Microscopic characteristics of the acoustic tumor in relationship of its nerve of origin.

The microscopic characteristics of a 0.9 cm vestibular schwannoma en bloc resected with its nerve of origin which occurred in a 54-year-old white woman presenting with a two-year history of a unilateral progressive sensori-neural hearing loss is described. The tumor originated in the inferior vestibular portion of the vestibular division of the VIIIth cranial nerve just medial to the internal auditory canal meatus at approximately the level of the glial-non-glial junction. The tumor demonstrated two distinctly different, yet simultaneous, modes of involvement with its nerve of origin: 1. inseparable cellular continuity; and 2. peripheral compression of the remainder of the nerve within the tumor capsule. Despite only slight microscopic continuity of the nerve histologically, electronystagmography showed no unilateral weakness on bithermal caloric testing, and pure tone and speech audiometry was only moderately depressed.