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.
- 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.
- 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).
- 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 250 and 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.
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.