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Segments of the Facial Nerve

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1. Intracranial (Cisternal) Segment — 17–24 mm

• Runs from the pontomedullary junction to the internal auditory canal (IAC) fundus.
• Travels through the cerebellopontine angle (CPA) cistern alongside CN VIII (vestibulocochlear nerve).
• Accompanied by the nervus intermedius, which carries parasympathetic, taste, and somatic sensory fibers.
• Sub-segments at the root exit zone (intraoperative classification): Root Exit Zone (RExZ) → Attached Segment (AS) → Root Detachment Point (RDP, Obersteiner-Redlich zone) → Cisternal Portion (CP).

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2. Meatal (IAC) Segment — 8–10 mm

• Runs from the fundus of the IAC in its anterosuperior quadrant to the meatal foramen (porus acusticus internus).
• The nerve has no fibrous sheath or endoneurium here; surrounded only by a thin arachnoid layer.
• Surgical landmark: Vertical crest (Bill's bar) separates the facial nerve from the superior vestibular nerve.

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3. Labyrinthine Segment — ~4 mm (shortest & narrowest)

• From the meatal foramen to the geniculate ganglion.
• This is the narrowest part of the fallopian (bony) canal — the nerve is most susceptible to compression and ischemia here (inflammatory swelling in Bell palsy, herpes zoster).
• Travels superior to the cochlea.
• Lacks epineurium and associated vascular plexus → vulnerable to injury.
• Blood supply watershed: vertebrobasilar (via AICA labyrinthine branches) meets the external carotid system (petrosal branch of middle meningeal artery).

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4. Tympanic (Horizontal) Segment — ~11 mm

• From the geniculate ganglion (first genu, acute posterior turn) to the second genu (pyramidal eminence level).
• Runs along the medial wall of the middle ear — superior to the oval window, below the lateral semicircular canal.
• The nerve acquires its fibrous sheath at and distal to the geniculate ganglion.
• Most common site of congenital bony dehiscence (above the oval window, in ~25% of ears) → risk of injury during otologic surgery and toxic effects of otitis media.
• Surgical landmarks: cochleariform process, supratubal recess.

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5. Mastoid (Vertical) Segment — ~13 mm

• From the second genu to the stylomastoid foramen.
• Chorda tympani branches off within this segment → carries taste from anterior 2/3 tongue and preganglionic parasympathetics to submandibular/sublingual glands.
• Stapedial branch also arises here.
• The facial recess (posterior tympanotomy) lies between the mastoid segment and the chorda tympani — a critical surgical corridor.
• Most variable segment in anatomy, especially in congenital aural atresia; the nerve runs more anterior/lateral in newborns (underdeveloped mastoid tip → risk during postauricular approaches and forceps delivery).
• Surgical landmarks: pyramidal eminence, short process of incus, prominence of lateral SCC.

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6. Extratemporal Segment

• Exits the stylomastoid foramen → enters the parotid gland.
• Divides into upper and lower trunks within the parotid, then into five terminal branches:

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Surgical Landmarks by Segment

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Key Clinical Points

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