Hemifacial spasms are a neurological condition which leads to short, irregular involuntary contractions of the muscles on one side of the face. The condition was first medically described in the late 19th century.

The twitches typically first appear with one of the eyelids, and can progress from there to the rest of the face (one side only).

It occasionally has an obvious cause such as a facial nerve injury or a tumour, but usually there is no known causative factor.

There are several theories regarding possible mechanisms of the disorder - but none is generally accepted - see Wikipedia

Also see Benign fasciculation syndrome (BFS)plugin-autotooltip__plain plugin-autotooltip_bigBenign fasciculation syndrome (BFS)

Benign fasciculation syndrome (BFS) is a neurological disorder characterized by fasciculation (twitching) of various voluntary muscles in the body. The twitching can occur in any voluntary muscle group but is most common in the eyelids, arms, legs, and feet. Facial twitching almost always affects just one side of the face - usually the left side. which, when affecting the face, typically only affects one side.