Distance from the labyrinthine portion of the facial nerve to the basal turn of the cochlea. Temporal bone histopathologic study.

The middle cranial fossa approach to lesions of the geniculate ganglion and internal auditory canal preserves cochlear function and affords access to the lateral internal auditory canal. The labyrinthine portion of the facial nerve tends to course near the basal turn of the cochlea, just beneath the middle cranial fossa floor, and is usually dissected in this approach. To determine the distance from the labyrinthine portion of the facial nerve to the basal turn of the cochlea, measurements were obtained in the temporal bones of 24 subjects (48 ears) 9 to 76 years of age. These subjects had no history of facial nerve or ear disease, and had normal audiograms. The distances ranged from 0.06 to 0.80 mm, with 21 of 24 right ears (87.5%) showing distances less than the standard size of the smallest diamond drills (0.6 mm), and 18 of 24 (75%) less than 0.5 mm. Incidental note is made of the distance from the geniculate ganglion to the ampulla of the superior semicircular canal, which ranged from 2.06 to 4.88 mm in the 48 specimens. These measurements can serve as guidelines for the surgeon working in the middle cranial fossa.