A hearing aid amplifies incoming sound but there must be functioning hair cells and an intact nerve to transmit that amplified signal to the central auditory pathways. A cochlear implant, on the other hand, attempts to replace a function lost by the cochlea, usually due to an absence of stimulable hair cells. In a normal hearing ear, the hair cells within the cochlea act as transducers of mechanical and hydraulic vibration of the tympanic membrane, ossicles of the middle ear and perilymph and endolymph of the inner ear to chemo-electric energy capable of stimulating the eighth nerve. The decrease in hair cells causing a sensory hearing loss results in the cochlea losing its ability to stimulate the eighth nerve. The cochlear implant replaces the function of the lost hair cells by converting mechanical energy (sound waves) into electrical energy capable of exciting the auditory nerve.

Cochlear implants are surgically placed within the inner ear, bypassing the hair cells of the cochlea anddirectly stimulating the endings of the auditory nerve. Although there have been many variations on the theme, the basic design of an implant system has remained relatively stable over the years. It consists of an external microphone, processor and transmitter and an internal receiver-stimulator and electrode array. The microphone captures incoming sound and converts it into electrical signals. The processor reconfigures, amplifies and manipulates the electrical signal into an analog or digital signal, which is then transmitted to the receiver/stimulator and electrode array of the implanted device. Subsequently, the electrodes stimulate neural populations in a pattern which is determined by the encoding strategy of a given implant. This dictates if electrodes are stimulated simultaneously or sequentially, in a monopolar or bipolar fashion, utilizing a slow or rapid rate of stimulation. The surgery required for the implant takes approximately three hours and has a very low rate of complications.

The Auditory Brainstem Implant (ABI) is a modified cochlear implant intended to be used to stimulate the cochlear nucleus in the brainstem of patients who have had their eighth nerves severed during surgery for removal of bilateral neurofibromata, as in patients with NFII. The cochlear implant linear array of electrode contacts is replaced by a small rectangular silastic paddle containing the 21 contacts. This is surgically inserted into the lateral recess of the fourth ventricle. The need for this device is much less common than that for a cochlear implant, but these patients are typically totally deaf and, although the benefit is not as great on the average as that of the cochlear implant, most recipients derive significant auditory perception.