facial nerve

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The Facial Nerve (Cranial Nerve VII)

Overview and Fiber Types

The facial nerve is the nerve of the second branchial arch and carries four functional fiber types:

Fiber Type	Function	Target
Special visceral efferent (SVE)	Motor to muscles of facial expression	Mimetic muscles, stapedius, stylohyoid, posterior digastric
General visceral efferent (GVE)	Parasympathetic secretomotor	Lacrimal gland, submandibular + sublingual glands, nasal seromucinous glands
Special visceral afferent (SVA)	Taste	Anterior 2/3 of tongue (via chorda tympani), palate + tonsillar fossa (via GSPN)
General somatic afferent (GSA)	Cutaneous sensation + proprioception	External auditory canal, conchal skin, facial muscles

The brainstem nuclei of origin are: motor nucleus (SVE), superior salivatory nucleus (GVE), and nucleus of the solitary tract (SVA/visceral afferent).

(Cummings Otolaryngology, p. 2451)

Course and Segments

The nerve travels through a bony canal - the fallopian canal - from the fundus of the internal auditory canal (IAC) to the stylomastoid foramen. It has five named segments:

1. Intracranial / Cisternal Segment

Exits the brainstem at the pontomedullary junction, travels with CN VIII to enter the internal auditory canal. Within the IAC the nerve lacks a fibrous sheath or endoneurium and is surrounded only by a thin arachnoid layer.

2. Labyrinthine Segment

First, shortest, and narrowest segment of the fallopian canal
Travels superior to the cochlea
Opens into the geniculate fossa - site of the geniculate ganglion (cell bodies of sensory neurons)
The overlying bone is dehiscent in ~25% of ears
Most vulnerable to injury because: (a) narrowest lumen, (b) lacks epineurium, (c) is a watershed zone between the vertebrobasilar (labyrinthine a.) and external carotid (petrosal branch of middle meningeal a.) systems
Surgical landmark: Bill's bar (vertical crest)

3. Tympanic (Horizontal) Segment

Begins at the first genu where the nerve turns acutely posterior and slightly inferior at the geniculate ganglion
Runs along the medial wall of the anterior attic, over the cochleariform process, and forms the superior wall of the oval window niche
Most common site of congenital bony dehiscence, especially above the oval window
Surgical landmarks: cochleariform process (transmastoid), supratubal recess

4. Mastoid (Vertical) Segment

Begins at the second genu at the pyramidal eminence (anteroinferior to the lateral SCC)
Gives off the chorda tympani (taste, anterior 2/3 tongue; parasympathetics to submandibular/sublingual glands) and stapedial branch at variable points
The space between the mastoid segment and chorda tympani forms the facial recess (posterior tympanotomy)
Surgical landmarks: pyramidal eminence, short process of incus, lateral SCC cortex, chorda tympani nerve

5. Extratemporal Segment

Exits via the stylomastoid foramen (receives blood from the aponeurosis of the posterior digastric - this vascular relationship should be preserved during nerve rerouting)
Enters the parotid gland, divides into upper and lower trunks
Further branches and anastomoses within the parotid

(Cummings Otolaryngology, pp. 2450-2452)

Five Terminal Branches (TZBMC)

After exiting the parotid gland:

Branch	Muscles Supplied
Temporal	Frontalis, orbicularis oculi (upper), corrugator
Zygomatic	Orbicularis oculi (lower), zygomaticus
Buccal	Buccinator, orbicularis oris, muscles of upper lip
Marginal Mandibular	Depressors of lower lip and chin
Cervical	Platysma

The buccal branch closely accompanies the parotid duct (surface marking: tragus to midpoint between upper lip and alar base). Injuries lateral to a vertical line through the lateral canthus are surgically repairable.

(Gray's Anatomy for Students, p. 1052; Bailey & Love, p. 430)

Named Branches Within the Temporal Bone

Greater superficial petrosal nerve (GSPN) - from geniculate ganglion; preganglionic parasympathetics to pterygopalatine ganglion → lacrimal gland + nasal glands; also carries taste from palate
Nerve to stapedius - from mastoid segment; dysfunction causes hyperacusis
Chorda tympani - from mastoid segment; taste anterior 2/3 tongue + preganglionic parasympathetics to submandibular ganglion → submandibular and sublingual glands

Surgical Landmarks by Segment

Segment	Landmark
Labyrinthine	Vertical crest (Bill's bar)
Geniculate ganglion	Retrograde dissection of GSPN (middle fossa approach)
Tympanic	Cochleariform process; supratubal recess
Second genu	Oval window
Mastoid	Pyramidal eminence; bisects lateral SCC; short process of incus; chorda tympani
Stylomastoid foramen	Cephalic edge + aponeurosis of posterior digastric

(Cummings Otolaryngology, Table 126.1)

Clinical Disorders

Bell Palsy (Idiopathic Facial Paralysis)

Incidence: 23-37 per 100,000/year; peaks >65 years (59/100,000); bilateral in only 0.3%
Diagnosis requires: (1) paresis of all muscle groups on one side, (2) sudden onset, (3) no CNS signs, (4) no ear/CPA disease
Etiology: Reactivation of latent HSV-1 in the geniculate ganglion (best current evidence); HSV-1 DNA found in both trigeminal and geniculate ganglia at autopsy
Pathology: Diffuse demyelination throughout intratemporal course; most severe at the labyrinthine segment and meatal foramen (Wallerian degeneration)
Treatment: Corticosteroids (mainstay) ± antivirals
Progressive facial paralysis is NOT Bell palsy - always image to exclude neoplasm

Ramsay Hunt Syndrome (Herpes Zoster Oticus)

Caused by varicella zoster virus (VZV) reactivation
Presents with facial palsy + vesicles in the ear canal/auricle (+ sometimes sensorineural hearing loss, vertigo)
Treat with steroids + acyclovir

Differential Diagnosis of Facial Paralysis

Acute onset: Bell palsy, herpes zoster, Guillain-Barré, Lyme disease, otitis media (suppurative or cholesteatoma), trauma

Chronic/progressive: Facial neuroma (schwannoma), parotid malignancy with perineural invasion, cholesteatoma, meningioma

Bilateral facial paralysis suggests metabolic, autoimmune, or infectious disease (including Lyme disease).

Management Algorithm for Facial Paralysis

Fig. 172.1 - Management algorithm. Partial palsy: observe ± steroids; if progressive, image. Complete palsy: start steroids (+ acyclovir if vesicles present), then ENoG - if ≥95% degeneration, consider surgical decompression. Posttraumatic: ENoG within 21 days; CT if severe fallopian canal disruption, consider surgical exploration. (Cummings Otolaryngology)

Electrodiagnostic Testing

Electroneuronography (ENoG): Compares compound action potential amplitudes side-to-side; ≥95% degeneration within 14 days of onset → threshold for considering decompression surgery
Topodiagnostic tests: Schirmer test (lacrimal = GSPN), stapedial reflex (nerve to stapedius), taste testing / submandibular flow (chorda tympani) - useful for localizing level of lesion but accuracy is limited

Grading of Facial Palsy

The House-Brackmann scale (Grades I-VI) is the standard clinical grading system. Key parameters include absolute movement, eye closure, resting asymmetry, and degree of synkinesis. Progressive or recurrent palsy should prompt CT/gadolinium-MRI of the entire facial nerve from brainstem to parotid.

Key Vulnerabilities Summary

Labyrinthine segment: Narrowest, no epineurium, watershed blood supply - most susceptible to inflammatory compression (Bell palsy, trauma)
Geniculate ganglion: Thin/dehiscent bone, tethered by GSPN, in path of temporal bone fractures
Tympanic segment: Most common site of congenital dehiscence; exposed by cholesteatoma
Mastoid segment: Variable course especially with congenital anomalies; at risk in postauricular approaches
Stylomastoid foramen: Superficially placed in neonates (underdeveloped mastoid tip) - vulnerable to forceps injury

(Sources: Cummings Otolaryngology Head and Neck Surgery; Gray's Anatomy for Students; Bailey and Love's Short Practice of Surgery, 28th ed.)

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