After entering the internal auditory canal (meatal segment), the facial nerve traverses the temporal bone in three intratemporal segments: the labyrinthine segment (shortest, from IAC fundus to the geniculate ganglion, gives the greater petrosal nerve), the tympanic segment (horizontal, from geniculate to posterior genu above the oval window, gives the nerve to stapedius), and the mastoid segment (vertical, from posterior genu to stylomastoid foramen, gives the chorda tympani). The geniculate ganglion at the first genu contains the cell bodies for taste and lacrimal secretion.
Knowledge of the intratemporal facial nerve segments is essential for temporal bone surgery. The tympanic segment is the most commonly injured in chronic otitis media, cholesteatoma, and temporal bone fractures because it lies exposed in the oval window niche. The labyrinthine segment has the tightest bony canal and is most vulnerable to oedema in Bell palsy. In cochlear implant surgery, the mastoid segment is the posterior limit of the facial recess approach to the round window. Skull base fractures produce facial palsy at different segment levels depending on fracture line location.
Cholesteatoma eroding the tympanic facial nerve canal at the oval window niche produces progressive facial paresis or palsy; CT demonstrates bony dehiscence of the Fallopian canal at this level and surgical canal wall-down mastoidectomy removes the cholesteatoma while the exposed nerve is decompressed and repaired under operating microscope magnification.
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