Bell's palsy
Bell's palsy is an acute idiopathic unilateral peripheral facial-nerve palsy; forehead involvement and isolated lower-motor-neurone weakness support the diagnosis, but eye protection and exclusion of stroke, Ramsay Hunt syndrome and other causes are essential.
Definition
Bell's palsy is an acute idiopathic unilateral peripheral facial-nerve palsy diagnosed clinically after assessment for stroke and other identifiable causes. It causes weakness of the forehead, eye closure and lower face on the affected side.
First principles
Peripheral facial-nerve weakness affects the whole side
A lesion of the facial nerve distal to the facial nucleus weakens the forehead, eye closure and lower face on the same side. A typical cortical stroke causes contralateral lower-face weakness with forehead sparing because the upper facial muscles receive bilateral cortical input. Forehead involvement is useful, but brainstem stroke and other neurological disease can also produce a lower-motor-neurone pattern, so the rest of the neurological examination matters.
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