The cochlear implant is an elective treatment for those who have lost all or most of their useful hearing in both ears. Figures collected by the MRC Institute of Hearing Research show that at the end of the year 2000, more than 1,500 adults and more than 1,400 children had received implants in the UK.
When there is a very severe to total sensorineural hearing loss it is usually because the hair cells in the cochlea are so damaged that they do not create these electrical signals, or enough of them, to make use of a hearing aid satisfactory. The cochlear nerve is usually intact. The cochlear implant is so named because it is implanted in the cochlea.
A cochlear implant is an electronic device. A small part of it is surgically implanted in the cochlea to provide direct electrical stimulation of the cochlear nerve (7). It gives a sensation of hearing. There is also an externally worn part. This is a speech processor (3) that is connected to a headpiece with a microphone (1) to pick up the sound waves. The processor converts them into minute electrical signals. The headpiece has a transmitter, which sends the electrical signals through the skin to the receiver and internal implant (5). The transmitter is held in place over the internally implanted receiver above the pinna, or outer ear, by a magnet.
A wire leads down from the receiver into the middle ear and then into the cochlea itself. The implant has eight or more electrodes depending on which device is used. The electrodes are placed at intervals along the part of the wire, which is called the electrode array. This is threaded into the cochlea (6). The electrical signals from the processor are sent to the electrodes. Like the hair cells in the normal ear, the electrodes at the beginning of the electrode array are usually stimulated by the high frequencies and those at the end of the array, near the apex of the cochlea, by the low frequencies. The signals then pass along the cochlear nerve and are decoded in the auditory centre of the brain and the implant user "hears".
Some cochlear implant users hear very well indeed. They can even use a telephone quite normally and go about their daily life reasonably well. Not everyone is so successful, but users who do not gain as much can still hear environmental sounds like dogs barking, birds singing and the doorbell. Best of all, most people find conversation easier and more relaxed because their hearing helps them to lip-read better even if they cannot understand speech through hearing alone.
Cochlear implants are suitable for adults and children who have a severe to profound sensorineural deafness in both ears, and cannot gain significant benefits from conventional hearing aids. Young children have special requirements, especially with regard to the assessment, selection and follow-up procedures.