Topography of vestibulocochlear nerve fibers in the posterior cranial fossa

Abstract

In the posterior cranial fossa, the separation of the vestibular and cochlear subdivisions of the eighth nerve is clear in about 75% of cases. Although in the remaining cases the vestibulocochlear cleavage plane is not visible macroscopically, the large-fibered vestibular subdivision and the small-fibered cochlear subdivision are well recognized in histological sections. Within the cochlear subdivision, some fibers of large caliber are mixed with fibers of small caliber in a region we named the "overlapping zone." We studied, histologically, cross sections of the intracranial portion of six eighth nerves at about 5 mm proximal to the porus acusticus. A computerized video system was used to measure the diameters of the fibers of the vestibular and cochlear subdivisions localized at different distances from the vestibulocochlear cleavage plane. The overlapping zone is located within the cochlear subdivision adjacent to the vestibulocochlear cleavage plane. It has a pear-like shape, with the larger part occupying the anterosuperior part of the cochlear subdivision. The mean cross-sectional area of this zone in our six samples is about 0.4 mm2, which is approximately 23% of the area of the cochlear subdivision. The thickness of the zone in the superior-inferior direction ranges from 0.23 mm to 0.55 mm. The parameters of the described overlapping zone should be taken into consideration in vestibular neurectomy, in which complete sectioning of the vestibular fibers is important.