Hemifacial spasm (HFS) is characterized as involuntary irregular clonic or tonic movements of the facial muscles innervated by the seventh cranial nerve on one side of the face and is most often a result of vascular compression of the facial nerve at the root exit zone (Wang & Jankovic,1998). Facial muscle twitches usually begin in the periocular region and can progress to involve the cheek and perioral muscles. Hemifacial spasm is usually unilateral; however, uncommonly, it can spread and affect the other side of the face. Atypical cases have been reported to initiate in the orbicularis oris and buccinator muscles and gradually spread upward to involve the orbicularis oculi (Ryu et al., 1998). Muscles involved in HFS include the orbicularis oculi, frontalis (rarely), corrugator, nasalis, zygomaticus, risorius, orbicularis oris, and sometimes the platysma. (See Figure 6.1).

Hemifacial spasm appears to be more prevalent in females; commonly begins in the fifth decade and tends to have a fluctuating course. In contrast to essential blepharospasm, symptoms often continue during sleep and can provoke insomnia. Emotion and stress tend to exacerbate facial twitching. Although benign, HFS can be disabling due to social embarrassment and from excessive closure of one eye interfering with vision. Symptoms can progress over time and facial weakness can develop. Hypertension is thought to be a risk factor for the development of HFS (Oliveira et al., 1999; Defazio et al., 2000, 2003).