The precise pathophysiology of Bell palsy remains an area of debate. The facial nerve courses through a portion of the temporal bone commonly referred to as the facial canal. A popular theory proposes that edema and ischemia result in compression of the facial nerve within this bony canal. The cause of the edema and ischemia has not yet been established. This compression has been seen in MRI scans with facial nerve enhancement.[14]
The first portion of the facial canal, the labyrinthine segment, is the narrowest; the meatal foramen in this segment has a diameter of only about 0.66 mm. This is the location that is thought to be the most common site of compression of the facial nerve in Bell palsy. Given the tight confines of the facial canal, it seems logical that inflammatory, demyelinating, ischemic, or compressive processes may impair neural conduction at this site.
Injury to the facial nerve in Bell palsy is peripheral to the nerve’s nucleus. The injury is thought to occur near, or at, the geniculate ganglion. If the lesion is proximal to the geniculate ganglion, the motor paralysis is accompanied by gustatory and autonomic abnormalities. Lesions between the geniculate ganglion and the origin of the chorda tympani produce the same effect, except that they spare lacrimation. If the lesion is at the stylomastoid foramen, it may result in facial paralysis only.