PP> Hemifacial spasm (HFS) consists of involuntary irregular clonic or tonic twitch-like movements of the facial muscles innervated by cranial nerve VII on one side of the face, and is most often a result of vascular compression of the facial nerve at the root exit/entry zone. Non-vascular causes of HFS are less common, and include facial nerve injury, Bell’s palsy, demyelination presumed to involve the facial nucleus and various tumors and space-occupying lesions in the cerebellopontine angle. PP> Twitches usually begin in the periocular region and can progress to the cheek, perioral and platysma muscles. Hemifacial spasm is almost always unilateral. Muscles involved in HFS include the orbicularis oculi, orbicularis oris and zygomaticus predominantly with frontalis, corrugator, nasalis, buccinators, risorius, depressor angularis oris, mentalis and platysma. PP> Injections of botulinum neurotoxin (BoNT) are the preferred treatment of HFS. They are successful in over 90% of patients, with relief lasting approximately 12 weeks, and repeat treatments remaining effective for many years. This chapter lists the pertinent facial muscles along with their principal mechanical actions; illustrates their anatomy under the skin, showing the recommended injection sites; and tabulates the dose ranges of the various toxins in each of the muscles. <COMP: I can’t get rid of the PP coding here without seeming to lose text>