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.

Various factors can trigger or exacerbate BFS, including anxiety, stress, medicinal drugs, recreational drugs, vitamin deficiencies and infections etc., but the general mechanism of BFS is unknown. It's also not known if it's a disease of the motor nerves, the muscles, or the neuro-muscular junctions.

(Note: As the name suggests, BFS is not a dangerous condition, but there are other conditions with somewhat similar symptoms that can be serious.)

Further info UK NHS

Rarely, patients can develop more serious twitching and muscle contractions which are known as Cramp Fasciculation Syndrome - it's cause is also unknown. See: Revue Neurologique Volume 161, Issue 12, Part 1,

Also see : Hemifacial spasmplugin-autotooltip__plain plugin-autotooltip_bigHemifacial spasm

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).