The cochlear implant; basic principles.

Abstract

In recent years the cochlear implant has been a subject of much discussion and controversy. The clinician has often been confused by the conflicting reports of success and failure. In this paper the development of the cochlear implant is reviewed and its present status summarized. It is hoped that the clinician may thereby gain an understanding of this device so that he can better evaluate its present and future status. Selection of Patients for Cochlear Implantation. The cochlear implant will benefit only those patients with hair cell loss who have remaining viable auditory neurons. In order to determine whether viable neurons remain, an electric current is passed through a small needle which is place into the promontory through the tympanic membrane. If patients experience an auditory sensation as a result of this electrical stimulation, it is felt that they are suitable candidates for a cochlear implant. Feasibility of Long-Term VIIIth Nerve Stimulation. Many questions have been raised regarding the feasibility of long-term stimulation of the auditory nerve. The first question raised was whether the auditory nerve would survive severe hair cell degeneration. Studies have shown that in most cases at least a few auditory neurons remain. The next question was whether the cochlear implant itself would destroy the remaining auditory neurons. Preliminary studies have shown that the nerve will survive the placement of electrodes both into the modiolus and the scala tympani. Several electrode materials and insulation have been found to be well tolerated, and there has been minimal damage from thermal or electrolytic processes; therefore, it appears feasible to stimulate the auditory nerve over a long period. Information Transfer by Electrical Stimulation. Single-channel stimulation produces only periodicity pitch, and information transfer is insufficient for speech discrimination. Experience to date indicates that it will be possible to produce both place and volley pitch by electrical stimulation with multiple electrodes in the scala tympani. These findings give promise for the feasibility of producing a device which will transfer sufficient information to produce speech discrimination. Present Status of the Cochlear Implant. To the present time 15 patients have been implanted with a unipolar electrode under the direction of the Ear Research Institute. These patients have all benefited from their devices. They are able to perceive background sounds and receive a cadence or rhythm to speech which makes the device helpful in lipreading. None of the patients have developed significant speech discrimination.