What is hemifacial spasm?

As the name implies, hemifacial spasm is an involuntary spasm of one half of the face.  It typically begins as a twitch or spasm of the eyelid or muscles surrounding the eye and can spread to the cheek and muscles under the chin. 

Hemifacial spasm is a neuromuscular disorder characterized by frequent involuntary contractions (spasms) of the muscles on one side (hemi-) of the face (facial). This disorder occurs in both men and women, although it more frequently affects middle-aged or old women. The first symptom is usually an intermittent twitching of the eyelid muscle that can lead to forced closure of the eye. The spasm may then gradually spread to involve the muscles of the lower face, which may cause the mouth to be pulled to one side. Eventually the spasms involve all muscles on one side of the face almost continuously. The condition may be caused by a facial nerve injury or a tumor or it may have no apparent cause. Rarely, doctors see individuals with spasm on both sides of the face. Most often, hemifacial spasm is caused by a blood vessel pressing on the facial nerve at the place where it exits the brainstem.

 This answer is based on source information from the National Institute of Neurological Disorders and Stroke.