Hemifacial Spasm (HFS) is an involuntary muscle contraction caused by a blood vessel repeatedly irritating and compressing the facial nerve (the 7th cranial nerve) near where it exits the brainstem. It usually starts as a fluttering of the eyelid and gradually spreads to the cheek, the corner of the mouth, and the jaw over months to years. It worsens with tension, fatigue, and lack of sleep, and in severe cases the eye may close completely and obstruct vision. Most cases are primary, but rarely there are secondary causes such as a brain tumor or vascular malformation, so imaging studies are needed early on to rule these out.