Curriculum · Neurology
Localising facial weakness: central versus peripheral
What it is
Facial weakness is separated by the level of the lesion. A lower motor neurone lesion of the VIIth cranial nerve gives weakness of the upper and the lower part of the face on the same side, and Bell palsy is an isolated lower motor neurone paresis of that nerve leading to facial weakness. A central lesion behaves differently: supranuclear input to the facial nerves comes from both cerebral hemispheres, so strokes and other central pathologies affecting the facial nerves typically spare the forehead. The forehead is therefore the divide between an upper motor neuron cause such as stroke and the many lower motor neuron causes.
Causes and risk
The idiopathic form is Bell palsy, and it is relatively common. The secondary causes are many.
- Trauma, such as a temporal bone fracture.
- Infection: herpes zoster, which with a vesicular rash around the ear is Ramsay Hunt syndrome; HSV reactivation; and malignant otitis externa.
- Tumors: parotid gland tumors and acoustic neuroma.
- Diabetes mellitus and sarcoidosis.
- HIV and multiple sclerosis also appear among the lower motor neuron causes, while stroke is the upper motor neuron cause.
Bilateral facial nerve palsy carries its own short list: sarcoidosis, Guillain-Barre syndrome, Lyme disease, and bilateral acoustic neuromas as in neurofibromatosis type 2. Bell palsy is common enough that it accounts for up to 25% of cases of bilateral palsy, but this represents only 1% of total Bell palsy cases.