What is Hemifacial spasm?

A disorder of the facial nerve characterized by unilateral involuntary paroxysmal contractions of the facial muscles, caused by high-frequency bursts of motor units lasting from a few msec to several seconds; reported causes include compression of the ipsilateral facial nerve near its exit from the pons by a vascular malformation, compression of the ipsilateral facial nerve by a posterofossa neoplasm, and idiopathic derivations.