Current trends in implantable hearing AIDS.

Abstract

When the research is examined historically, there appear to be three rationales for developing and fitting implantable hearing aids (IHA). These include improved (a) sound reception, (b) cosmetics, and (c) fitting of patients who could not derive optimal benefit from conventional amplification. With the introduction of modern completely-in-the-canal (CIC) hearing aids, the cosmetic concern is no longer a major motivation. (Johnson and Danhauer, 1997). Dumon et al (1995) clearly stated the ultimate goal of IHA research is the creation of a “… totally implantable middle ear implant (MEI), for severe and average [moderate] sensorineural deafness.” Until the technology is available to provide a totally IHA, Goode et al (1995) defined an implantable hearing aid as a class of electronic devices that are “wholly or partially implanted through surgery and … [are] designed to ameliorate hearing loss.” This definition not only covers the various middle ear implants, but also the temporal bone implants. This latter group is generally referred to as bone anchored hearing aids. Both of these types of IHAs will be discussed in great detail in this issue of Trends. Like any dynamic area of research, spin-offs of this technology are already available in related areas, such as a middle ear pump system designed to release small amounts of antibiotics for chronic otitis media (Maassen et al, 1996), and as a “transparent link [to the temporal bone] in the study of tinnitus…” (Downing, 1996). Goode et al (1995) and Ko et al (1995) delineated some of the parameters that an IHA should possess. As a general set of requirements these researchers suggested that IHAs should: provide sufficient gain and output for the patient's needs, not reduce residual hearing by having minimal loading or contact with middle ear structures, not limit daily activities such as swimming, not predispose the patient to recurrent ear infections, have proven long-term stability and tolerance to anatomical changes, have a demonstrated biocompatibility of the implanted materials and package, have minimal movement of the implanted parts in order to minimize wear and tear, be adjusted/programmed and removed easily with an external battery, have safety of the surgical procedure demonstrated, and allow the patient the ability to wear a conventional hearing aid if for some reason the IHA breaks or does not provide sufficient benefit. Given the definition of IHAs, there is no reason to assume that the various devices should be restricted to only one type of hearing loss. Indeed, many of the various IHA projects that are underway around the world can be applied to sensorineural, conductive, and mixed hearing loss, despite the original intent of the researchers. The discussion of IHAs in this manuscript is divided into two main sections; (a) bone anchored hearing aids and (b) middle ear implants (MEI). As the names suggest, a portion of a bone anchored hearing aid is surgically implanted in the temporal bone, whereas the middle ear implants are to be found partially implanted in the middle ear cavity.