Ching, Teresa Y.C.
Providing binaural hearing is a vital component of aural rehabilitation, and bilateral auditory input is essential to binaural hearing. While binaural fittings have traditionally applied to hearing aids, monaural stimulation has commonly pertained to cochlear implants. People who receive a cochlear implant in one ear can, nonetheless, obtain bilateral auditory input by wearing a hearing aid in the non-implanted ear (binaural-bimodal hearing). The demand for such binaural-bimodal fittings is increasing, as improvements in cochlear implant performance have led to the extension of implantation from profoundly deaf people to those with considerable residual hearing in both ears.
In this article, I will draw on recent research evidence to address the question of who should use binaural-bimodal hearing. The article will examine binaural advantages in relation to the degree of a patient's residual hearing. It will also analyze data from three National Acoustic Laboratories (NAL) studies to determine the relationship between hearing thresholds in the non-implanted ear and degree of benefit. Next, a clinical method for checking binaural interference will be proposed. Finally, I will discuss how to provide optimal bimodal fittings.
Normal-hearing people use auditory inputs from both ears to localize sounds and to understand speech better in adverse listening conditions. The advantages of binaural hearing are well known and detailed discussions, including references, are presented elsewhere.1–3 Briefly, the effects of binaural hearing may be attributed to head-shadow, “binaural redundancy” and “binaural squelch.”
Because of the size of the head, the level and time/phase of sounds reaching the two ears differ, depending on the location of the sound source relative to the ears. When sounds are audible in both ears, the brain can use interaural level and time differences to determine where sounds come from. When one ear is closer to the primary talker or farther from the dominant noise, the listener can selectively attend to the ear with a better signal-to-noise ratio. This head-shadow effect provides on average a 3-dB advantage.
Even when speech and noise come from the same direction, being able to listen twice to the same sounds (binaural redundancy) gives a binaural advantage of about 1 to 2 dB. The brain can also combine the speech and noise from both ears to reduce the impact of noise on understanding speech by about 2 dB on average. This is referred to as “binaural squelch.”
Hearing loss limits a person's ability to understand speech, especially in noisy situations. This problem may be alleviated for a cochlear implant user who also wears a hearing aid in the non-implanted ear. Although there is evidence to suggest the possibility of a loudness mismatch between the outputs of the two devices,4 numerous studies using various methodologies and test materials have shown binaural benefits with binaural-bimodal hearing (see Ching et al.,5 for a detailed review).
Figure 1 summarizes data from eight studies on adults6–13 and six on children.14–19 Binaural speech benefits are expressed as the difference between the speech scores obtained when listeners wore a cochlear implant and a hearing aid (CIHA) and the scores obtained when they wore a cochlear implant alone (CI)
These data were obtained by presenting speech test material and noise in the sound field. Each data point represents the difference scores averaged across all subjects in one study in relation to the three-frequency average thresholds of the subjects.
Binaural word and sentence recognition in quiet was similar to monaural performance in three studies, as shown by the data points on the zero line. All of the other studies revealed superior binaural speech recognition over monaural performance, especially when speech and noise were spatially separated. The benefits can be explained in terms of the combined effects of head-shadow and binaural redundancy. Since head-shadow is a physical phenomenon, it will occur whenever sounds are audible in both ears and each ear can extract information from the signal it receives, whether through electrical or acoustic stimulation of the auditory system.
A recent study measured the effects of binaural redundancy while controlling for head-shadow by presenting signals through the direct audio input sockets of the respective bimodal hearing devices.20 It found that adults with acquired hearing loss understood speech better in noise with bimodal listening than with a cochlear implant alone, whereas the same was not observed in children with congenital hearing loss. The same study found that users of bimodal hearing devices were unable to benefit from binaural squelch when it was measured by presenting speech and noise with and without an interaural delay via direct audio input to the respective devices. Because current cochlear implants do not adequately preserve fine-timing information, the listener cannot make use of inter-aural time difference cues for speech perception.
Two NAL studies that investigated consonant perception of children15 and adults21 revealed that binaural-bimodal fittings provided complementary information for speech perception in noise. Significantly more information relating to voiced-voiceless distinctions and manner of articulation were received with bimodal listening than with a cochlear implant alone, both for children (p <0.002) and for adults (p <0.003). Figure 2 shows relative information transmission averaged for 11 children (left panel) and 21 adults (right panel) when listening with CI compared to CIHA. The vertical bars denote 0.95 confidence intervals.
Information about voicing and manner were not effectively received through electrical stimulation, possibly because current cochlear implants and speech processing systems do not provide adequate information about spectral properties of complex sounds.22
A simple way of complementing information from electrical stimulation in one ear is via acoustic amplification in the contralateral ear. The enhanced reception of voicing and manner information with bimodal hearing is a noteworthy advantage, especially when one considers that congenitally deaf children rely on the auditory input for development of speech and language skills.
Hearing loss also impacts on a person's ability to localize sounds. Hearing-impaired listeners localize sounds better on the horizontal plane when they receive bilateral auditory inputs than when they hear sounds in one ear only (see Byrne & Noble23 for an overview). In a similar vein, people who use a cochlear implant and a hearing aid in contralateral ears locate sounds more accurately than when they use a cochlear implant alone.9,10,15,16
Is binaural-bimodal fitting advantageous in everyday communicative situations? Several studies addressed this question by using a range of questionnaires. Most persons surveyed have reported that bimodal hearing was advantageous. Subjective benefits included greater naturalness of sounds and better quality of people's voices5,9,10,15,16 than are provided by a cochlear implant alone. Parents also commented on their children's increased awareness of environmental sounds when they wore bimodal hearing devices.
Other potential advantages of wearing two devices include access to a second device when one fails. Someone who wears two devices can also use two directional microphones and may benefit from advances in noise-reduction technology that use bilateral processing. In some cases, wearing a hearing aid helps mask tinnitus in the non-implanted ear. Furthermore, fluctuations in CI performance have been observed in some CI users,24 but auditory functional performance may be less affected if a hearing aid is used with the cochlear implant.13
Last but not the least in importance is avoiding auditory deprivation25,26 in the non-implanted ear by providing auditory stimulation via a hearing aid. Access to auditory input after the onset of hearing loss has been identified as important for achieving a higher level of performance after implantation.27
Possible disadvantages of binaural-bimodal fitting relate to costs and to binaural interference. Two devices cost more than one. Adjusting a cochlear implant and a hearing aid requires more time and skill than adjusting a cochlear implant alone, and maintenance is also more costly. In children, management may also require more counseling and support from educational facilities.28
Binaural interference has been reported in a small number of patients fitted with bilateral hearing aids.29 It may result from either cochlear distortions30 or abnormal transfer between the left and right hemispheres of the brain.31
These same considerations may be expected to apply to binaural-bimodal fittings. If cochlear distortion can cause binaural interference (as evidenced in a study that simulated cochlear distortion and presented the sounds to listeners with normal hearing), then it seems entirely possible that a cochlear implant, which undoubtedly distorts the sounds in some way, and transduction of acoustic vibrations in the opposite cochlea, which undoubtedly distorts the sounds in some other way, could cause binaural interference. However, the fact that so many people find a bimodal fitting useful is convincing evidence that the benefits outweigh any effects of differential distortion.
When binaural interference is suspected, the clinician may check for it by comparing the patient's speech perception while wearing a cochlear implant alone with speech perception when he/she wears an implant with a hearing aid. The clinician should present speech and noise from the same loudspeaker placed at 0° azimuth of the patient to minimize the positive effects from head diffraction and squelch so that these do not obscure any potential negative effect arising from binaural interference.3
If binaural interference is confirmed and bimodal fitting is excluded, head-shadow advantages may be provided by using bilateral microphone input to a single cochlear implant.32
Who should GET A binaural-bimodal fitting?
This question may be examined in two parts, viz, in how many cases would bimodal listening be beneficial and which patients would benefit?
How many patients would benefit?
Obviously the answer to this question may vary among populations. Questionnaire surveys of adults who received a cochlear implant prior to 2000 at two cochlear implant centers revealed that 19% (20 out of 105 respondents12) and 10% (9 out of 87 respondents9) continued to use a hearing aid with a cochlear implant.
Two factors may have led to these low percentages. One is that many of the clients received implants when implantees commonly had very little remaining hearing in the opposite ear. The other factor is that these clients may have followed the recommendations of their clinicians to quit using a hearing aid after implantation in the contralateral ear without assessing whether it provided additional benefit.
In a recent survey of adults who have residual hearing in the non-implanted ear (based on pre-operative aided thresholds),33 75% tried wearing a hearing aid with a cochlear implant after implantation (53 out of 71 adults) and 80% (42 of 53 adults) continued to use bimodal hearing devices for 8 hours or more on an average day. All those who continued to use bimodal hearing rated the bimodal condition to be superior to a cochlear implant alone in all situations examined.
Which patients would benefit?
It would be useful if we could decide in which cases to consider bimodal fitting on the basis of such clinical information as the pure-tone audiogram and speech discrimination testing.
It appears from Figure 1 that the absolute levels of the unaided thresholds (averaged over 500, 1000, and 2000 Hz), within the range of 85 to 110 dB HL, do not have a consistent effect on the degree of benefits. Because only averaged thresholds were available from the studies, it was impossible to relate hearing thresholds at specific frequencies directly to binaural benefits.
To examine this question, I analyzed data from three NAL studies. The data on hearing thresholds and binaural speech perception in noise (with speech and noise from 0° azimuth) from two studies of children15,16 and one study of adults10 were combined. I performed forward stepwise regression analysis, using the CIHA score as dependent variable and hearing thresholds at 500 and 1000 Hz, low-frequency average thresholds (250, 500, and 1000 Hz), three-frequency average thresholds (500, 1000, and 2000 Hz), and CI scores as independent variables.
The CIHA scores were significantly related to CI scores (beta = 0.93, p <0.001) and hearing thresholds at 500 Hz (beta = −0.12, p <0.04). In Figure 3, the left panel shows that children and adults obtained higher speech perception scores when using a cochlear implant with a hearing aid than with a cochlear implant alone. The improvement was only limited by ceiling effects.
The right panel shows CIHA scores as a function of hearing threshold level at 500 Hz. Even when hearing loss was severe or profound at 500 Hz in the non-implanted ear, listeners were able to extract some speech information from an audible signal in that ear and combine it with information from electrical stimulation in the implanted ear for speech perception. This is consistent with research evidence showing that, although the amount of speech information that can be extracted from an audible signal decreases with increased hearing loss, the degradation is less severe at the lower frequencies than at the high frequencies. On average, a person with a 100-dB hearing loss at 500 Hz can extract about half the information available to a normal-hearing person from the same amount of audible signal.34
Further stepwise regression analysis was carried out with speech perception benefits (CIHA — CI scores) as a dependent variable, and hearing threshold levels at 500 and 1000-Hz, low-frequency average, and three-frequency average as independent variables. The results indicated that only the hearing threshold level at 500 Hz was significantly related to benefits (beta = −0.4, p <0.006). As shown in Figure 4, children and adults with better hearing thresholds at 500 Hz in the non-implanted ear derived greater binaural benefits in speech perception on average.
How should bimodal devices be fitted?
Simply putting a hearing aid into the non-implanted ear of anyone who has a unilateral cochlear implant cannot be expected to produce consistently good results, any more than putting two hearing aids on anyone who has two hearing-impaired ears would always be successful. It is reasonable to treat a binaural-bimodal fitting as two monaural fittings initially, and then use a systematic procedure that considers the combined result of using both devices simultaneously to fine-tune the fitting.
This approach was adopted in the development of a procedure for adjusting a hearing aid with a cochlear implant.15,35 In this procedure, stable mapping parameters are first established for the cochlear implant speech processor and hearing aid parameters are selected according to the NAL prescription.36–38 A fine-tuning procedure is then implemented so that the frequency response of the hearing aid is adjusted to maximize use of residual hearing, and the gain for low-, medium-, and high-input levels are adjusted to give the same overall loudness as in the implanted ear. This procedure has been validated with children, and evidence showing that the fine-tuning procedure led to greater binaural benefits has been published.15,35 An online course on the research basis and a practical step-by-step guide on this procedure for bimodal fitting may be accessed via Cochlear College (www.cochlearcollege.com).
The advantage of this approach is that it ensures that each device is optimally adjusted and that combined device usage gives comfortable and balanced loudness. A point to remember is that some binaural-bimodal hearing device users (like some bilateral hearing aid users) prefer to use only one device in some situations.13,28 Therefore, the monaural fitting must be optimal.
On the other hand, it is interesting to consider if it might be advantageous to develop procedures that adjust a hearing aid and a cochlear implant together. This is viable with recent developments in signal processors for use in both devices,39 but little research on combined fitting is available.
The validated procedure described above can be used with children older than about 6 years of age. Here, two case studies are presented to illustrate that the fine-tuning procedure is an integral part of (re)habilitation. The benefits in these cases are representative of the group results achieved.
In the first case, greater benefits were obtained by fine-tuning a hearing aid with a cochlear implant.15 In the second case, re-introduction of hearing aid use after some years of non-use still brought about binaural benefits.16
In cases where use of hearing aids was consistent prior to implantation but discontinued after implantation in the contralateral ear, it is necessary to allow time for familiarization. The hearing aid for the non-implanted ear should be selected using a validated prescription, and fine-tuning should not take place until consistent usage of at least 4 hours a day has been established.
CASE 1—SF, 8.5 years
SF was diagnosed with moderate to severe hearing loss in the left ear and severe-to-profound hearing loss in the right ear at 3 years of age. He was fitted with bilateral hearing aids soon after diagnosis. His hearing deteriorated over the next 2 years, and he received a Nucleus 24 system in his poorer (right) ear at 5.7 years. He continued to wear a hearing aid in his left ear. Six months after implantation, his hearing aid was fine-tuned with his cochlear implant by using the NAL procedure.
Figure 5 presents his unaided audiogram in the non-implanted ear together with evaluations of performance at 3 months after hearing aid fine-tuning. Details of the evaluations have been described elsewhere.15 Briefly, speech perception was assessed by presenting sentences in babble noise at a signal-to-noise ratio of 10 dB, localization ability was assessed by presenting pink noise pulses from an array of 11 loudspeakers spanning 180°, and functional performance was evaluated by administering a questionnaire to parents (PEACH), who were asked to observe the child over a 1-week period when the relevant device condition was used.
Evaluations were carried out when SF wore a cochlear implant with a hearing aid before fine-tuning (CIHAworn), a cochlear implant alone (CI), a hearing aid alone (HA), and a cochlear implant with a hearing aid that had been fine-tuned with her cochlear implant (CIHAopt). The parents and the child were blind as to whether the “worn” or the “optimized” condition was used during each trial period.
The speech scores for the two CIHA conditions were similar, and both were significantly better than the CI scores. There was a significant reduction in localization errors when CIHAopt was used compared with CI alone. The most important benefit was in real-life situations where SF's parents observed that he was much more effective in noisy environments than when in the CIHAworn and CI alone conditions.
CASE 2—GD, 8.5 years male
GD was diagnosed with bilateral moderate to profound sensorineural hearing loss at 1.5 years. He was fitted with hearing aids at 1.8 years with excellent compliance. At 3 years of age, a Nucleus 22 multi-channel cochlear implant was implanted in the poorer (left) ear. GD was instructed to discontinue hearing aid use in the contralateral ear.
Five years elapsed before the parents became aware of research on binaural-bimodal benefits, and arranged for GD to receive a hearing aid. The hearing aid was adjusted according to the NAL-RP prescription, and GD wore it with his cochlear implant for at least 4 hours a day. A month later, the hearing aid was fine-tuned with his cochlear implant.
Binaural effects were measured 3 months after use of bimodal hearing. Figure 6 shows GD's unaided audiogram in his non-implanted ear together with his performance with a cochlear implant alone (CI), a hearing aid alone (HA), and a cochlear implant with an optimized hearing aid (CIHA).
There was significant improvement in speech-perception scores and a significant reduction in localization errors when CIHA was used over CI alone. In everyday life, parents observed that GD was more effective in speech-communicative situations in noise and more responsive to environmental sounds. They also reported that he was more attentive to speech, his speech-production skills improved, and he was better at locating the source of sounds.
Conclusions and RECOMMENDATIONS
The evidence to date supports binaural-bimodal fitting of all recipients of unilateral cochlear implants who have residual hearing in the non-implanted ear, even when the degree of hearing loss is severe to profound. People with less hearing loss at 500 Hz in the non-implanted ear derived greater binaural speech-perception benefits than those with more severe hearing loss.
Binaural/bimodal fitting should no longer be regarded as a special technique for a few selected unilateral cochlear implant wearers, but should be considered as the management of choice for people who have residual hearing in the contralateral ear. It is recommended that binaural-bimodal fitting be provided to everybody. If there is any suspicion of binaural interference, the clinician can check by presenting speech and noise at 0° and comparing the speech scores obtained with monaural and binaural stimulation.
In cases, such as young children, where subjective feedback is limited, one not-yet sufficiently examined method of ascertaining if there is binaural interference might be to use aided cortical auditory evoked potentials.40 Because evoked cortical responses are affected by the combined stimulation of the two ears, a comparison of the responses arising from bilateral and unilateral stimulation may enable the synergistic or destructive effects of stimulation of each ear to be inferred.
Providing binaural-bimodal fittings should be standard practice, and performance with a cochlear implant and a hearing aid in opposite ears should be the appropriate baseline (rather than a single cochlear implant) against which to judge if bilateral implants offer any advantage. Similarly, binaural-bimodal hearing seems a relevant comparison condition (in addition to a single implant) to judge the benefit of a monaural-bimodal fitting.
I would like to thank Mandy Hill, Paula Incerti, and Colleen Psarros for their contributions to the NAL studies reported in this paper. The research was supported by the Cooperative Research Centre for Cochlear Implant and Hearing Aid Innovation, Australia. Thanks are also due to Harvey Dillon who provided helpful comments on an earlier version of this paper.
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© 2005 Lippincott Williams & Wilkins, Inc.