Vestibular nerve compression in eighth-nerve tumors.

The superior vestibular nerve was studied histologically in 25 patients who underwent removal of inferior vestibular nerve schwannoma. In most cases, the nerve fibers were structurally normal but the endoneurial space showed various degrees of capillary stasis and/or extravasation of red blood cells. Increased fibrosis of the endoneurial space was seen in about two-thirds of the cases; it was mild in all except two cases. The endoneurial space appeared edematic in about one-third of the cases. Wallerian degeneration of individual fibers was occasionally observed in most cases and was severe in the two cases with moderate or severe endoneurial fibrosis. In about one-third of the cases there were large numbers of myelinated nerve fibers with thinned myelin sheaths. The observed structural changes are compatible with those seen in human and experimentally induced chronic compressive or entrapment neuropathies. Thus, early symptoms of eighth-nerve tumors might develop due to direct compression of the eighth-nerve trunk within the internal auditory canal.