25th June 2023

CN VII – Facial Nerve

The facial nerve is a mixed nerve that travels through the internal acoustic meatus and splits into five major branches (frontal, zygomatic, buccal, marginal mandibular, and cervical) within the parotid gland, which then go on to supply the motor functions of facial expression, autonomic impulses to lacrimal and salivary glands, stapedius muscle innervation that protects against loud noises and sensory function to the anterior two-thirds of the tongue.

Disorders of the CN VII lower motor nucleus will result in an ipsilateral facial asymmetry, with absence of transverse wrinkles of the forehead, loss of nasolabial fold, drooped corner of the mouth and loss of tone of the orbicularis oculi leading to eversion of the lower eyelid on the affected side. There is also loss of the efferent part of the conjunctival reflex, leading to uncontrolled tearing and inability to close the eye, which can result in eventual corneal ulceration. Bell’s Palsy is a classic example of an ipsilateral lower motor neuron CN 7 disorder.

Contralateral and bilateral innervation

CN VII demonstrates the effects of bilateral innervation from upper motor neuron sources as it receives stimulation from both the ipsilateral and the contralateral corticobulbar tracts.

The upper muscles of facial expression receive stimulation from both the ipsilateral and the contralateral corticobulbar tracts while the lower muscles receive stimulation only from the contralateral corticobulbar tract. A pathology disrupting this stimulation, e.g. an upper motor neuron lesion such as stroke, would cause loss of function of the contralateral lower muscles of facial expression but sparing of the upper muscles of facial expression as they would still be receiving stimulation from the ipsilateral upper motor neuron component.

Bell’s Palsy

A Bell’s Palsy is the classic example of an isolated lower motor neuron cranial nerve palsy. It manifests as a unilateral complete paralysis of the facial muscles of one side of the face. While the aetiology of a Bell’s Palsy is not completely clear, it has been associated with several conditions, specifically diabetes mellitus, herpes simplex type 1 viral infection, pregnancy, herpes zoster viral infection (shingles), hypertension, Lyme disease and autoimmune disease.

Due to the unilateral complete palsy of the facial muscles with no forehead sparing, this leads to ocular complications due to loss of the ability to blink and protect the exposed eye, dysarthria drooling, and difficulty eating and drinking due to the loss of facial muscle actions, as well as loss of ancillary functions of the facial nerve which include loss of function of the stapedius muscle in the ear leading to tinnitus and hyperacusis and loss of taste as a result of the facial nerve’s innervation to the anterior two thirds of the tongue.

During the recovery from Bell’s palsy, abnormal connections can form during nerve regeneration where efferent fibres from the superior salivary nucleus synapse with parasympathetic fibres of the facial nerve, causing tears to be produced during salivation in what is known as Bogorad syndrome or “Crocodile tears syndrome.”. This is similar to, but distinct from Frey syndrome, where sweating occurs with salivation due to abnormal connections between sympathetic and parasympathetic nerves associated with the auriculotemporal nerve (branch of mandibular division V3 of Trigeminal nerve), usually with an iatrogenic aetiology, e.g. parotid surgery.

Treatment of a Bell’s Palsy is with high dose oral prednisolone (a 10-day tapering course starting at 60mg Prednisolone) to reduce facial nerve inflammation. Investigations for conditions associated with Bell’s palsy should be considered where clinically suspected, e.g. Hba1c testing for diabetes, Lyme disease antibody titres. Antivirals such as aciclovir may be considered but should not be used in isolation. Eye protection is very important to replace the functions that serve to protect the cornea; lubricating eye drops and eye patches with ophthalmology follow-up is typically indicated.

Ramsay Hunt Syndrome

Ramsay Hunt syndrome is the manifestation of the reactivation of dormant varicella-zoster virus (shingles) with spread to the facial nerve. It is more common in immunocompromised individuals, e.g. patients with HIV, patients on immunotherapy, chemotherapy, or radiotherapy. Clinically, it manifests as a vesicular, erythematous, painful rash, in or around one ear with an ipsilateral ‘Bell’s palsy’ and can affect CN 5 and CN 8 with symptoms such as vertigo, tinnitus and hearing loss. The vesicles can be confined only to the auditory canal with facial pain and otalgia; all patients with facial pain and palsy should have an otoscopic exam.

Treatment is focused on the following:

• Disease modifying: Antivirals, e.g. acyclovir, famciclovir, together with corticosteroids
• Symptomatic: carbamazepine for neuralgia, antihistamines, and anticholinergics for vertigo
• Eye care as in the treatment of a Bell’s palsy

Parotid tumour

The extracranial motor branch of the facial nerve splits into 5 branches in the parotid gland – the temporal branch, zygomatic branch, buccal branch, marginal mandibular branch, and cervical branch. A lesion in the parotid gland, e.g. trauma/iatrogenic during surgery or a tumour, can result in a lower motor neuron facial nerve palsy.

Vestibular schwannomas

A vestibular schwannoma, also known as an acoustic neuroma, is a benign tumour developing from the vestibulocochlear nerve. It is the most common cerebellopontine angle tumour (85%).

Cerebellopontine angle (CPA) tumours are the most common tumours in the posterior cranial fossa, specifically in the triangular space in the posterior cranial fossa, which is bound by the tentorium superiorly, medulla posteromedially and petrous part of temporal bone posterolaterally, through which run the anterior inferior cerebellar artery, CNs 5, 6, 7, and 8. Tumours in this area can cause ipsilateral LMN signs in CN 7, with overlapping CN5, 8, and 9 palsies. Thus a patient presenting with such a collection of ipsilateral signs, including the loss of corneal reflex, should prompt suspicion of a CPA lesion. Bilateral vestibular schwannomas are associated with neurofibromatosis type 2.

Pontine lesions

The facial cranial nerve nuclei of CN7 lies in the pons and is closely associated with the nuclei of the abducens nerve (CN6). A lesion in the pons, e.g. infarct or tumour, can cause an ipsilateral CN 7 and CN 6 palsy, with contralateral hemiplegia due to being above the decussation of the corticospinal tracts.

Bilateral facial palsy

A rare manifestation of facial palsy, these can be sequential, i.e. unilateral progressing to bilateral facial palsy or concurrent. A wide range of causes of unilateral facial palsy can have bilateral facial palsy, albeit rarely. As noted above, bilateral CPA tumours, or large pontine lesions (stroke/tumour) or systemic disease with multi nerve affecting pathologies such as Lyme disease, syphilis, sarcoidosis, demyelinating disease like Guillain Barre syndrome, vasculitis, and diabetes mellitus can be a cause. Rarer causes include hereditary congenital facial palsy, Melkersson-Rosenthal Syndrome (recurrent antihistamine resistant oedema of lips, cheeks, and eyelids with recurrent facial palsy and cheilitis) and Moebius-Poland syndrome (congenital facial and abducens palsy with pectoral and limb deformities).