Habilitative Audiology Program

A cochlear implant is an electronic device that provides a sensation of hearing to individuals who are deaf. It does not provide "normal" hearing, but the device can allow children to detect and differentiate among sounds.

It consists of two main parts: an external part worn somewhat like a hearing aid, and an internal part which must be implanted by a surgeon.

The external part of a cochlear implant is the speech processor. It has a "microphone" worn over or behind the ear. A cord leads from the microphone to the speech processor. The speech processor codes the sound input into electrical signals which are sent to the "transmitter," a thin plastic piece about one inch in diameter containing a magnet placed on the side of the head behind and slightly above the ear.

The transmitter sends the signals across the skin to the internal part of the implant (the "receiver/stimulator"), which is under the skin.

The receiver/stimulator sends the signals into the electrode array, which is a one-inch long wire surgically inserted into the inner ear. The electrode array consists of an array of electrode bands, each of which can provide a tiny current to the inner ear, to replace the function of the damaged or missing hair cells which ordinarily would stimulate the nerve endings of the auditory nerve.

Implants from different manufacturers differ in the number of channels, programming strategy, and appearance of the externally worn device. Behind-the-ear processors are available to eliminate the need for a body pack for most cochlear implant users once they reach approximately kindergarten age or sometimes earlier.

Does a cochlear implant provide normal hearing?

No. A cochlear implant provides a limited sense of hearing. However, most individuals with good language abilities can integrate this sensation with visual cues to understand spoken language. Many cochlear implant users can learn to understand spoken sentences without looking at the person who is talking, particularly if there is not background noise.

Who can benefit from a cochlear implant?

Adults and children who used to have normal hearing or partial hearing, who learned to talk before they became deaf, often benefit from a cochlear implant. These individuals have the advantage of remembering what speech used to sound like, so they can use the sound sensation from the cochlear implant to supplement their lipreading ability.

Children who were born deaf ("congenitally deaf") or who became deaf before they learned to talk ("prelinguistically deaf") also can benefit from a cochlear implant, although their progress in learning to understand spoken language with the implant will be slower than for individuals who became deaf later.

To be a candidate for a cochlear implant, a child must have a severe or profound sensorineural hearing loss in both ears. The average hearing level in the speech frequency range (500-2000Hz) must be poorer than 70 decibels in both ears without hearing aids, and with hearing aids the child must not be able to recognize single words clearly out of context without looking at the talker's face.

If the child is under 24 months of age, the hearing loss must be 90 decibels or greater in both ears. If the child has more hearing than this range, then the child might receive more benefit from a hearing aid than from a cochlear implant.

If the child has the ability to recognize words with a hearing aid, then a cochlear implant may not provide any more benefit.

At what age should a child receive a cochlear implant?

The Food and Drug Administration allows cochlear implants for children beginning at age 12 months. A congenitally deaf child who is going to have a cochlear implant should have the surgery before the age of four years, or earlier if possible. This early implantation gives the child the best chance to learn to use sound while language skills are developing.

Some congenitally or prelinguistically deaf children who receive cochlear implants when they are older do not develop the ability to recognize speech with the implant, and ultimately may reject its use. However, a school-age deaf child who makes maximal use of hearing aids and who already uses spoken language may benefit from a cochlear implant.

Children who once had normal hearing or partial hearing, and then became deaf, may be implanted as soon as it is clear that the child's hearing is not going to recover and that there is no benefit from a hearing aid. Older children and teenagers who lose their hearing must participate in the decision whether to have a cochlear implant.

Factors which favor a beneficial outcome from a cochlear implant are:

• normal cochlear anatomy;
• surgery at a young age;
• good underlying language abilities;
• a solid base of language development prior to surgery;
• a high level of motivation and commitment on the part of the family to keep frequent appointments, maintain the device, and encourage listening skills;
• an appropriate educational program which incorporates listening activities into the curriculum;
• regular speech/language therapy given by a clinician with specific expertise and experience in the area of spoken language development in deaf children using cochlear implants.

Determination of the appropriateness of the available program of education and therapy is part of the decision process regarding whether a particular child may benefit from a cochlear implant.

The benefit from a cochlear implant may be limited by a child's previous language deprivation or by a particular child's disorder in language acquisition skills. The anatomy of the child's ear and auditory nerve also may limit sound reception and clarity with an implant.

Children who have cochlear dysplasia (congenital malformation of the inner ear, such as a Mondini deformity) may not be able to accommodate the entire electrode array in the cochlea, and may have fewer electrodes usable to stimulate different sensations of hearing for different frequencies.

Children who have had meningitis as the cause of their hearing impairment may have ossification (bony growth) in the cochlea which may require extra drilling for electrode placement, and may limit the extent to which the electrode array can be inserted into the cochlea. This ossification usually can be seen on a CT scan of the temporal bones. Although the auditory nerve, which will be stimulated by the implant, may be seen on an MRI prior to surgery, the function of the auditory nerve can not be predicted accurately.

The surgery is performed under general anesthesia and takes three to six hours. The child stays in the hospital one or two nights after the surgery. A parent may sleep in the child's hospital room.

During the surgery, the receiver/stimulator and electrode array are implanted. An incision is made behind the ear for the surgeon to expose the area of the round window, a tiny membrane at the separation between the middle ear and the inner ear. Then the surgeon places the receiver/stimulator in a small area on the side of the head where the bone has been drilled thinner to make a place for the receiver/stimulator to fit, outside the skull but under the skin.

The brain is not exposed or penetrated during the surgery. The electrode array is inserted into the inner ear, and the receiver/stimulator is fixed in place over the bone. Electrical recordings are made to show that the electrodes are providing stimulation. The skin is surgically closed over the implant.

When the external parts of the implant are not being worn, the implant is not visible from the outside and the patient cannot hear.

What are the risks of the surgery?

The risks of anesthesia are the same as for any surgery. Surgery to the inner ear also carries the risks (although uncommon) of facial nerve paralysis, loss of taste sensation, dizziness, or ringing in the ear.

The surgery does destroy any ability the individual may have had to hear with a conventional hearing aid in that ear. It is possible that at some point in the future, the implant may stop working and may need to be replaced in another operation.