this is an affordable and effective solution for children with OME waiting for treatment or recovery and for children with a fluctuating hearing loss.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 supuration might lead to a serious complication: cholesteatoma (benign collection of keratinized epithelium in the middle ear that can destroy vital structures).

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 supprative 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), which led to a significant reduction  in outpatient visits (Hol et al., 2004). This is an important advantage in terms of cost-benefit as well as in patient’s convenience.

Another amplification option is a MEI, like the VSB device, with the actuator coupled to one of the cochlear windows. This requires an infection-free middle ear. MEIs have 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. 

With the Sophono device (transcutaneous ‘BCD’) applied in 10 patients with CSOM, a mean ‘effective gain’ of -13 dB was found (mean PTAbc was 29 dB HL, PTAbc was 16 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. MEIs 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 presbycusis-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 mobility of the ossicular chain (referred to as ‘stapedotomy’). This is considered as a safe and effective intervention that reduces the AB-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 its actuator 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 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 three studies in which stapedotomy was not performed or unsuccessful (Burell et al., 1996; Lenarz et al., 2013; Busch et al., 2013).

5 Slide1

Figure 5.1. The mean device-aided thresholds presented versus the mean SNHLc of study groups of patients with otosclerosis using: BTEs (red symbols, 3 studies, n=27), Baha (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 in patients with a stapes prosthesis (green symbol, 2 studies, summed n=7) and, finally, Codacs device (black round, 3 studies, n=44). Standard deviations (not indicated) varied between 4 dB and 6 dB in either direction. Target line for aided thresholds is presented as introduced in section 4.3, chapter 4

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 less effective. This suggests that FMT applied in the classical way is a better option, which is in accordance with the experimental findings of Hüttenbrink et al. (2008).

For patients with advanced otosclerosis there is still a debate on the best option; 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 cochlear implantation, should still be considered.. Stapedotomy with a BTE resulted in several of the CI (cochlear implant) candidates in adequate speech perception. Another recent observation was that patients suffering from advanced otosclerosis, fitted with the Codacs device had speech in noise scores much better than those of CI users, matched with respect to their SNHLc  (Klundt et al., 2015). This suggests that acoustic stimulation might outperform electric stimulation in patients with a severe SNHLc in advanced otosclerosis.

The other issue when considering cochlear implantation is that speech recognition after cochlear implantation, might be below the expected ‘normal’ values, caused by the otosclerotic cochlea. Bone formation within the cochlea might lead to problems with insertion of the electrode array of the CI and broad current spread might occur owing to poor bone quality of the cochlear shell. This might lead to unwanted stimulation of the nerves facialis when activating the CI (see Chapter 9, section 9.2.3 for some more details).

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

MRI compatibility of the studied devices has been described in paragraph 5.1.2.

Stability. Stability figures have been presented in paragraph 5.1.2, 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 (after stapedotomy), it is expected that stability factors will not be worse than those 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 MEI 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. Basic considerations; A new device-fitting model and device choice for elderly

4.1 Introduction

An attempt is made to develop a fitting model based on a ‘acceptable partial’ use of the
patient’s 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 patient’s ‘dynamic range of
hearing’ should be audible with a device; ensuring 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 hearing device makes use of wide-dynamic
range compression as a kind of automatic volume control.

This criterion is referred to as the ‘DR>35 dB rule’. A second criterion is now introduced,
namely that at least 2/3 of the patient’s ‘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 ‘dynamic range of hearing’ with the device equals
about 2/3 of the unaided range, 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 taken from Table 2.1. The second and third columns of
this table indicate up to what mean SNHLc value the different devices fulfill the ‘DR&gt;35
dB rule’ or ‘the DR 2/3’. These values deviate from those advocated by the
manufacturers; typically by 10 to 20 dB.

figure 4.1

Figure 4.1. The minimal desired mean MPO versus mean SNHLc. Red line presents the
minimal desired MPO if the device-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 pure conductive hearing loss: indeed Sylvester et al. (2013) studying a group of
Sophono users came to a similar conclusion. 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 &gt; 35 dB rule), the application range is broader, see
third column. As said before, a ‘working range’ of 35 dB might be just sufficient for proper
amplification of speech, in combination with wide-dynamic range compression (Rheinfeldt
et al., 2015). ‘Wide-dynamic range compression’ increases the amplification of low-level
sounds while keeping the high-level sounds at loud but comfortable levels. ’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. When applying ‘BCDs’ or MEIs, ‘wide-dynamic range
compression’ is also used, although the limited ‘dynamic range of hearing’ is not caused
by physiological conditions but technical restrictions of the used 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.

Importantly; 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’. This agreement
(between Figure 3.3.c and Table 4.1 data) validates the MPO-based ‘max SNHLc-
values’ for application, as presented in the Table 4.1.

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 amplification 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 mean SNHLc, the active
‘Bonebridge’ in patients with mean SNHLc below 25 dB HL and the
percutaneous ‘BCDs’ and VSB in patients with mean SNHLc up to 50
dB HL. These values are more conservative (less optimistic) than those claimed by the manufacturers and might change when more powerful
processors are released.

4.2. Implanting elderly patients; the best solutions

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 see Table
4.1). 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. It should be noted 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,
which is approx. 20 dB (mean value 0.5 to 4 kHz; see ee Table A9.1, Chapter 9, and
related text). Taken this 20 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’ and the VSB seem to be the better choice.
When choosing for a particular treatment, effectiveness in the long run 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 might change.

Effectiveness in the long run is an often ignored but important factor
when counselling specifically elderly patients (if implant surgery is
involved).

4.3 Attempt to formulate a prescription procedure

So far, the evaluation of the capacity of various amplification options aimed at the
MPO, as a low MPO restricts the ‘aided dynamic range of hearing’ of the patient. As said
before, to deal with that limitation, compression is often used. Another technical limitation might be device noise as patients with predominant conductive hearing loss might hear it,
owing to normal cochlear sensitivity. In that case ‘expansion’ (reduced amplification for
soft sounds) can be used to make the noise inaudible (Dillon, 2012). Information on the
noise level of a device should be clearly indicated on datasheets, as is available for
percutaneous ‘BCDs’. Problems with the noise floor have been reported with some VSB
processors (e.g. Linder et al., 2009), which, according to the manufacturer, have been
solved in newer sound processors. For the Cochlear Codacs device, the noise floor is
approximately 40 dB HL (Cochlear’s data sheets), which can be lowered to 25 dB HL.
This should explicitly be taken into account when counselling this device for a given
patient.

To choose the best device for a given patient is of utmost important. Note that when
considering the application of a MEI, 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 both a ‘BCD’ and a MEI, with its
actuator coupled to one of the cochlear windows, directly stimulate the cochlea. This
simply implies that we can use our knowledge on fitting conventional hearing devices
(e.g., BTEs) in pure sensorineural hearing loss; thus we can make use of the well-known
validated prescription rules, like the NAL and DSL rules (Dillon, 2012).

To develop a practice-based prescription procedure, we used published data. The
selected studies, used in Chapter 3 were used again. For each of the included studies,
the ‘amplification at cochlear level’ was calculated as a function of frequency. That
amplification, referred to as ‘effective gain’, is, by definition, the frequency specific bone-
conduction threshold (cochlear threshold) minus the device-aided threshold (for more
information, see Appendix 2.3). To obtain a relative amplification 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 NAL-RP, that ratio should be approx. 0.45
(with some minor corrections; Dillon, 2012).

Twenty papers were included, which provided all the data needed for the calculations.
Figure 4.4 presents the ‘gain ratios’, presented per device type and the degree of
cochlear loss (mean SHNLc). To deal with the discontinuities that we found when
evaluating Figures 3.2 and 3.3, 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, 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
Hypothetical target for gain Predominant conductive 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 0.45, the value taken from the NAL-RP rule (valid at 1, 2, 4 kHz; Dillon, 2012). 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 hearing 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 (SNHLc) should be ’compensated’, equally as prescribed for sensorineural hearing loss. For group 1 with predominant conductive hearing loss, thus without a SNHLc (close to) 0 ,  the amplification (and thus the ‘gain ratio’) should be (close to) 0. However, negative ‘gain ratios’ are seen (Figure 4.4.A), as we also saw in Figures 3.2 and 3.3. Related to this, Dillon argued that  in patients with a conductive hearing loss, not the whole AB gap should be compensated, but only partially (Dillon, 2012, Chapter 10.4). Following that reasoning, it is suggested that, based on our data (Figs 3.2 and 3.3), the device-aided thresholds might be 25 dB HL instead of the ideal value of (closer to) 0, for patients with a (sub) normal cochlear function. More details are found in Snik et al. (2019).

Table 4.3 presents the desired device-aided thresholds and ‘effective gain’, based on the data presented in Figure 4.4.

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, amplification at 0.5 kHz might be set 8 dB lower than that at the higher frequencies (to deal with upward spread of masking; Dillon, 2012). 

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. 

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, as listed in 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’ explicitly into account.