Facial nerve
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Facial Nerve (Cranial Nerve VII)
![Facial Nerve [VII] on the Face - Terminal branches and branches before entering the parotid gland](https://cdn.orris.care/cdss_images/f6e64e76fccc22f60c0b2a31de6757253c3dc155da58063439a61d1e9bbba19c.png)
Overview
The facial nerve (CN VII) is a mixed nerve derived from the second branchial arch. It carries five functional components:
| Component | Type | Function |
|---|---|---|
| Motor (SVE) | Special visceral efferent | Muscles of facial expression, stapedius, stylohyoid, posterior digastric |
| Parasympathetic (GVE) | General visceral efferent | Lacrimal, submandibular, sublingual glands; nasal/palatal mucosa |
| Taste (SVA) | Special visceral afferent | Anterior 2/3 tongue (chorda tympani); soft palate/tonsillar fossa (GSPN) |
| Somatic sensation (GSA) | General somatic afferent | Skin of EAC, concha, part of auricle |
| Visceral afferent (GVA) | Visceral sensory | Mucosa of nose, pharynx, palate |
Brainstem nuclei: motor nucleus (pons), superior salivary nucleus (parasympathetics), nucleus tractus solitarius (taste/visceral afferent). The geniculate ganglion contains the cell bodies of sensory neurons.
- Cummings Otolaryngology Head and Neck Surgery
Course and Segments
1. Intracranial (Cisternal) Segment
Emerges at the pontomedullary junction, travels to the internal acoustic meatus (IAM) alongside CN VIII. Within the IAC the nerve lacks a fibrous sheath and is surrounded only by a thin arachnoid layer.
2. Intratemporal Segments (Fallopian / Facial Canal)
The fallopian canal runs from the fundus of the IAC to the stylomastoid foramen. It has three named segments with key landmarks:
| Segment | Features | Surgical Landmark |
|---|---|---|
| Labyrinthine | Shortest and narrowest; travels superior to cochlea; leads to geniculate fossa | Vertical crest (Bill's bar) |
| Geniculate ganglion | First genu here; GSPN exits anteriorly; dehiscent in ~25% of ears | Middle fossa retrograde dissection of GSPN |
| Tympanic (horizontal) | Medial wall of anterior attic; superior to oval window; most common site of congenital dehiscence | Cochleariform process; oval window |
| Second genu | At pyramidal eminence; nerve turns inferiorly | Oval window |
| Mastoid (vertical) | Most variable segment; stapedius and chorda tympani branch off here | Pyramidal eminence; bisects prominence of lateral SCC; short process of incus |
The nerve exits through the stylomastoid foramen.
- Cummings Otolaryngology Head and Neck Surgery
3. Extratemporal Segment
Immediately after exiting the stylomastoid foramen, the nerve:
- Gives off the posterior auricular nerve (occipitalis, posterior auricular muscle)
- Branches to the posterior belly of digastric and stylohyoid
- Enters the parotid gland, divides into:
- Temporofacial (upper) trunk
- Cervicofacial (lower) trunk
From these two trunks, five terminal branch groups emerge from the borders of the parotid:
| Branch | Motor supply |
|---|---|
| Temporal | Frontalis, upper orbicularis oculi, corrugator supercilii |
| Zygomatic | Lower orbicularis oculi |
| Buccal | Buccinator, orbicularis oris, upper lip muscles |
| Marginal mandibular | Depressors of lower lip and chin |
| Cervical | Platysma |
The branches travel between the SMAS and deep fascia, entering muscles from their lateral underside. Motor nerves of the face run deeper than sensory nerves or axial vasculature.
- Gray's Anatomy for Students; Dermatology 2-Volume Set 5e
Important Branches Within the Temporal Bone
- Greater superficial petrosal nerve (GSPN): Carries preganglionic parasympathetics to the pterygopalatine ganglion → lacrimal gland, nasal/palatal glands. Also taste from soft palate.
- Nerve to stapedius: Arises from mastoid segment; paralysis → hyperacusis.
- Chorda tympani: Arises from mastoid segment; carries taste from anterior 2/3 tongue and preganglionic parasympathetics to submandibular ganglion → submandibular and sublingual glands. Travels with the lingual nerve (V3).
Blood Supply
The labyrinthine segment is a watershed zone between:
- Vertebrobasilar system (labyrinthine branches of AICA)
- External carotid system (petrosal branch of middle meningeal artery + stylomastoid artery from posterior auricular artery)
This makes the labyrinthine segment especially vulnerable to ischemia during edema or trauma.
Clinical Correlations
Upper Motor Neuron (UMN) vs. Lower Motor Neuron (LMN) Palsy
| Feature | Central (UMN) | Peripheral (LMN) |
|---|---|---|
| Weakness distribution | Lower face only | Entire face (upper + lower) |
| Forehead wrinkling | Intact (bilateral cortical representation) | Lost |
| Eyelid closure | Intact | Impaired (lagophthalmos) |
| Taste | Normal | Often impaired (if proximal to chorda tympani) |
| Hyperacusis | No | Possible (if proximal to stapedius branch) |
Central lesion locations:
- Cortex / internal capsule: voluntary movement lost, emotional movement spared
- Extrapyramidal (basal ganglia): emotional movement lost, voluntary intact
- Pons (ipsilateral): CN VI palsy + contralateral hemiparesis (Millard-Gubler syndrome)
Peripheral lesion locations:
- CPA angle: acoustic neuroma, meningioma - gradual onset, CN VIII involvement
- Temporal bone: fracture, cholesteatoma, otitis media, herpes zoster (Ramsay Hunt)
- Parotid: tumor, trauma, surgery
- Wills Eye Manual; Adams and Victor's Principles of Neurology
Bell's Palsy
The most common facial nerve disorder - incidence ~23/100,000/year, equal sex distribution. Cause: HSV type 1 reactivation in the geniculate ganglion (HSV-1 DNA found in endoneurial fluid at surgery, Murakami et al.). Ramsay Hunt syndrome = VZV reactivation → facial palsy + vesicular rash on pinna/palate/tongue + sensorineural hearing loss.
Clinical features:
- Acute onset; 50% reach maximal paralysis within 48 h; practically all within 3-4 days
- Retroauricular pain may precede by 1-2 days
- Impaired taste (indicates lesion proximal to chorda tympani junction)
- Hyperacusis (indicates lesion proximal to stapedius branch)
- Epiphora or dry eye
Prognosis: 70% recover completely within 1-2 months; 85% near-normal function. Early return of taste (first week) and early motor recovery (days 5-7) are favorable signs. Gadolinium-enhanced MRI shows abnormal enhancement; more intense enhancement = worse prognosis.
- Adams and Victor's Principles of Neurology, 12th Edition
Surgical Dangers
- Parotidectomy - the nerve's intimate relationship with the parotid makes it the most at-risk structure
- Temporal bone fractures - geniculate ganglion tethered by GSPN; labyrinthine segment vulnerable to compression
- Children < 5 years - mastoid tip undeveloped, so the nerve trunk is superficial behind the earlobe
- Facial lacerations - injuries lateral to a vertical line from the lateral canthus are surgically repairable
- Buccal branch - closely follows the parotid duct and may be co-injured
- Bailey and Love's Surgery; Dermatology 2-Volume Set 5e
Synkinesis (Aberrant Regeneration)
After injury and recovery, misdirected regrowth causes simultaneous movements of unrelated muscles - e.g., corner of mouth contracts when eye closes, or excessive lacrimation when eating ("crocodile tears" = aberrant reinnervation of lacrimal gland by fibers originally destined for salivary glands).
Grading of Facial Palsy
The House-Brackmann scale (I-VI) is the standard clinical grading system, where I = normal and VI = complete paralysis with no movement.
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