Cochlear Implant Technology

Editor: This workshop was presented by David R. Friedland, MD PhD and Christina Runge-Samuelson PhD. Both are Associate Professors in the Department of Otolaryngology and Communication Sciences at the Medical College of Wisconsin in Milwaukee.

This is part one of two parts.

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The first part of this presentation was a review of a normally functioning auditory system. Sound initially encounters the outer ear or pinna, which channels and filters sound to the external ear canal. The canal is resonant around 3000 Hz and amplifies sounds near that frequency.

At the end of the ear canal is the tympanic membrane or eardrum, which is a cone-shaped translucent membrane about one square centimeter in size.

Sound causes the eardrum to vibrate, and that vibration is passed on to the middle ear, which consists of the hearing bones, called the hammer, anvil, and stirrup. These amplify the vibration of the eardrum; the last one presses on the cochlea, and its motion causes the fluid within the cochlea to vibrate.

Moving cochlear fluid causes the inner and outer hair cells to vibrate, which causes them to release chemicals which stimulate the auditory nerve. The location of the hair cells along the cochlea correspond to a particular frequency, with lower frequencies being deeper within the cochlea. The auditory nerve transmits signals to the brain, where the signals are interpreted as sound.

There are two fundamental kinds of hearing loss: conductive and sensorineural.

Conductive hearing loss is caused by a problem in transmitting sound vibrations through the outer or middle ear. Examples include problems with the hearing bones or a hole in the eardrum. The causes of conductive hearing loss can often be fixed, so conductive hearing loss is usually treatable,

Sensorineural hearing loss is caused by problems within the cochlea or the auditory nerve. Missing or damaged hair cells are the most common cause. Sensorinerual hearing loss is not repairable, so hearing aids and/or cochlear implants are the standard treatments.

The next part of this workshop focused on the audiogram, with explanations of what sounds (including speech sounds) are in various parts of the chart. The basics are that low frequencies appear on the left side of the audiogram, and high frequencies on the right. Also soft sounds are near the top and loud sounds near the bottom.

The sounds a person can just barely hear at a variety of frequencies are plotted on the audiogram, and the points connected with a line. This is done separately for each ear. The key point is that a person is able to hear any sounds below their lines, and not able to hear sounds above their lines. Also note that the farther down the lines are on an audiogram, the worse the person's hearing.

When considering who could benefit from a cochlear implant, a logical place to start is with an audiogram that plots a person's responses WITH hearing aids along with the speech banana (a region of the audiogram that displays the location of speech sounds). If the lines representing a person's hearing lie entirely above the speech banana, the person should be able to hear all speech sounds using hearing aids, and would not be a candidate for a cochlear implant.

On the other hand, if a person's corrected response lines lie at least partially below the speech banana, that person would likely benefit from a cochlear implant.

Most people have the greatest hearing loss in the high frequencies, which is also the location of many of the speech sounds that contain important grammatical information. An 's', for example, distinguishes between singular and plural, and it is a soft, high frequency sound. So a person with high frequency hearing loss might very well NOT be able to hear this important speech sound using hearing aids.

Having access to these "grammatical" speech sounds is important for adults to understand speech, but that access is even more important for children, who are just learning the language and the grammar. A child who never hears 's' sounds will likely struggle with singular vs plural grammar issues.

People with severe to profound hearing loss might be candidates for a cochlear implant, and should consider an evaluation if they are interested. Note that there are no upper or lower age limits for evaluation. A 93-year old has been implanted. While children may be implanted at the age of twelve months, they can be evaluated at any age, and are often identified as candidates by a few months.

The evaluation process includes audiometric testing, radiographic testing, and "attitude" testing.

Audiometric testing includes a standard audiogram and speech recognition testing using sentence and/or word tests. It also includes a hearing aid trial to ensure that top quality hearing aids don't result is satisfactory hearing. Finally, one or more tests are conducted to ensure that the auditory nerve is responding appropriately.

Radiographic testing involves either an MRI or a CAT scan, and is performed to ensure that a person has a functional cochlea and auditory nerve.

The "attitude" testing is performed to ensure that a person has reasonable expectations. A recipient may need to do some aural rehabilitation, may still need captions, may not be able to converse comfortably on the telephone, etc.

The evaluation process for children is a bit different than that for adults. As much of the same testing as possible is performed, so the main difference is that the "attitude" testing involves expectations for the child and the willingness of the parents to commit to working on the development of auditory skills.

A cochlear implant contains three components.

The processor is the external component that normally looks like a behind-the-ear hearing aid. It contains the microphone that picks up sound, which is then digitized. The various manufacturers have different strategies for processing speech. Finally, the digitized signal is sent to the receiver via a coil.

The receiver is just under the skin in a pocket that the surgeon created during the surgery. It receives the digital signal from the processor and further processes it in preparation to instructing the electrode array how to stimulate the auditory nerve.

The third component is the electrode array, which is threaded into the cochlea. Arrays contain between twelve and 24 electrodes, depending on the manufacturer. Each electrode corresponds to a particular frequency, with those farther into the cochlea representing lower frequencies.

The number of channels an implant has is influenced by the number of electrodes, but is not necessarily the same. Some implants, for example, are able to fire two electrodes simultaneously, which creates a "virtual" channel whose frequency is between that corresponding to the two electrodes.

Here's Part Two