The National Institute of Neurological Disorders and Stroke (NINDS) defines a hemifacial spasm as a neurological disorder where the patient has involuntary muscular contractions on one side of the face.
Middle-aged women and elderly women are diagnosed with hemifacial spasm more often than men. The Mayfield Clinic states that 92 percent of patients have symptoms starting near their eye (eyelid twitching), then a progression down the face (resulting in a pulling of the mouth to one side). The other eight percent of patients have reverse symptoms thats begin near the chin and spreads up the face toward the eyes.
Hemifacial spasm is caused by nerve damage, a tumor, or Bell's palsy, though the NINDS points out that there may be no apparent cause. The most common cause is damage to the facial nerve where a blood vessel presses against the nerve at the nerve's exit from the brain stem. Decompressing the nerve through surgery is the main treatment option and 85 percent of patients can be cured.