Hemifacial spasm (HFS) is uncontrolled contraction of the muscles on one side of the face, usually including the eyelids. The initial symptom may be twitching of the eyelids, with progression to involve the muscles on one entire side of the face. The severity of symptoms may vary from mild fluttering to forceful contraction. Unlike blepharospasm, this condition occurs during sleep.
HFS is sometimes caused by irritation of the facial nerve at the base of the skull. This irritation may be the result of an abnormal blood vessel pulsating against the facial nerve. When the facial nerve is irritated, it causes the facial muscles to contract and spasm. Less than 1% of cases are caused by a tumor. Therefore, your physician may recommend magnetic resonance imaging (MRI).