Bilateral or unilateral amplification: Is there a difference? A brief tutorial

The literature contains many examples of the terms bilateral and binaural fittings used interchangeably (i.e., Carter, Noe & Wilson, 2001; Chmiel, et al, 1997; Dutt et al, 2002; Dermody & Byrne, 1995). In fact, however, humans are fit with hearing aids on either one ear (a unilateral fitting) or on both ears (a bilateral fitting), and generally, bilateral fittings have an impact on binaural hearing (the integration of two signals arriving from each ear independently). Standardization of this terminology should help to clarify future discussions and is essential to avoid confusion. The choice of when it is better to fit unilateral or bilateral hearing aids has been the subject of research, discussion and controversy for over fifty years (Carhart, 1946; Dirks & Carhart, 1962). It would seem a simple statement of fact and logic, that since humans have two ears, two ears wearing hearing aids would be better than one ear wearing one aid. However, evidence to date has not always been so factual, nor have the results been so logical. Dillon (2001) cites over 100 studies relevant to this topic. In 1999, 68% of hearing aid fittings in the U.S. were bilateral, up by 13% over the previous 10 years (Kochkin, 2000). Further, in 1999, prior to the modernized hearing aid plan, the United Kingdom (10 15%) and Finland (20%) fit fewer bilateral aids than Denmark (50 60%) and Norway (60%), with Sweden at 30% (Joint Nordic-British Project, 2001). Today, however, the number of bilateral fittings in England in some regions is 40% 80%. Why is there such variance? Shouldn’t everyone automatically receive and benefit from bilateral amplification? Unfortunately, the answer to that question is equivocal: some people benefit from two aids, and some do not. Generally speaking, hearing is the sensory system of import for oral communication, providing essential information about the environment that serves to alert and inform. Oft times these two functions interact. Thus, from a purely mechanical point of view, the ideal hearing aid enhances oral communication and permits perfect localization of sound in the environment. Consequently, tests for proper function of a hearing aid should include measures of speech intelligibility, sound quality (clarity, loudness, fullness, brightness, etc.), binaural redundancy (hearing the sound in both ears), noise tolerance and loudness discomfort levels as well as localization (in the vertical, lateral and horizontal planes). Further, consideration should be given to patient attitudes and physical comfort with the aid. The more normal the responses to all of these issues, the more satisfied the user and the more successful the fitting. Patients, as individuals, have unique auditory impairment and a unique set of personal, social, psychological and physical needs and characteristics. This means each fitting, and the results obtained from it, will be different, no matter how consistent the actual fitting process. Further, it should always be understood that any findings or opinions regarding the success or failure of any fitting may be completely independent of the actual benefit derived from the device itself. Measures of unaided versus aided threshold sensitivity are not good predictors of success. Two patients with similar degrees of threshold sensitivity often present with variant degrees of speech recognition in noise (Lyregaard, 1982). Patient attitudes, personality, the impact of the listening environment, perception of handicap and expectations from a hearing aid have all been significantly correlated to overall satisfaction and usefulness. So, even if formal audiometric measures do not show significant benefit from one fitting over another, the patient may feel otherwise (Humes, 1999; Cox & Alexander, 1999; Walden & Demorest, 1984). We know that audiometric measures do not reflect the value of a bilateral fitting. Real Ear Measurements, an excellent mechanism for determining the physical response of the ear itself, also do not adequately address questions of binaural interaction, how sound is perceived, comfort, or any of the myriad of psychological factors that could influence a successful outcome. Finally, current clinical procedures measure speech intelligibility and auditory localization. These excellent measures may not be entirely appropriate when assessing bilateral versus