Mencher, G.T. & Davis, A. (2006). Bilateral of unilateral amplification: is there a difference? A brief tutorial. International Journal of Audiology 45 (S1), S3-11.
This editorial discusses the clinical implications of an independent research study. This editorial does not represent the opinions of the original authors.
The decision to fit binaural hearing loss with bilateral hearing aids is influenced by a number of factors. The recommendation of two hearing aids may be contraindicated for financial reasons or because of near-normal hearing or profound loss in one ear, but the consensus among clinicians is that bilateral amplification is preferable for individuals with aidable hearing loss in both ears. Mencher and Davis examine a variety of considerations that may affect bilateral benefit, including speech intelligibility in noise, localization and directionality, sound quality, tinnitus suppression, binaural integration and auditory deprivation. Research in these areas is discussed with reference to clinical indications for hearing aid fitting.
The authors begin with a clarification of the terms binaural and bilateral. They explain that a bilateral fitting refers to the use of hearing aids on both ears, whereas binaural hearing refers to the integration of signals arriving at two ears independently. They point out that standardization of these terms should help avoid confusion in the discussion of bilateral versus unilateral hearing aid fittings.
Because speech is the most important mode of everyday communication, studies of hearing aid benefit typically employ speech intelligibility measures in quiet and noisy conditions. Early studies investigating aided speech intelligibility yielded conflicting reports, with some in favor of bilateral amplification (Markle & Aber, 1958; Wright & Carhart, 1960; Olsen & Carhart, 1967; Markides, 1980) and others showing no difference between unilateral and bilateral fittings (Hedgecock & Sheets, 1958; DeCarlo & Brown, 1960; Jerger & Dirks, 1961).
Many early studies were criticized for methodological choices that could have obscured bilateral benefits such as the use of a single noise source or test materials that were not representative of everyday, conversational speech. More recent work has examined speech intelligibility under conditions that more closely approximate real-world conditions, with multiple noise sources and sentence-based test materials. For instance, Kobler & Rosenhall (2002) studied intelligibility and localization for randomly presented speech from locations surrounding the listener, in the presence of speech-weighted noise from multiple loudspeakers. They found that bilateral amplification improved performance over unilateral fittings and unaided conditions. Their findings confirmed earlier work by Kobler, Rosenhall and Hansson (2001) in which bilateral benefits were reported for speech recognition, localization and sound quality; as well as more investigations such as that of McArdle and colleagues (2012).
Sound localization has implications for everyday environmental awareness as well as speech perception. Studies of auditory scene analysis (Bregman, 1990) underscore the importance of localization for identifying and attending to specific sound sources. This has specific relevance for understanding conversation in complex environments in which the speech must first be identified and separated from competing sound sources before higher level processing can occur (Stevens, 1996). Therefore, the effect of hearing aids on localization is likely to impact an individual’s overall ability to understand conversational speech in a noisy environment.
The physical presence of a hearing aid and earmold obscures some pinna-based localization cues; the use of bilateral hearing aids should aid horizontal localization under some circumstances. Individuals with moderate to severe hearing loss who wear only one hearing aid may hear some sounds only on the aided side, whereas binaural time and intensity cues may be preserved in a bilateral fitting. Current hearing aids with bilateral data exchange that can account for interaural phase and time cues may offer additional binaural localization benefits. Individuals in social situations are likely to be conversing with one or more people at roughly the same vertical elevation. Therefore, preservation of horizontal localization cues with bilateral hearing aids may outweigh the loss of pinna cues and may have more relevance for speech intelligibility, especially in noisy conditions.
Sound quality encompasses a number of attributes that include clarity, fullness, loudness and naturalness. Bilateral hearing aid use may improve the quality of these attributes. Balfour & Hawkins (1992) examined eight sound quality judgments for listeners with mild to moderate hearing loss, tested with unilateral and bilateral hearing aids. Subjects judged the sound quality of speech in quiet and noise, and music in a test booth, living room and concert hall. Subjects had a significant preference for bilateral hearing aids for all sound quality dimensions, with clarity being ranked as the most important. This finding is in agreement with Erdman & Sedge (1981) who reported that clarity was the most significant benefit of bilateral amplification. Naidoo & Hawkins (1997) reported bilateral benefits for sound quality and speech intelligibility in high levels of background noise.
Tinnitus suppression is another area in which bilateral hearing aid use appears to offer an advantage over unilateral fittings. A questionnaire by Brooks & Bulmer (1981) found that 66.52% of bilaterally aided respondents experienced reduction in tinnitus versus only 12.7% with unilateral aids. Surr, Montgomery & Mueller (1985) reported that about half of their subjects with tinnitus experienced partial or total relief from tinnitus with hearing aid use. Melin, et al. (1987) found that there were differences in tinnitus relief based on the number of hours of use per day. Taken together, these studies suggest that individuals who suffer from tinnitus may experience from relief with hearing aids and are more likely to do so with consistent, bilateral hearing aid use.
There is evidence to suggest that some individuals experience better results with unilateral fittings. Binaural integration of simultaneous auditory signals in asymmetric hearing loss may have negative implications for bilateral hearing aid use. This was first described by Arkebauer, Mencher & McCall (1971) who reported that amplified signals presented to two asymmetrically impaired ears resulted speech discrimination that was worse than the better ear alone and similar to the poorer ear alone. Hood & Prasher (1990) simulated bilateral hearing loss and found poorer speech discrimination ability when dissimilar distortion patterns were sent to each ear and significant improvement when identical distortion patterns were sent to the two ears. The results were interpreted to suggest that an inability to process incongruent or dissimilar speech input from both ears could contribute to the rejection of two hearing aids. Jerger et al. (1993) reported similar findings and explained that stimulation of the poor ear was interfering with the response of the better ear. He posited that binaural interference could affect approximately 10% of elderly hearing aid users. Binaural interference may be caused by age-related atrophy or corpus collosum demyelination resulting in poor inter-hemispheric transfer of auditory information (Chmiel et al. 1997) and individuals experiencing binaural interference may be likely to perform better with one hearing aid.
Auditory deprivation is commonly cited when recommending bilateral hearing aids. First described in 1984 by Silman, Gelfand and Silverman, it was noted that in unilateral fittings on bilaterally impaired individuals, speech discrimination in the unaided ear was reduced relative to the aided ear. Pure tone thresholds and speech reception thresholds were not affected. Gelfand, Silman & Ross (1987) again found reduced speech recognition scores over time (4-17 years) for unilaterally aided individuals. Hurley (1999) also reported that unilaterally aided subjects were more likely to experience monaural reductions in word recognition scores compared to bilaterally aided subjects. Subsequent studies determined that the auditory deprivation effect was reversible and subjects who were later fit with a second aid experienced improved word recognition (Silverman & Silman, 1990; Silverman & Emmer, 1993; Silman, et al., 1992). Byrne & Dirks (1996) expanded on the concept of auditory deprivation, reporting that it may also affect localization and intensity discrimination. Though more research in this area may be warranted, Mencher & Davis note that the best way to treat auditory deprivation is to avoid it, with bilateral amplification as part of the solution.
Though research provides insight into possible predictors of success, an important measure of success can be found in post-fitting reports of unilateral and bilateral hearing aid users. Self-assessments of hearing handicap and disability can help hearing aid users express how their hearing aids affect important activities and everyday communication. Some investigations using self-assessment techniques have revealed a preference for bilateral hearing aid use (Chung & Stephens, 1983, 1986; Stephens et al, 1991) whereas others have revealed a preference for unilateral fittings (Cox, 2011). Because many factors contribute to an individual’s preferences and perceived success, self-assessments should be used in combination with verification measures and consideration of individual attributes, such as age, experience with hearing aids, audiometric configuration and speech discrimination ability. Most patient questionnaires target specific topics such as satisfaction, comfort, usage patterns and speech intelligibility, so it may be useful to combine measures to gain comprehensive information about a patient’s experience. The Speech, Spatial and Qualities of Hearing Scale (SSQ; Gatehouse & Noble, 2004), for instance, measures hearing disability in a variety of circumstances. Because it also examines directional, distance and spatial perception, the SSQ may provide more insight into the effect of bilateral versus unilateral amplification on hearing in everyday situations.
Mencher and Davis’s review suggests that there are numerous likely benefits to bilateral amplification for most, but not necessarily all, individuals. Bilateral hearing aids may offer an improved signal-to-noise ratio, reduced annoyance from tinnitus, improved sound quality and better localization in complex listening environments. Hearing aids are typically dispensed with a trial period of 30-60 days with relatively low financial risk to the patient in the event of a return. Therefore, it seems sensible to recommend bilateral fittings for candidates with bilateral hearing loss, knowing that a return of one hearing aid will be possible if contraindications to bilateral hearing aid use should arise during the trial period. Close monitoring of performance and comfort during the trial is essential, especially for individuals with asymmetrical hearing loss or a history of unilateral hearing aid use. In these cases, it may be necessary to reduce gain and output or increase compression in the poorer or previously unaided ear, to accommodate the likely inter-aural differences in acclimatization rate. Ears with more hearing loss and/or less aided experience generally take more time to adapt to amplification, so gradual adjustment and focused counseling may be necessary to eventually achieve satisfactory binaural balance.
The authors conclude by pointing out that the only way to know if a patient is successful with their hearing aids is to ask them! The interaction of several factors such as sound quality, localization, noise tolerance, loudness discomfort and physical comfort will contribute to patient satisfaction. Ultimately, clinicians should develop a clinical strategy that employs objective and subjective measures to truly document benefit and satisfaction with the hearing aid fitting—be it unilateral or bilateral.
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