Here is more information on implantable hearing aids.
Nowadays, when the term "implant" is applied to a hearing device, what usually comes to mind is a cochlear implant, a device designed to directly stimulate the auditory nerves located in the inner ear. There is no doubt but that this device has been of inestimable value for people with major damage to their inner ears.
However, there are two other general types of implantable hearing devices as well, those designed for people whose inner ears are either completely normal, or still functional in spite of moderate to severe damage.
The first type is the Bone-Anchored Hearing Aid (BAHAŽ); and the other falls under the general category of middle-ear implants (MEI).
Bone-Anchored Hearing Aids
A bone-anchored bearing aid is basically a modern development of a bone-conduction (b/c) hearing aid, something that has been with us for over 60 years. In a traditional b/c hearing aid, the user places a tight, elastic, metal band over his or her head. A bone-conduction vibrator (oscillator) is fixed to one end of the band and placed behind the ear and connected to a body-worn hearing aid via a wire cord. These oscillators are essentially the same devices still being used during routine audiometric testing. People wearing such b/c hearing aids would frequently complain of discomfort and headaches because of the constant pressure of the vibrator behind their ears.The people who were candidates for b/c hearing aids were those whose hearing losses were primarily conductive, that is the sound was not being "conducted" through either the ear canal or middle ear because of some impediment to the transmission of the sound vibrations. This impediment could be caused by ear-canal infections (external otitis), chronic drainage from the middle eat or a congenital malformation of the outer or middie ears - anything that precluded the insertion of an earmold in the ear canal.
In these instances, it would be appropriate to fit the person with a b/c hearing aid in order to by-pass the outer and middle ears and directly vibrate the cochlea through the skull. In other words, instead of amplified sound being transmitted through the ear canal to the inner ear via a traditional hearing aid, tiny skull vibrations would deliver the same sound signals to the inner ear (the cochlea) via bone conduction. Once sound signals from either source arrive at the inner ear, they are analyzed by the auditory system in exactly the same way.
The BAHAŽ requires that a titanium screw be inserted into the temporal bone behind the ear. This sounds rather fearsome, but I'm informed that it is really a minor surgical procedure. After about three months, the process of "osseointegration" and healing is completed. Osseointegration refers to a process in which an organic connection occurs between implanted material (the titanium) and living tissue. This type of connection provides for a much more efficient way of transferring energy from an external vibrator through the skull to the cochlea than traditional b/c oscillators. The titanium screw terminates in an abutment into which the hearing aid can easily be attached and detached.
The hearing aid itself (there are several models, ranging from ear level to body-worn types) contains the usual hearing aid circuitry and can be modified and upgraded independent of the inserted portion which is permanent. The vibrations produced by the BAHAŽ are transmitted to the embedded titanium screw and passed along to the cochlea via the bone conduction pathway.
Unlike previous generations of b/c hearing aids, because of its vibratory efficiency, the BAHAŽ is also appropriate for people with mixed hearing losses; i.e. part conductive and part sensorineural (up to about a sensorineural component of about 45 dB). The ear-level instrument contains a direct audio input jack into which assistive listening systems or an external telephone coil can be plugged. Further information about the BAHAŽ can be obtained from www.entific.com.
Middle Ear Implant
A middle ear implant (MIE) is basically a hearing aid that, instead of delivering amplified sounds through the ear canal, is designed to directly vibrate one of the three tiny bones in the middle ear (the ossicular chain). There are at least six such devices in various stages of development, though only one, the Symphonix, has received FDA approval. I understand that several other middle-ear implants are undergoing the FDA approval process at the current time and that we can soon expect other such devices to enter the market. In the meantime, for this report, I'll focus on the Symphonix.The Symphonix MEI consists of two parts, one internal and one external. The internal portion is surgically implanted in the temporal bone behind the ear; in much the same way as a cochlear implant. A conductor link (a wire) that terminates in a tiny transducer is led from this implanted component to the middle ear. There the transducer is crimped to the incus, the center bone of the middle-ear ossicular chain. Much of the effectiveness of the Symphonix depends upon the operation of this tiny electromagnetic transducer, termed the Floating Mass Transducer (FMT).
The external portion contains the microphone, battery, and the signal processing circuitry. It is held in place over the implanted component (termed, the "receiver") via magnetic attraction, again similar to cochlear implants. When a sound signal arrives at the MEI microphone, it is processed and transmitted across the skin through the receiver and conductor wire to the FMT. The FMT vibrates the ossicular chain, thus stimulating the cochlea. In other words, rather than the eardrum passing vibrations on to the ossicular chain, this system "shortcuts" the process somewhat by directly driving the bones of the middle ear when an electrical signal is applied to the FMT. Unlike the BAHAŽ, the MEl is intended for people with no middle-ear problems and up to a moderate to severe sensorineural hearing losses. While cosmetics may be the underlying motivation for people who select this device, there are other reasons why it may be a desirable option for some individuals. Its major advantage is that nothing is inserted in the ear canal.
Some people, because of extreme allergic reactions, or a chronic infection in the ear canal, cannot insert a foreign body (an earmold) into their ear canals. For them, an MEl provides the advantages of amplified sound signals while keeping the ear canals open.
Another possible advantage is the fact that acoustic feedback (an audible "squeal") is virtually eliminated. Feedback occurs when amplified sounds escape from the ear canal and are picked up by the hearing aid microphone. This sound is again amplified through the hearing aid, more sound escapes from the ear canal, the microphone again picks up this sound thus generating the feedback cycle. With an MEl, on the other hand, since the output is vibrations passed directly to the middle-ear bones and not audible sound, the potential feedback cycle is interrupted before it can begin. Finally, because nothing is blocking the ear canal, the occlusion effect is eliminated. The occlusion effect refers to a "booming" sensation in one's own voice (like "talking in a barrel") that may occur when the sound vibrations produced in the ear canal by one's own voice are prevented from escaping by a hearing aid or an earmold.
In the course of gaining FDA approval, the company was required to sponsor a large-scale clinical investigation. The results of this multi-center study support the notion of the MEI in many respects. From a subjective standpoint, the people who tried this MEI reported improved benefit in many listening situations, such as in noise, reverberation, and in the sound of their own voices (eliminating the occlusion effect). The subjects also reported that overall comfort and sound quality were greater with the MEI than with their own hearing aids. Objectively, however, there was no improvement in speech perception capabilities (though it also didn't decrease these capabilities either). In other words, it has not been experimentally demonstrated that the device helps people actually hear better, only that they feel more comfortable with this device than with traditional hearing aids. Still, this is not inconsequential, particularly since there arc people who cannot comfortably wear a conventional hearing aid for the reasons outlined above.
More information about this MEI can be obtained from www.symphonix.com.
Acknowledgements:
This article was supported in part by grant #RH133E980010 from the U.S. Department of Education, NIDRR, to the Lexington Center.
Mark Ross, Ph.D., is an SHHH member from Storrs, Connecticut, and writes the column "Developments in Research and Technology" in each issue of Hearing Loss.