Facial nerve (VII) lesions - Disorders of cranial nerves

Facial nerve (VII) lesions

Anatomy

The facial nerve has motor and sensory components. The motor nerve cell bodies are in the facial nerve nucleus in the pons. The nerve enters the internal auditory meatus and passes laterally within the petrous temporal bone to the medial wall of the middle ear. Here the sensory nerve cell bodies cause a swelling called the geniculate ganglion and give off the nerve to stapedius and chorda tympani (taste and lacrimation) before exiting the skull through the stylomastoid foramen.

Each facial nucleus supplying the forehead muscle (frontalis) receives some innervation from each hemisphere, so that unilateral upper motor neurone lesions cause sparing of the forehead, whereas unilateral lower motor neurone lesions cause forehead involvement.

Function

Muscles of facial expression and taste of the anterior two third of the tongue.

Specific causes

·        Lower motor neurone (all of one half of the face affected) – Bell’s palsy, herpes zoster, polio, otitis media, skull fracture, parotid tumours.

·        Upper motor neurone (forehead spared) – stroke, tumours.

Clinical features

The features of facial nerve palsy depend on the level of the lesion. If both lacrimation and taste are unimpaired, the lesion is below the stylomastoid foramen. Hyperacusis (hearing sounds louder than normal) suggests a lesion proximal to the stapedial branch.