Hemimasticatory spasm (HMS) is a rare disorder of the trigeminal nerve characterized by paroxysmal involuntary contractions of the unilateral jaw-closing muscles. HMS has been frequently described in association with facial hemiatrophy or localized scleroderma. A 42-year-old female presented with involuntary paroxysmal spasms of the left face, of 6 months duration. Her lower face on the left was markedly hypertrophied without skin lesions. An electrophysiological study indicated that the masseter reflexes and masseteric silent period were attenuated on the affected side. Surface electromyography demonstrated irregular bursts of motor unit potentials at high frequencies up to 200 Hz. Magnetic resonance imaging of the head showed marked hypertrophy of the left masseter muscle. Biopsy of the hypertrophied masseter muscle was normal. Repeated local injections of botulinum toxin noticeably reduced the size of the hypertrophied muscle as well as improved the patient’s symptoms.

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