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

Abstract

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, tranversely subscrially 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.

The extracisternal approach in vestibular schwannoma surgery and facial nerve preservation

“…However, the cisternal arachnoid membranes anatomy and its relationship with cranial nerves are controversial, especially because of the difficulty preserving the membranes on cadaveric specimens. The epiarachnoid or subarachnoid origin of VS has been subject of discussion and the presence of an arachnoid surgical plane between tumor and cranial nerves questioned by some authors 17,18,19,20,21,22,23,24,25 . This is not what we have experienced during the surgical procedures, where the arachnoid membrane is used as a protection to preserve the facial nerve anatomically and functionally.…”

An update on unilateral sporadic small vestibular schwannoma

“…44 The location of the tumor's origin is described as the neurilemmal-neuroglial junction (Obersteiner-Redlich zone) within the internal auditory canal. 48,67 Considering the origin of the tumor, the most obvious hypothesis for hearing loss is the mechanical and/or neurotoxic effect that a growing tumor might have on its neighboring neurovascular structures. 57,81 The data, however, do not always show a linear correlation between the rate of tumor growth and deterioration of hearing.…”

Neuroanatomical correlation of the House-Brackmann grading system in the microsurgical treatment of vestibular schwannoma

“…However, evidence from histological studies of the interface between a VS and the cochlear nerve from en bloc-resected VS tissue suggests that no well-defined connective tissue structure exists between the cochlear nerve and tumor tissue. 60,67,68,104,105 The histological data on the facial nerve and tumor interface are much more scarce, however, due to efforts to preserve facial nerve function among surgeons. Nevertheless, studies exist that examine the interface between facial nerve and tumor, and all demonstrate lack of such interface in at least parts of the tumor, if not in all observable parts of the tumor histologically.…”