A Neurological Look at Cochlear Implants

The featured speaker at the March 8, 2003 meeting of San Diego's Cochlear Implant Association International (CIAI) - Pioneer Chapter meeting was Dr. Starr, a neurologist from the University of California at Irvine. He and his colleagues are deeply involved in the study of how the nervous system processes acoustic information, and especially with how it processes inputs from a cochlear implant (CI). Dr. Starr is currently working to define an objective measure of what happens in the brain when a CI user turns his processor on.

Dr. Starr began by differentiating between central processing disorders and peripheral processing disorders. Central processing disorders are those that occur in the brain, while peripheral disorders occur elsewhere (e.g. the cochlear hair cells). Either type of disorder can cause hearing loss; because the treatments are generally different for the different disorder types, it's important to identify the disorder before specifying treatment.

Modern science allows neurologists to measure the response of various parts of the nervous system to various stimuli. An example is to measure the response of various parts of the brain to the stimulus provided by a CI. One interesting result of these studies is that some CI users have responses in the brainstem, while others don't. Dr. Starr mentioned a potential correlation between this phenomenon and a user's ability to benefit from a CI. Those without a brainstem response may derive less benefit from the use of a CI.

One of Dr. Starr's current projects is to compare two groups of CI users. One group consists of persons with hair cell degeneration, which is the common cause of hearing loss normally referred to as "nerve deafness". The people in the other group suffer from auditory neuropathy, a condition in which their ears work perfectly well, but disorders in the auditory nerve or the brain prevent them from understanding speech.

People with auditory neuropathy have the following characteristics:
1. Their audiogram is generally flat or has a low frequency loss.
2. Their speech comprehension scores are worse than would be expected based on the audiogram.
3. Their ability to understand speech in the presence of noise is degraded.
4. Otoacoustic emissions are present.
5. The auditory brainstem response (ABR) is absent or weak, even though patients report that they hear the clicks that cause the response.
6. Amplification (hearing aid) is typically ineffective.

Auditory neuropathy is present in about 10% of newborns with hearing loss. Virtually all older adults with significant hearing loss suffer some auditory neuropathy, but it may not be their dominant hearing problem.

What we call "hair cells" are really extensions of the auditory nerve. There are two types. The outer hair cell provides mechanical gain. The cochlear fluid moves in response to sounds, and that motion causes minute movements in the outer hair cell. In response, other portions of the outer hair cell undergo much larger movements. The other type is the inner hair cell. It responds to the large movement of the outer hair cell by producing a chemical called a neurotransmitter, which begins the process of sending an electrical signal to the brain.

Q. What is your opinion regarding bilateral cochlear implants?
A. People would hear better with bilateral implants, but most users are doing pretty well with just one. We are making progress on treating hearing loss and it's hard to say what might be available in a few years. Because CI surgery does damage some of the inner ear structures, it may preclude future treatments. My advice, especially for younger people, is to save one ear for future developments.