Gross and microscopic anatomy of the eighth cranial nerve in relationship to the solitary schwannoma.

Surgical advances over the past two decades, allowing the discovery of smaller solitary schwannomas of the VIIIth cranial nerve and their extirpation with preservation of the VIIth cranial nerve, have lead to attempts to resect the tumor without completely resecting its nerve of origin. This paper reports the macroscopic and microscopic observations of 22 solitary schwannomas of the VIIIth cranial nerve as they relate to the nerve of origin. Nine tumors were removed, en bloc with the VIIIth nerve, transversely subserially sectioned and studied by light microscopy. Four of the en bloc cases and 13 additional cases were studied by biopsying the VIIIth nerve branches, lateral, and the VIIIth nerve trunk medial to the tumor. The results suggest that the VIIIth nerve is involved throughout the extent of the tumor and, in some cases, beyond the tumor limits with intraneural invasion by tumor cells. There is an immediate dispersion of VIIIth nerve fibers to extremely small aggregates dispersed peripherally about the tumor surface. These fiber aggregates are not surgically dissectable from the tumor because of the difficulty in identifying the fibers and because of tumor infiltration of normal appearing nerves. The pure tone threshold average of 500-2,000 Hz. tends to correlate more closely with the number of cochlear fibers present, lateral to the tumor, than do other auditory tests. The percent of unilateral vestibular weakness does not correlate with the number of fibers present in the superior vestibular nerve. In summary, these histologic data suggest that complete tumor resection from VIIIth nerve fibers may be beyond our present technological capabilities. They further suggest severe limits to accurate identification of candidates for this conservation surgery.