High freguency hearing loss in due to a decrease in the number of peripheral neurites (dendrites to some), in aging ears, was described in 1933 by Crowe, Guild, and Polvogt in 1933, but received little attention possibly because it was only published in the Bulletin of the Johns Hopkins Hospital. They found 24 cases out of 740 pairs of bones examined. More recently, using electron microscopy, Nadol(1979) and Spondlin and Schrott(1990) each described a similar case.

The difficulty with quantitation of the number of neurites in the osseous spiral lamina may also explain why a decrease in their numbers as a cause of presbycusis has not been investigated. In our laboratory the neurites that innervate the part of the cochlea that is most affected in presbycussis, are cut longitudinally and impossible to count in 20 micron thick sections. Dr Margaret Chen tried using confocal microscopy but the inherent auto-fluorescence of celloidin embedded tissue made accurate indentification of neurites impossible. We have observed, as reported by Crow, Guild, and Polvogt, that as the number of fibers in the fascicles decreases, the fibers coalesce into smaller bundles. We therefor counted the number of fibers in various sized bundles in temporal bones that had been treated with osmium, that preserves the myelin sheeths, and used the conversion factor to estimate the number of fibers in sections in which the fibers were cut longitudinally.

Anterior basal segments of two cochleae, one with reduced neurites on the left, and one with a normal level for comparison.

Cross section of osseous spiral lamina from bone prepared with osmium. Under high power individual fibers can be determined. (400X)

We found a statistically significant decrease in the number of nerve fibers in the anterior basal segment of the cochlea, compared to other segments in older persons with down sloping audiograms.