Hemifacial spasm

[Britney]

Hemifacial spasm is painless involuntary twitching of one side of the face due to malfunction of the 7th cranial (facial) nerve and/or the area of the brain that controls it (called a center or nucleus). This nerve moves the facial muscles, stimulates the salivary and tear glands, enables the front two thirds of the tongue to detect tastes, and controls a muscle involved in hearing.

Twitching may occur only occasionally at first but may become almost constant.
Doctors diagnose hemifacial spasm based on symptoms but do magnetic resonance imaging to check for other disorders that can cause similar symptoms.
Hemifacial spasm is treated with botulinum toxin or another drug, but if drugs are ineffective, surgery may be necessary.

(See also Overview of the Cranial Nerves.)
Hemifacial spasm affects men and women but is more common among middle-aged and older women.
The spasms may be caused by

An abnormally positioned artery or loop of an artery that puts pressure on (compresses) the facial cranial nerve where it exits the brain stem

[Vania]

US spelling of haemifacial spasm