Lesions of the facial nerve (CN VII)

The seventh cranial nerve is routinely tested in the unconscious ICU patient as the motor component of the corneal reflex. In the conscious patient, one may also test facial nerve function by asking the patient to perform a standard series of grimaces. The usual series involves asking the patient to raise their eyebrows, close their eyes very tightly, puff out their cheeks, make a cheesy grin, and protrude their lower lip in a pout. The seventh nerve was one among about four cranial nerves asked about in Question 21.2 from the first paper of 2014 (other involved were CN IX, CN XII, CN VII, CN VI).

Lesions of the facial nerve

Obvious features

• Facial paralysis:
  • Supranuclear "central" lesions spare the forehead and brow
  • Peripheral lesions take out the whole hemiface

Subtle features

• Failure of lacrimation
• Failure of salivation
• Loss of taste in the anterior 2/3rds of the tongue
• Loss of sensation from tympanic membrane, part of external auditory canal, lateral surface of ear, and area behind the ear.

Causes of a unilateral lesion of the facial nerve

Unilateral lesion:

• Peripheral (complete) lesions: ipsilateral paralysis
  • Trauma
  • Tumour (cerebellopontine angle)
  • Otitis media
  • Parotidectomy
  • Meningitis
  • Diabetic neuropathy
  • Bell's Palsy
• Central (forehead-sparing) lesions:
  • Traumatic brain injury
  • Tumour
  • Stroke
  • Maxillary sinusitis
  • Tumour
  • Aneurysm of the internal carotid artery
  • Cavernous sinus thrombosis

Causes of an isolated bilateral lesion of the facial nerve

Bilateral lesions are freakishly rare in isolation

• Guillain-Barre Syndrome
• Lyme disease
• Meningitis
• Melkersson-Rosenthal syndrome (a rare neurological disorder characterized by facial palsy, granulomatous cheilitis, and fissured tongue)
• Diabetic neuropathy
• Bilateral neurofibromas.

Obviously, the facial nerve forms the efferent component of the corneal reflex, and a bilateral absence of the corneal reflex is to be expected in the context of braindeath.