6. Bilateral application should always be considered
Bilateral hearing refers to hearing with two ears. When listening with two ears instead of one, at least four advantages can be distinguished: 1) loudness summation, 2) use of acoustic head shadow to hear better in noisy places, 3) directional hearing and 4) binaural squelch. In normal hearing listeners binaural hearing is obvious, based on accurate processing of the bilateral inputs, leading to a ‘fused’ percept (binaural hearing). That is not necessarily the case for patients using hearing devices; firstly, a short introduction of these four advantages.
- Loudness summation refers to improved hearing owing to summation of sound as heard by the two ears: the input perceived by either cochlea is summed, leading to an increase of perceived loudness of approx. 5 dB.
- Improvement in speech recognition in noise owing to effective use of head shadow. Let us assume that the speech is coming from the front and noise from the left. The speech is heard equally well by either ear, but the noise is not. At the right ear, the noise is perceived attenuated (by acoustic head shadow) compared to the left ear. Therefore, the right ear will have the better speech to noise ratio. Selective listening with the right ear, thus ignoring the left ear, leads to better speech perception.
- Improved localization of sounds, in the horizontal plane. To identify where sounds come from, the two ears have to work together to detect interaural difference cues caused by the different positions of the two ears with respect to the sound source. These interaural differences comprise interaural loudness differences (ILD; owing to acoustic head shadow) and interaural time differences (ITD, difference in arrival times of the sound at the two ears). Head shadow attenuates primarily high frequency sounds ( > 3000 Hz). As a consequence, high-frequency sounds are perceived louder by the ear nearest to the sound source, creating an ILD. Below 1500 Hz, head shadow causes little attenuation. Instead, ITDs (or, related, interaural differences in phase) are the relevant cue to localize a sound source. Sounds arrive earlier at the ear nearest to the sound source, creating the ITD. Owing to the limited distance between the two ears, the ITD varies between 0 ms (sound presented in the front of the listener) to a maximum of 0.7 ms (e.g. sound presented e.g. at the very left, perceived by the two ears).
- Binaural squelch refers to central de-masking. Assume a subject is listening to speech coming from the front and noise coming from the left. The speech in the two ears will be nicely in phase (ITD=0). However, the noise is perceived differently by the two ears, not only because of the head shadow, but also because of ITD, caused by the different travelling times of the noise to the left and right ear. The difference in ITDs caused by the speech and by the noise can be used to perceptually separate the speech and noise signals, referred to as binaural squelch.
Bilateral application of BTEs in patients with bilateral sensorineural hearing loss mostly leads to binaural hearing. However, the outcome might be worse than in normal hearing subjects who are listening with two ears (concerning directional hearing, see e.g. Bogaert et al., 2006). Nevertheless, e.g. Boymans et al. (2008) showed an obvious benefit of BTEs in a large group of patients with bilateral sensorineural hearing loss, with regard to directional hearing and the effective use of head shadow.
6.2 Binaural hearing with bone conduction devices
When using the air-conduction route, the two ears can be stimulated independently because they are acoustically well isolated from each other. That is not the case for bone-conduction stimulation. The skull transmits the bone-conducted vibrations quite effectively, with little damping. Therefore, with one BCD, not only the ipsilateral cochlea is stimulated but also the contralateral cochlea. This is referred to as cross hearing. Attenuation of vibrations from one cochlea varies widely (range more than 20 dB), with a median of just 3 to 5 dB; variation between subjects is significant as well as within subjects, between frequencies (Stenfelt, 2012). In other words, with bone-conduction stimulation, the cochleae are poorly acoustically isolated, however, just sufficiently to enable to detection of interaural differences to some extend (Stenfelt, 2005).
Table 6.1 presents data on binaural effects when changing from unilateral to bilateral listening in patients using bone-conduction devices. A summary of published Nijmegen papers is presented on Baha application (bilateral Baha in bilateral conductive/mixed hearing loss and unilateral Baha application in unilateral conductive hearing loss with a second normal hearing ear). Data are added as obtained in a group of normally hearing controls, listening with two versus one ear; with, in the latter case, one ear plugged and muffed (Agterberg et al., 2011). All the patients were evaluated with one and the same protocol, and they were using linear (analogue) Baha bone-conduction devices. Binaural summation and speech in noise were assessed using speech.. Directional hearing was tested with short (1s) narrow-band noise bursts (Agterberg et al., 2011).
Table 6.1, first row, presents the mean result (and standard deviations) of the ‘binaural’ factors as measured in normal hearing controls. The binaural summation score is the difference between speech recognition scores obtained with one ear versus two ears. Head shadow and binaural squelch have been measured in one experiment. The combined score is included in the Table (column 3). Firstly, speech recognition in noise was measured while the subject was listening with one ear (in unilateral cases with their normal ear, in bilateral cases with their left ear only, thus Baha of the right ear turned off). Speech was presented in front of the subject and the noise at the side of the only hearing ear. Secondly, the Baha at the patient’s other ear was activated (in the normal hearing subjects, one ear had been blocked for the first measurement, which is now unblocked). The presented score is the improvement between these two situations. The asterisks indicate values that differ significantly from 0. The next two columns show directional hearing results for low frequency sounds (relying on detection of ITDs) and for high frequencies sounds (relying on the detection of ILDs), respectively. The mean absolute error is shown (0 means perfect localisation, chance level is 800). Concerning the directional hearing results: statistically significance was assessed by comparing the scores obtained in the unilateral listening situation (not shown) and bilateral listening situation. Thus the first row of the table presents norm values for our set-up.
Let us consider bilateral Baha application in bilateral hearing loss. According to the table, row 2, bilateral Baha application does lead to the expected improvement in binaural summation (column 2) of, approximately, 4 dB. This result is in agreement with the literature (Janssen et al., 2012, review). Although not measured, binaural hearing in control subjects is around 5 dB. Obviously, the inevitable cross stimulation of bone-conduction stimulation plays a minor role when considering binaural summation.
|N||Binaural summation (dB)||Head shadow & squelch (dB)||Directional hearing, 0.5kHz (degrees)||Directional hearing, 3 kHz (degrees)|
|Controls||10||n.a.||4.6 ± 1.6*||7 ± 7^||8 ± 10^|
|Bilat CHL, bilat Baha, acquired||17||4.0 ± 2.1*||2.5 ± 1.8*||26 ± 8^||25 ± 8^|
|Unilat CHL, acquired||13||2.2 ± 1.5*||3.0 ± 1.7*||16 ± 10^||20 ± 12^|
|Unilat CHL, congenital||10||1.3 ± 2.4||1.1 ± 1.9||30 ± 13||31 ± 18|
Note. N.a.: not available, CHL: conductive hearing loss.
* significantly different from 0 (p<0.05); ^significantly improved compared to the unilateral listening condition (p<0.05)
Considering row 3, Baha application in acquired unilateral conductive hearing loss, a small but significant binaural summation score was found, however, not for the unilateral congenital cases using Baha (row 4). Their score was not significantly different from zero. Column 3 shows the binaural squelch/head shadow data. The bilateral conductive hearing loss patients (row 2) profit significantly as a group, however, profit is approx. half that of the controls (2.5 vs. 4.6 dB). For unilateral acquired conductive hearing loss, the outcome is approx. the same (3.0 dB), while for the congenital cases, no significant benefit was found (rows 3 and 4). Thus, on the average, head shadow doesn’t help the patients with congenital onset to better understand speech in noisy conditions, according to this study.
Columns 4 and 5 comprise the outcome of the directional hearing tests; the mean absolute difference in degrees between the perceived location and the real location of the activated loudspeaker are presented. For normal hearing controls, an error of 7-80 was found, for the bilateral Baha users approximately 250 and for the patients with unilateral acquired hearing loss using Baha, approximately 200. These three groups showed significantly improved scores when changing from unilateral (mean unaided scores 46-660) to bilateral hearing. Probably, disturbing cross hearing is the reason for the difference in score between the controls and bilateral Baha-users. The unilateral congenital cases showed the worst scores, see next paragraph.
6.3 Unilateral congenital conductive hearing loss; elaborated outcomes
The localisation scores of the patients with congenital unilateral conductive hearing loss with their Baha active were largely unchanged, thus close to their unaided scores (340 and 390 for 0.5 and 3 kHz, respectively; thus mean improvements of just 40 and 80). The most likely explanation is that these patients (without the Baha), being unilaterally hearing since birth, use monaural loudness cues and spectral cues much more effectively than the other patients and normal hearing controls (see also Agterberg et al., 2012; Vogt et al. 2020). Vogt et al. even showed that with the percutaneous BCD turned on, these patients remained relying on monaural cues for sound localisation, despite the second input (via the BCD).
In an attempt to understand the rather poor but variable results of the patients with congenital onset of their hearing loss, we decided to study whether or not age at intervention played a role. The hypothesis was that maybe a sensitive period exists; the earlier implantation, the better the binaural results would be. Figure 6.1 shows the individual summed outcome of the three binaural tests (the head shadow test and the two directional hearing scores) after expressing the outcomes into z-scores, using the standard deviations of the normal hearing group. Results were available from 20 patients with unilateral congenital conductive hearing loss using a Baha for at least 3 months (data taken from Kunst et al., 2008). Figure 6.1 shows the summed z-scores of these 20 patients as a function of age at implantation. For reference purposes the same procedure was applied to the data of the patients with acquired unilateral conductive hearing loss (Table 6.1, row 3); a mean summed z-score of just over 10 was found in the latter group. The Figure shows that a value of 10 is achieved by just one of the patients with congenital unilateral hearing loss. Obviously, also negative summed z-scores were found, indicating a worse result with Baha than unaided. A significant effect of age at intervention is not seen, while the spread in results was large.
Figure 6.1. Binaural advantage based on the use of head shadow and directional hearing. Summed z-scores are presented as a function of age at intervention. Baha users and non-users are indicated by different symbols (data from Nelissen et al., 2015).
Fitting of a (percutaneous) BCD in patients with unilateral congenital conductive hearing loss to enable binaural hearing might result in a poor outcome; age at implantation doesn’t seem to play a role (in the age-range studied, from 4 years into adulthood).
Furthermore, Nelissen et al. studied non-use in this group of 20 patients using Baha, with a follow-up of more than 5 years. Non-use turned out to be rather common, as indicated in Figure 6.1; 65% non-users. An association was found between ‘long-term’ non-use and the ‘short-term’ (one-year post-intervention) summed binaural advantage score. Non-use was not related to age at intervention. It seems that several of these patients were not able to integrate the new input with that of the normal hearing ear, what is needed for a binaural percept. Thus, not surprisingly, the mean reason to stop as indicated by the patients was complaints about interfering sounds/noise when using the Baha device in situations with background sounds. As patients with unilateral acquired conductive hearing loss do benefit from Baha application (summed z-score of 10 on the average), they seem to profit from their previously developed binaural abilities.
Recently a review of the literature was published (Vogt et al. 2021) focussing on post intervention binaural hearing abilities in children with congenital aural atresia. Interventions comprised not only a percutaneous BCD but also all other types of implantable BCDs, active middle ear implants like the VSB as well as atresia repair. Surprisingly, all outcomes were consistently disappointing, irrespective of the type of intervention (concerned binaural summation, use of head shadow and binaural squelch and directional hearing). Vogt et al. discussed factors that might have played a role like age at treatment, whether the treated ear was a ‘lazy’, not fully maturated, ear as well as the role of the remaining asymmetry in hearing thresholds, as was seen generally. It was argued that the major problem might be the remaining asymmetry in hearing, which typically varied between 20 and 35 dB. The asymmetry in hearing might affect negatively the development of binaural hearing and consequently leads to complaints about competing sounds. Children often reduce the gain of the device to deal with that. Low volume means more asymmetry in hearing; a chicken and egg problem. Special training to teach the child to use the bilateral inputs effectively seems to be indicated.
Concerning the binaural advantage of an active middle ear implant in unilateral conductive hearing loss, it should be noted that in contrast to a BCD, cross stimulation with an active middle ear implant device is absent. Two publications were found in which outcomes obtained with the VSB middle ear implant were compared to that with BCDs in patients with congenital unilateral conductive hearing loss (Agterberg et al., 2014; Vogt et al. 2018). Agterberg et al. compared results of patients fitted either with a Baha (n=4) or with a VSB (n=4). All were experienced device users. An anticipated better result with the cross-stimulation-free VSB device in a directional hearing experiment could not be established. Same conclusion was drawn from a comparison between the VSB and an active transcutaneous BCD, the Bonebridge device (Vogt et al., 2018). This suggests that cross hearing might not be a dominant factor, further research is warranted.
Although unilateral congenital hearing loss in children might lead to developmental delays (in speech language development and academic performance), outcomes of today’s treatment options are grossly disappointing (Vogt et al., 2021). Nevertheless, some children experience improved hearing and are happy with that. No conclusive evidence exists regarding the role of age at implantation, degree of asymmetry in hearing after the treatment nor what intervention is most effective.
6.4. Bilateral application of BCDs and AMEI
Bilateral application in bilateral conductive hearing loss works, see Table 6.1, which is in agreement with Janssen et al. (2012) and Zeitooni et al, 2016. The latter study was carried out in normal hearing controls; binaural advantages were compared as obtained with stimuli/speech presented bilaterally over headphones (air-conduction route) and, separately, bilaterally by bone-conduction transducers (bone conduction route). The authors reported profit in both conditions, however the binaural advantage (using speech in noise) was approx. two times better with headphone presentation. This factor of 2 is in fair agreement with the difference as presented in column 3, Table 6.1, when comparing the outcomes of the normal hearing controls and the bilateral Baha users.
A further question is whether or not (adult) patients with bilateral congenital conductive or mixed hearing loss do profit from fitting bilateral BCDs. Bosman et al. (2001) reported that the ‘binaural advantage’ as found in their patients with acquired bilateral conductive hearing loss (summarized in row 2, Table 6.1) and those of 6 congenital cases were grossly the same. This might suggests that early binaural auditory experience is not a prerequisite for effective use of bilateral Bahas in bilateral conductive hearing loss. More recently, Den Besten et al. (2020) published data on a group of 7 children with bilateral congenital conductive hearing loss, all experienced users of bilateral percutaneous BCDs. They found that all children showed improved sound localisation when using two instead of one BCD, however, localisation scores were poor in most of the children. When (just) considering lateralization, a more obvious advantage was seen. The mean absolute error was 370 ± 170, while with unilateral Baha the error was 640 ± 120 (reference value normal hearing children: 100). One additionally child was tested with bilaterally acquired conductive hearing loss. In this case, sound localisation was close to the score of the controls, suggesting that children with previous binaural experience might profit more from bilateral BCDs than congenital cases. However, this is a preliminary conclusion.
Den Besten et al. also presented data on none-use. Almost 85% of their total group of bilaterally implanted children (n=33) used both percutaneous BCD devices all the time.
Although transcutaneous BCDs have been applied bilaterally, no systematic prospective evaluative study has been found in literature (autumn 2020). As far as we know, studies are also missing on binaural hearing after bilateral application of middle ear implants, like the VSB, in patients with conductive or mixed hearing loss. Wolf-Magele et al. (2016) studied 10 bilateral VSB users, however 6 of them with sensorineural hearing loss, 4 with mixed loss and Koci et al (2016) reported directional hearing measurements in 10 bilateral VSB users; 8 of them had sensorineural hearing loss. Reported results of these mixed groups of VSB users are favourable, but the outcomes are most probably not representative for patients with conductive or mixed hearing loss. So, specific studies are needed.
Note. A special remark has to be made concerning binaural summation in patients with bilateral conductive hearing loss using (two) implants with limited maximum output (MPO; thus all BCDs and MEIs on the market autumn 2020; see chapter 2). Generally, improved hearing has been observed owing to binaural summation, in the order of 4 dB. This is of major importance. During the initial fitting of two BTEs or two CIs in sensorineural hearing loss/deafness, when switching on both ear-individually-fitted devices for the first time, overstimulation might occur owing to binaural summation. Generally, to deal with that, the volume and maximum output of the individual (thus per ear) fitted devices is lowered, and, quite often, the software does this automatically. Evidently, for devices with limited MPO, such a lowering of gain and output is contra-productive and should be avoided.
In bilateral conductive or mixed hearing loss, the application of bilateral BCDs leads to binaural hearing, although not optimal, with somewhat better result in acquired cases than congenital cases. Device compliance is high.
At last, so far, the patients in the bilateral BCD studies had rather symmetric bone-conduction thresholds. It is not clear what might happen in case of asymmetric bone-conduction thresholds. The theoretical problem is that the better ear might be hindered or even over-stimulated by the a BCD device fitted at the side of the worse cochlea. No data are available on this issue. When using active middle ear implants, such a problem will not occur.
6.5. Binaural hearing and adaptive sound processing
Today’s BCDs and VSBs make use of, amongst others, adaptive sound processing like expansion- and/or compression amplification, noise reduction, feedback reduction and adaptive directional microphones. Interaural cues (ILDs and ITDs) might be distorted by independently working digital sound processors. Sound processing times are relatively long (3 to 9? ms) compared to the maximum possible ITD (0.7 ms) and the bilateral devices are not synchronized. All these factors deteriorate interaural cues, resulting potentially in poor horizontal localization (Bogaert et al., 2006; Beck & Sockalingam, 2010). In principle, these advanced sound processing options are not really necessary when fitting patients with good cochlear function (conductive or mixed hearing loss with mild to moderate SNHLc), because the auditory system can still sort out the important information. On the other hand, some of these processing options might be necessary, to deal with limitation of the devices such as feedback, the unnatural position of the microphone, audible microphone noise and the limited MPO.
It is not evident that patients with conductive hearing loss or mixed hearing loss with a minor to moderate sensorineural hearing loss component do profit from advanced adaptive sound processing. Side effects like deteriorated spatial hearing might occur. Conclusive research is lacking.
New references concerning the update January 2021
den Besten CA, Vogt K, Bosman AJ, Snik AFM, Hol MKS, Agterberg MJH. The Merits of Bilateral Application of Bone-Conduction Devices in Children With Bilateral Conductive Hearing Loss. Ear Hear. 2020;41(5):1327-1332.
Vogt K, Wasmann JW, Van Opstal AJ, Snik AFM, Agterberg MJH. Contribution of spectral pinna cues for sound localization in children with congenital unilateral conductive hearing loss after hearing rehabilitation. Hear Res. 2020;385:107847.
Vogt K, Frenzel H, Ausili SA, Hollfelder D, Wollenberg B, Snik AFM, Agterberg MJH. Improved directional hearing of children with congenital unilateral conductive hearing loss implanted with an active bone-conduction implant or an active middle ear implant. Hear Res. 2018;370:238-247.
Vogt K, Desmet J, Janssen A, Agterberg M, Snik A. Unexplained variance in benefit of treatment of congenital unilateral aural atresia. Audiol Neurotol 2021; in press
Zeitooni M, Mäki-Torkko E, Stenfelt S. Binaural Hearing Ability With Bilateral Bone Conduction Stimulation in Subjects With Normal Hearing: Implications for Bone Conduction Hearing Aids. Ear Hear. 2016;37(6):690-702.