Anatomic considerations in vestibular neuritis.

HYPOTHESIS The authors believe that anatomic differences render the superior division of the vestibular nerve more susceptible to injury during vestibular neuritis. The purpose of the study was to investigate anatomic differences between the superior vestibular nerve and singular nerve canals. BACKGROUND Previous studies of temporal bones have revealed vestibular nerve degeneration in patients with vestibular neuritis. Although the cause of this degeneration has not been established, it has been noted that the superior division of the vestibular nerve is preferentially affected, with sparing of the inferior division. The superior vestibular nerve and the singular nerve, a branch of the inferior vestibular nerve, both pass through canals interlaced with bony networks before reaching the peripheral receptors. METHODS The authors performed histologic analysis of 40 normal temporal bones randomly selected from their temporal bone library. With a micrometer, measurements were taken of the individual canals. The ratio of the total bony spicule component to the total canal width was obtained for both the superior vestibular nerve and the singular nerve. The length of the canals was also measured. Arteriole:arteriolar canal ratios of the superior vestibular nerve and singular nerve were obtained. RESULTS The bony channel of the singular nerve had an average length of 0.59 mm, and the average length of the superior vestibular nerve was 2.30 mm (p < 0.001). The ratio of total bony spicule width to total canal width was significantly smaller (p < 0.05) for the singular nerve (0.30 mm) compared with the superior vestibular nerve (0.34 mm). The arteriole: arteriolar canal ratio was significantly smaller (p < 0.05) for the singular nerve (0.45 mm) than for the superior vestibular nerve (0.54 mm). CONCLUSION The bony canal of the superior vestibular nerve is longer than the singular nerve canal. Additionally, the superior vestibular nerve and arteriole travel through a relatively narrower passage than the singular nerve and its vascular supply. From an anatomic standpoint, this renders the superior division of the vestibular nerve more susceptible to entrapment and possible ischemic labyrinthine changes.