Selective retrosigmoid vestibular neurectomy without internal auditory canal drill-out: an anatomic study

It is well established that selective vestibular nerve section by means of the retrosigmoid or posterior fossa approach can be accomplished with or without drill-out of the internal auditory canal (IAC) by virtue of the presence or absence of a surgically accessible cleavage plane between the vestibular and cochlear nerves. Some reports have indicated that a majority of patients would be amenable to successful separation of the vestibular nerve from the cochlear nerve medial to the IAC, thus obviating the need for IAC drill-out and associated complications. However, other reports have indicated routine difficulty in finding a satisfactory vestibulocochlear cleavage plane within the cerebellopontine angle. This in situ cadaver study was undertaken to determine whether normal anatomic relationships support the hypothesis that selective vestibular nerve section can be accomplished by means of the posterior fossa approach without the need for concomitant IAC drill-out in a majority of circumstances.

A retrosigmoid approach to the posterior fossa was performed bilaterally on 36 intact human cadavers. After displacement of the cerebellum, an operating surgical microscope was used to visualize the cerebellopontine angle in the surgical position. The ability to develop a satisfactory cleavage plane between the vestibular and cochlear nerves without the need for drill-out of the IAC was established in each case.

Seventy-two vestibulocochlear nerve bundles in 36 intact human cadavers were analyzed. A vestibulocochlear nerve cleavage plane within the cerebellopontine angle amenable to neurectomy medial to the porus of the IAC was observed in 81% left and 69% right vestibulocochlear nerve bundles (average, 75%). The facial nerve was found deep or anterior to the vestibulocochlear nerve bilaterally in all cases examined. The anterior inferior cerebellar artery, or a branch of the artery, was found to cross the plane between the facial and vestibulocochlear nerve bundles within the lateral cerebellopontine angle in 47% of the cases on the left and in 50% of cases on the right.

A vestibulocochlear nerve cleavage plane amenable for selective vestibular nerve transection without drilling the IAC was found in 75% of the 72 cerebellopontine angles studied. The facial nerve consistently lies deep or anterior to the vestibulocochlear nerve within the cerebellopontine angle with the retrosigmoid approach. These findings support the rational and feasibility of avoiding drill-out of the IAC in the majority of circumstances when performing selective vestibular neurectomy by means of the posterior fossa approach for Ménière's syndrome and other vestibular disorders.