Eighth nerve in acoustic neuromas. Special reference to superior vestibular nerve function and histopathology.

Eighth nerve specimens were removed during acoustic neuroma surgery, and findings were related to cochlear and especially to superior vestibular nerve function as studied by conventional caloric testing in 21 cases. The origin of the tumor was in eight cases the inferior and in two cases the superior vestibular nerve, and in eight cases the vestibular nerve without more specific localization. In three advanced cases the exact site of origin could not be determined. Leaving age-related changes out of account, the utricle and horizontal canal ampulla appeared normal in all ears except one in which the ampulla was replaced by tumor tissue. In 11 ears the superior vestibular nerve was histologically intact and three of these ears also showed normal caloric responses. In seven ears there was a canal paresis, and in three no reaction was obtained. The ten ears with partial or total nerve invasion by tumor either showed a weak reaction or no response at all. It is likely that an early conduction block arises in both cochlear and vestibular nerves, and reduced reactions to appropriate stimuli do not warrant conclusions that the numbers of anatomically intact nerve fibers possibly are reduced.