Dr. Ross on Hearing Loss
Implantable Hearing Aids
by Mark Ross, Ph.D.
Much has been written about cochlear implants, those marvelous devices that provide useful sound experiences for people with profound or severe-to-profound hearing losses. There is no question but that these systems have opened up the auditory channel in ways we could only dream about years ago. Much less has been written about other types of implantable hearing aids, those designed for people who still possess useful residual hearing. While their potential advantages are less dramatic than those found with cochlear implants, they are an alternative to conventional amplification for people who need and desire such systems. These hearing aids can be separated into two general types: bone-anchored devices and middle ear implants.
Bone-Anchored Hearing Aids (BAHAs)
A bone-anchored hearing aid (BAHA) is basically a logical progression of the classic bone-conduction (B/C) hearing aid. This type of hearing aid was frequently used years ago, when middle ear infections were much more common and very difficult to treat either medically or surgically. As a matter of fact, because of middle ear problems, more than one-third of the students in schools for the deaf in the 1930's wore B/C oscillators when using their hearing aids or the group hearing auditory training system. Because of frequent "drainage" from their ear canals, these children, and other people with middle ear problems, could not wear earmolds that would block the ear canal. The B/C hearing aid was able to bypass this problem by delivering usable amplified sounds directly to their cochlea via bone conduction. True, a B/C hearing aid was not overly comfortable, what with the vibrator being held tightly in place by a coiled spring situated over the top of the head. But the advantages were still there, in spite of frequent complaints of "pressure" and "headaches."
Although there have clearly been significant advances made in the medical and surgical treatment of middle ear disorders, and one now rarely sees a conventional B/C hearing aid, there still is a need in this day and age for such a hearing aid. Basically, there are three types of people who can benefit from bone conducted sound.
The first are children born with the kind of physical abnormalities in the outer and/or middle ears that prevent an earmold from being inserted into the ear canal (e.g., those with Treacher-Collins or Goldenhar syndromes). The second type is people with middle ear infections that are resistant to medical or surgical therapy and who have been unable to wear conventional hearing aids. The third type of potential candidates for a B/C hearing aid are those who have chronic eczema or inflammation in the ear canal, perhaps secondary to an allergic reaction to the earmold, that precludes delivering sound via the classic air conduction route.
To fit a BAHA, it is necessary to insert a titanium screw into the temporal bone behind the ear. It takes about three months for osseointegration to occur (this refers to the growth of a solid connection between the bone and the implanted screw). Because the screw does not completely go through the bone, there is no restriction in activities such as swimming or showering. An external mount is connected to the screw, into which a removable ear-level hearing instrument is plugged. The hearing aid itself, being independent of the implant screw, can be modified as desired with no need to change the implanted portion in any way. The hearing aid processed signals are delivered via bone conduction to the inner ear, bypassing the normal route through the ear canal which remains unoccluded. The fact that the ear canal can remain open is a major advantage of a BAHA.
Because of its efficiency in transferring energy from the hearing aid to the skull, a BAHA can be used by people with mixed hearing losses, those with bone conduction thresholds better than about 45 dB, as well as people with purely conductive hearing losses. According to one report, the BAHA results in improvements of about 20% in speech perception scores compared to a conventional bone-conduction hearing aid. It is important to consider all other amplification options before deciding on a BAHA, particularly for children who are in the process of initially learning speech and language. As of last year, approximately 7000 people have been fitted with these hearing aids. The company has received clearance from the FDA for the device to be employed clinically. Further information about the BAHA can be obtained from www.entific.com.
Middle Ear Implants (MEI)
A middle ear implant (MEI) is basically a hearing aid, but one in which the receiver ("loudspeaker") or the entire hearing aid is surgically inserted into the middle ear. Access to the middle ear is accomplished by a post-aural approach, similar to that of cochlear implants. There are at least six such devices now being developed, though only one has received FDA approval (the Symphonix device more on this one below). While they are all different in some respects, all of them have one feature in common: they all place an output transducer (the receiver) on the ossicular chain in the middle ear. (The ossicular chain is a series of three, tiny, interlocking bones that are responsible for transmitting sound vibrations from the eardrum to the inner ear.) Better sound fidelity is said to result when a receiver can be located directly on the ossicular chain and thus can vibrate the hearing mechanisms directly.
Candidates for middle ear implants must have a sensorineural hearing loss and a normally functioning middle ear. Unlike cochlear implants, these devices are designed for people with significant residual hearing. There are several stated advantages to a MEI. One is the elimination of acoustic feedback, a particularly salient advantage for those hearing aid users who have continual, and unresolved, problems with acoustic squeal. A MEI eliminates feedback because its output is a vibratory signal and not a sound, thus breaking up the potential acoustic feedback cycle. Another advantage relates to the fact that no earmold now need be inserted into the ear canal. This can be particularly advantageous to those for whom an earmold produces an allergic reaction. The elimination of an earmold will also preclude the occlusion effect, where one's own voice sounds muffled or as if one was "talking in a barrel." People wearing hearing aids frequently make this complaint, the origin of which is the blockage of the ear canal by an external object.
The third "advantage" is said to be cosmetic, although this may not be quite so clear-cut. While there is nothing visible in the ear canal, for most MEIs it is necessary to employ an external signal processor located behind the ear (which connects to an internal component). However, two of the systems now being developed are totally implantable and invisible. One uses the eardrum as a microphone - which is rather a neat idea - while the other actually implants a small microphone in the wall of the ear canal. The rest of the necessary electronics are surgically implanted within the temporal bone behind the ear. As of yet, neither of these has received final FDA approval for clinical application in the U.S. Thus, the cosmetic advantage still remains problematic for the one system that has been approved. As a rather personal bias, we would hate to see a decision being made for an MEI solely for cosmetic reasons.
The one MEI that has received preliminary FDA approval is the Vibrant Soundbridge developed by Symphonix Devices, Inc, albeit only for those who are 18 years of age or older. Potential candidates should have at least a moderate to severe sensorineural hearing loss, a normally functioning middle ear, and prior experience with conventional amplification. The device is separated into two components, external and internal:
The external portion is the part that remains outside of the body. It consists of a microphone, the signal processing electronics, and a battery. It is inductively coupled to the internal component, in the same way as a cochlear implant. The company is now in the process of developing other, more advanced "soundbridges," all of whom would be compatible with the internal portion.
The internal portion consists of an internal receiver, a conductor link, and the Floating Mass Transducer (FMT). The internal receiver is implanted in the temporal bone behind the ear, again much like a cochlear implant. The conductor link (a wire) is led from the receiver to the middle ear. There it is connected to the FMT. The FMT, smaller than a grain of rice, is crimped to the incus, one of the ossicles in the middle ear. The FMT is an electromagnetic transducer, consisting of a magnet surrounded by a coil of wire. When an alternating electrical current is led to the wire coil, it sets the magnet in motion which in turn vibrates the entire ossicular chain. Audio signals from the external component are transmitted across the skin and converted to an electrical current by the internal receiver. It is these electrical signals that are delivered to the FMT. For the overwhelming majority of subjects, implanting the Vibrant Soundbridge will not significantly affect residual hearing.
Twelve sites around the country participated in the clinical trials. The results were sufficiently positive for the FDA to grant preliminary approval to the Soundbridge, with final approval expected shortly. These results demonstrate that the vast majority of subjects were more satisfied with the MEI than with their own hearing aids. The patients reported significant benefit in many listening situations in areas where hearing aid users usually have great difficulty, namely in background noise, reverberation, and in the sound of their own voice. Patients also commented that overall comfort and sound quality was greater with the Soundbridge than with their own hearing aids. Speech perception scores with the MEI were similar to those obtained with the patient's own hearing aids.
Considering the number of MEIs now being developed, some now engaged in the FDA approval process and some still in the preliminary stages, it does appear that this type of hearing aid will find a useful niche within the armamentarium of personal amplification devices now available. As we indicated above, we would hate to see someone making a decision to obtain one solely for cosmetic reasons but, still, this is a decision that people must make for themselves. From a technological viewpoint, there is no doubt that MEIs are a noteworthy demonstration of the scientific capabilities of the modern era. The technological/surgical package that researchers have been able to combine is truly impressive.