Hemifacial spasm is an involuntary muscle contraction occurring on one side of the face, in one or more muscles innervated by the facial nerve. The disorder can be caused by compression of the facial nerve (cranial nerve VII), such as by an artery or tumor, or it can be idiopathic. Sometimes it occurs after Bell palsy. The condition is more prevalent in older women and Asian populations.

Muscle contractions are frequent but irregular. They are typically brief (a second or so) but can be more prolonged and "tonic," ie, for tens of seconds. They typically start in the eyelid and spread to other muscles in the face. Spasms may be worse in the setting of stress, anxiety, or fatigue.

Symptoms may be controlled by botulinum toxin injections, and in some cases, surgical microvascular decompression is performed.