Introduction
The facial nerve provides motor innervation to the muscles of facial expression. The chorda tympani branch supplies taste to the anterior two-thirds of the tongue. The facial nerve also carries parasympathetic innervation to the lacrimal glands, sinuses and the nasal cavity.
Injury to the facial nerve causes ipsilateral facial weakness. There is flattening of the nasolabial fold and drooping of the corners of the mouth. The patient may complain of hyperacusis and a decreased sense of taste.
If the damage is peripheral (lower VII), the forehead will be involved and there will be an inability to close the eye.
In central lesions, there is sparing of the forehead muscles (frontalis and orbicularis oculi) as these are innervated bilaterally.
The nerve leaves the brainstem between the pons and medulla. The motor part travels in the facial canal in the petrous part of the temporal bone close to inner ear. From here arise two branches: the nerve to stapedius and the chorda tympani.
The nerve leaves the skull at the stylomastoid foramen between the styloid and mastoid processes of the temporal bone then travels through the parotid gland, dividing into 5 branches as it leaves (temporal, zygomatic, buccal, marginal mandibular, cervical).
Test facial movements by asking the patient to:
- “Screw up your eyes”
- “Raise your eyebrows”
- “Show me your teeth”
Test taste to the anterior two-thirds of the tongue (seldom necessary).
Look for evidence of herpes in the external ear.
Upper motor neurone lesions of the CN VII (forehead sparing) require a CT head to exclude cerebrovascular events and other intracranial causes such as tumours, particularly cerebellopontine angle tumours.
A lower motor neurone lesion in the absence of any other clinical features or trauma is likely to be a Bell’s palsy. This does not usually need a CT scan, however, infections such as mumps, herpes zoster and measles should be considered.
Again, CT head following trauma will look for fractures of the petrous temporal bone.
Bell’s palsy is an idiopathic condition, which causes unilateral facial weakness with inability to close the eye. Possible aetiologies include viral infection and vascular compression as the nerve leaves the stylomastoid foramen.
Ramsay Hunt syndrome is a herpes zoster infection of CN VII motor ganglion. It causes a lower VII palsy associated with soft palate lesions, and taste disturbance.
Guillain Barre syndrome (GBS) can cause bilateral facial palsies, as can botulism.
Infections such as mumps, measles, chickenpox, otitis externa and media, encephalitis and mastoiditis can all cause a facial palsy.
Neoplastic causes include cerebellopontine angle tumours (see CN VIII) and parotid tumours.
Fractures of the petrous temporal bone cause damage to CN VII and VIII as they travel in the facial canal.
There may be an associated cerebrospinal fluid leak from the ear and a ruptured tympanic membrane.
The facial nerve can be injured after it exits the skull, either by blunt or penetrating trauma to the face or during parotid surgery.
Penetrating injury to the middle ear or barotrauma (scuba diving, altitude) can cause a facial nerve palsy.