Facial nerve

The facial nerve is one of the key cranial nerves with a complex and broad range of functions.

Although at first glance it is the motor nerve of facial expression which begins as a trunk and emerges from the parotid gland as five branches (see facial nerve branches mnemonic here), it has taste and parasympathetic fibersfibres that relay in a complex manner.

The facial nerve is the only cranial nerve that may show normal post-contrast enhancement, although this applies only to the labyrinthine segment up to the stylomastoid foramen. Enhancement of the other segments of the facial nerve and other cranial nerves is considered pathologic. Refer to normal facial nerve enhancement on MRI for more information.

Gross anatomy

Nuclei and brainstem tracts

Three brainstem nuclei contribute to the facial nerve: one motor, one secretomotor, and one sensory:

• facial nerve nucleus in the pons; as the motor fibres of the facial nerve loop posteriorly over the abducens nerve nucleus, they form the facial colliculus in the floor of fourth ventricle
• superior salivary nucleus, next to the facial nucleus supplies secretomotor parasympathetic fibres
• nucleus of tractus solitarius, lateral to the dorsal nucleus of the vagus nerve, supplies taste fibres that eventually end up in the chorda tympani

General course

The facial nerve has six named segments (see (facial nerve segments mnemonic):

1. intracranial (cisternal) segment - zero branches
2. meatal (canalicular) segment (internal auditory canal): 8 mm long, zero branches
3. labyrinthine segment (IAC to geniculate ganglion): 3-4 mm long, 3 branches (from geniculate ganglion)
4. tympanic segment (from geniculate ganglion to pyramidal eminence): 8-11 mm long, zero branches
5. mastoid segment (from pyramidal eminence to stylomastoid foramen): 8-14 mm long, 3 branches
6. extratemporal segment (from stylomastoid foramen to division into major branches): 15-20 mm, 9 branches

Intracranial (cisternal) segment

The nerve emerges immediately beneath the pons, lateral to the abducens nerve and medial to the vestibulocochlear nerve and is joined by the nervus intermedius, which has emerged lateral to the main trunk. Together the two travel laterally through the cerebellopontine angle to the internal acoustic meatus. The cisternal segment has no branches.

Meatal (canalicular) segment

Having been joined by the nervus intermedius, they are located in the anterior superior quadrant of the internal auditory canal, above the falciform crest and anterior to Bill's bar. The meatal segment has no branches.

Labyrinthine segment

As the facial nerve and nervus intermedius pass through the anterior superior quadrant of the internal acoustic meatus it enters the Fallopian canal, passing anterolaterally between and superior to the cochlea (anterior) and vestibule (posterior), and then runs bends posteriorly (the anterior or first genu) at the geniculate ganglion (where the nervus intermedius joins the facial nerve and where fibers for taste synapse - see function below). It is here that three branches originate:

• greater superficial petrosal nerve
• lesser petrosal nerve
• external petrosal nerve

The labyrinthine segment is the shortest only measuring 3-4 mm. It is also the narrowest and the most susceptible to vascular compromise (see blood supply below).

Tympanic segment

As the nerve passes posteriorly from the geniculate ganglion it becomes the tympanic segment (8-11 mm in length) and is immediately beneath the lateral semicircular canal in the medial wall of the middle ear cavity. The bone of the Fallopian canal is often dehiscent in the area of the oval window in 25-55% of postmortem specimens, having mucosa in direct contact with the nerve. The nerve passes posterior to the cochleariform process, tensor tympani and oval window. Just distal to the pyramidal eminence the nerve makes a second turn (posterior or second genu) passing vertically downwards as the mastoid segment. The tympanic segment has no branches.

Mastoid segment

The mastoid segment, measuring 8-14 mm in length, extends from the posterior genu to the stylomastoid foramen, through what is confusingly referred to as the Fallopian canal. It runs in the medial wall of the auditusaditus ad antrum of the mastoid antrum. It gives off three branches:

• nerve to stapedius
• chorda tympani: terminal branch of the nervus intermedius carrying both secretomotor fibres to the submandibular gland and sublingual gland and taste to the anterior two thirds of the tongue
• nerve from the auricular branch of the vagus nerve (CN X): pain fibersfibres to the posterior part of the external acoustic meatus hitchhike from the jugular foramen

Extra-temporal segment

As the nerve exits the stylomastoid foramen, it gives off a sensory branch that supplies part of the external acoustic meatus and tympanic membrane. It then passes between the posterior belly of the digastric muscle and the stylohyoid muscle, supplying both and the enters the parotid gland. Lying between the deep and superficial lobes of the gland the nerve divides into two main branches at the pes anserinus (Latin: duck foot) - superior temporofacial and inferior cervicofacial branches. From the anterior border of the gland, five branches emerge (from superior to inferior):

• temporal (which should not be confused with the deep temporal nerves of the mandibular division of the  trigeminal nerve)
• zygomatic (which should not be confused with the zygomatic nerve of the maxillary division of the  trigeminal nerve)
• buccal (which should not be confused with the buccal nerve of the mandibular division of the trigeminal nerve)
• mandibular (marginal) (which should not be confused with the mandibular division of the trigeminal nerve)
• cervical

See facial nerve branches mnemonic here.

The temporal branch runs with the superficial musculoaponeurotic system (SMAS) over the zygomatic arch. This branch is at risk during surgery in this region. To avoid damage procedures should be deep to the SMAS (e.g. zygomatic fracture repairs).

The mandibular branch, in 80% of cases, runs along the lower border of the mandible (thus also referred to as marginal branch). In 20% of cases however it can be up to 2 cm below the margin of the mandible. It is crucial to be aware of this if surgery in the submandibular region is being performed. Injury to this branch will result in paralysis of mouth depressors.

Function

Taste

Taste fibersfibres to the anterior two thirds of the tongue originate in the nucleus of the tractus solitarius (NTS), travel in the nervus intermedius (preganglionic) where they join the facial nerve at the geniculate ganglion and synapse. Postganglionic fibersfibres travel with the facial nerve and are given off as the chorda tympani, which eventually joins the lingual nerve (branch of the trigeminal nerve (CN V)).

Parasympathetic

Preganglionic fibres originate in the superior salivary nucleus and join the facial nerve at the geniculate ganglion having travelled with the nervus intermedius. They do not synapse in the ganglion, but rather pass through to be distributed between:

1. greater superficial petrosal nerve anastomosing in the pterygopalatine ganglion and supplying the lacrimal gland
2. lesser petrosal nerve anastomosing in the otic ganglion and supplying the parotid gland (along with fibersfibres from the inferior salivary nucleus which arrive via the glossopharyngeal nerve (CN IX) supply to the tympanic plexus)
3. chorda tympani anastomosing in the submandibular ganglion and supplying the submandibular gland and sublingual gland

Somatic motor

The muscles of facial expression are supplied by the branches of the terminal facial nerve.

Blood supply

The facial nerve receives its arterial supply from three main sources:

1. labyrinthine artery: a branch of the AICA, supplies the meatal segment. It is supplemented by direct twigs in the cisternal position, directly from the AICA
2. superficial petrosal artery: a branch of the middle meningeal artery which passes retrogradely along the greater superficial petrosal nerve
3. stylomastoid artery: a branch of the occipital artery, which passes retrogradely into the stylomastoid foramen

The labyrinthine segment is the most vulnerable to ischaemia as the connections between the labyrinthine artery and superficial petrosal artery are very tenuous, with each artery essentially being end arteries.

Related pathology

Disruption or compromise of the nerve leads to facial palsy.

Bilateral conditions

• Melkersson syndrome
• Möbius syndrome
• Guillain-Barré syndrome
• leprosy
• HIV infection related facial nerve palsy: may precede seroconversion

Unilateral conditions

• congenital facial palsy
• Bell palsy
• sarcoidosis & other granulomatous disorders
• infection
  • leprosy: especially with paralysis of upper face
  • chronic otomastoiditis
  • Lyme disease
  • HIV infection related facial nerve palsy
  • Herpes zoster (Ramsay Hunt syndrome)
• neoplasm(s) and masses
  • facial nerve schwannoma
  • facial nerve haemangioma
  • facial nerve choristoma
  • CNS lesions
  • perineural spread especially from head and neck SCC and adenoid cystic carcinoma of the parotid gland
  • compression from vestibular schwannoma
• trauma: especially temporal bone fractures
• cardiofacial syndrome: typically lower lip or complete facial palsy
• familial facial palsy