Morphometric analysis of the cochlear nerve in man.

Fiber diameters were analyzed in the meatal segment of the cochlear nerve from 7 temporal bones obtained from 7 patients. Two patients had normal hearing for their age. Two had sustained noise exposure and one had presbyacusis of predominantly neural type. The cochleae displayed characteristic degeneration patterns. The other two manifested hearing loss of unspecified type. The fiber diameters ranged from 0.5 to 11 microns. The diameter distribution was unimodal in all seven nerves. The means of the diameters ranged from 4.2 to 5.5 microns. They were significantly different between patients with age-related normal hearing on the one hand and patients with noise induced hearing loss and neural presbyacusis on the other. The findings are discussed in relation to changes in nerve conduction speed and hearing loss; a possible correlation between the fiber diameter distribution and the tonotopical arrangement of the cochlea is suggested.