Degeneration pattern of human first-order cochlear neurons.

In the present study, quantitative analysis of the cochlear neurons in the osseous spiral lamina, the modiolus and the internal auditory canal of the same cochlea was performed. Forty-five temporal bones were obtained from 25 patients and prepared by means of microdissection. Ten patients had age-related normal hearing (ARNH) assuming that the 5 children without audiogram had normal hearing. Fifteen patients had sensorineural hearing loss due to various causes. The present study has shown that in young individuals the numbers of cochlear neurons are almost identical at all 3 sites. In patients over 60 years with ARNH, the loss of peripheral nerve processes is always severer than the loss of central nerve processes. This finding suggests that the central processes degenerate at a much slower rate or not at all. Furthermore, 4 different peripheral degeneration patterns were described. The factors responsible for the different degeneration behaviors are still not understood and need further investigation.