The muscles of facial expression are controlled by branches of the facial nerve.
Functions of the facial nerve:
- Motor: muscles of facial expression
- Sensory: skin of the external ear
- Special sensory: taste to anterior 2/3 of the tongue
- Autonomic: submandibular and sublingual salivary glands
- Arises at the pontomedullary junction from two roots:
- Motor root
- Nervus intermedius: sensory and visceromotor fibres
- Passes through the cerebellopontine angle in the subarachnoid space next to CN VIII
- Enters the internal auditory meatus
- Passes through the facial canal to the geniculate ganglion
- Within the petrous part of the temporal bone the facial nerve gives off several branches
- Greater petrosal nerve: secretomotor innervation lacrimal glands
- Nerve to stapedius: motor innervation to stapedius
- Chorda tympani: taste fibres to anterior 2/3 of tongue, secretomotor innervation to submandibular and sublingual glands
- The remainder of the facial nerve passes through the stylomastoid foramen, passes through the parotid gland, and divides into terminal branches supplying the muscles of facial expression.
Clinical features of facial nerve lesions
General features of facial nerve lesions:
- Weakness of the muscles of facial expression
- Hyperacusis (due to stapedius weakness)
- Impaired corneal reflex (afferent CN V, efferent CN VII)
- Dry eyes
- Dry mouth
- Special sensory:
- Loss of taste in anterior 2/3 of the tongue
- UMN vs LMN:
- An UMN lesion refers to a lesion affecting the motor outflow of the 7th nerve at any level above the primary motor neuron.
- Muscles of the forehead and eye closure (frontalis and orbicularis oculi) receive extensive bilateral innervation from both cerebral hemispheres. This is logical – eye closure in anticipation of noxious stimuli is essential to protect the eyes. Bilateral innervation of these muscles can be thought of as an evolutionary safety net.
- LMN lesions of the 7th nerve lead to ipsilateral weakness of the entire face.
- UMN lesions affect only the lower face due to contralateral innervation.
- Other cranial neuropathies: the presence of an accompanying ipsilateral CN VIII +/- CN V palsy implies a cerebellopontine angle lesion
Differential diagnosis of a facial nerve lesion
Causes can be classified according to where they affect the nerve.
- Affecting the whole length of the nerve
- Bell’s palsy
- Guillan-Barre Syndrome
- Herpes Zoster (Ramsay-Hunt syndrome)
- Lyme disease
- Cerebellopontine angle
- Vestibular schwannoma
- Middle ear
- Otitis media
- Parotid gland
- Parotid tumour