Length of the Narrow Bony Channels May Not be the Sole Cause of Differential Involvement of the Nerves in Vestibular Neuritis

Objective: To measure the vestibular nerve bony channels, applying a 3D measurement to account for the oblique trajectory of the singular nerve.

Background: The clinical syndrome vestibular neuritis affects structures innervated by the superior vestibular nerve more commonly than the inferior vestibular nerve. Anatomical differences such as a longer, narrower bony channel of the superior vestibular nerve may increase its susceptibility to entrapment.

Main outcome measures: Length of the narrow segment of each vestibular nerve in which the nerve occupies more than 80% of the bony channel was measured.

Results: Forty six normal ears sectioned in the axial plane were measured. The narrow channel for the lateral semicircular canal (SCC, mean [SD] 2.94 ± 0.54) mm was longer than that of the singular nerve innervating the posterior SCC (1.95 ± 0.58 mm [p < 0.0001]), which also exceeded that of the utricular nerve (1.45 ± 0.36 mm [p < 0.0001]). The nerve to the superior part of the saccule (i.e., Voit's nerve) was 1.14 ± 0.48 mm and that of the inferior saccule was 0.52 ± 0.37 mm.

Conclusions: The length of the narrow bony channel for the singular nerve is longer than previously reported and exceeds the utricular nerve. Comparing these data with the frequency of clinical lesions in recent literature suggests that, although bony channel length may contribute to differential involvement of the vestibular nerves, other factors may increase susceptibility of the superior vestibular nerve, including redundancy in innervation of the saccule and posterior SCC and anastomoses between the facial nerve and the superior vestibular nerve through which reactivated herpes virus may spread.