Quantitative analysis of cochlear sensory cells and neuronal elements in man.

The myelinated radial fibres in the osseous spiral lamina and the myelinated fibres in the cochlear nerve in the internal auditory canal as well as the sensory cells were counted in cochleae from 15 dissected temporal bones from 8 patients. Light microscopy was carried out on semithin sections of epoxy resin embedded tissue. Audiometry had been performed within 6 months prior to death. Three patients had normal hearing for their age group, 2 had slight presbyacusis and the remaining 3 had sustained noise injury. All specimens clearly had fewer fibres in the spiral lamina than in the internal auditory canal. The cochleae from patients with normal hearing for their age group had a difference in the nerve fibre counts of up to 34%. A case of sensorineural presbyacusis showed 31%, and a case of neural presbyacusis, 47% difference. The greatest difference was found in a case of acoustic trauma, the range in this group being between 25% and 55%. The lower the number of fibres in the spiral lamina, the greater was the difference in all but two specimens. A slow retrograde degeneration, i.e. beginning in the peripheral process of the cochlear nerve, could be an explanation for these findings.