The Unrecognized Rotation of the Vestibular and Cochlear Nerves from the Labyrinth to the Brain Stem: Its Implications to Surgery of the Eighth Cranial Nerve

Abstract

The cochlear and vestibular nerves rotate 90 degrees from the inner ear to the brain stem. Most of the rotation occurs within the internal auditory canal (IAC); only minimal rotation occurs in the cerebellopontine (CP) angle. At the labyrinthine end of the IAC, the cochlear nerve--which at first lies anterior to the inferior vestibular nerve (saccular nerve)--rapidly fuses with the inferior vestibular nerve. It then rotates to become inferior as the nerves leave the porus acousticus. The cochleovestibular (C-V) cleavage plane lies in a superior-inferior direction in the lateral IAC and rotates to become anterior-posterior in the CP angle. In 25% of patients in whom no C-V cleavage plane can be seen, it is not possible to completely transect all vestibular fibers. The surgical implications are that the most complete vestibular neurectomy can be done only in the lateral IAC, the cochlear and inferior vestibular nerves, because of their intimate association, should not be separated in the mid-IAC, in order to prevent damage to the cochlear nerve, and to create a complete denervation of the vestibular labyrinth, only the posterior ampullary nerve along with the superior vestibular nerve should be transected.

Functional Results and Quality of Life After Retrosigmoid Vestibular Neurectomy in Patients With Ménière’s Disease

“…Despite the fact that Silverstein shows that approximately 25% of VIIIth nerves cannot be split specific to separate all vestibular nerve fibers accurately without injuring cochlear fibers, symptomatic relief of sufficient character seems to occur at least as often as 75% of the time (14).…”

“…The boundary with the posterior location of the cochlear nerve, which is initially difficult to recognize, is progressively identified using a color difference of the cochlear and vestibular partitions. A slight color difference may be a result of fiber size differences (14). The last fibers of the vestibular nerve are severed with the angled 0.3-mm microhook until the 2 ends of the vestibular nerve are clearly retracted.…”

Functional Anatomy of the Human Cochlear Nerve and Its Role in Microvascular Decompressions for Tinnitus

“…This results in a 90-degree rotation (54). This anatomic knowledge is important because it affects the tonotopic structure of the cochlear nerve.…”

Microanatomy of the cranial segment of the vestibulocochlear nerve. Possible correlations with the symptoms of neurovascular compression syndrome

“…вклейке). Это наблюдение не согласуется с данными других исследователей, отмечавших в этом отделе ПУН нижнее и даже нижнезаднее расположение КН [1,3,5,16,18,19].…”