Retro-cochlear Hearing Loss from the Viewpoint of Cochlear Implantation

Abstract

Characteristics of hearing disorders due to retro-cochlear lesions have been recognized as disproportionally poor discrimination of speech to the hearing level in pure tone audiometry and disturbances in tone decay test, Bekesy audiometry and binaural hearing test. There have been numerous patients whom we diagnosed as having retro-cochlear hearing loss because they had these characteristic. However, we have encountered quite a few individuals who failed to obtain speech understanding after cochlear implantation (CI) in spite of poor speech discrimination before surgery. From these clinical observations and review of recent literature, it appears that true retro-cochlear hearing disorder is far less frequent than we thought and that the retro-cochlear pathway may not have an important role in the process of speech recognition, although the cochlea plays an important role in transforming speech sound into electric signals. The main function of the retrocochlear tract may be to detect the direction of the sound source, as the vestibular tract contributes to equilibrium of the body in a reflex manner. Thus, in perception of speech, the cochlea and the auditory cortex play more important roles than the retro-cochlear pathway does. From the clinical points of view, cochlear implantation can be indicated even for patients who have any discrepancy between the result of pure tone audiometry and speech discrimination test.