Jaw closing spasm--a form of focal dystonia? An electrophysiological study.

The case of a 36 year old man suffering from unilateral right jaw closing spasms over two years is reported. Permanent spasm with trismus severely impeding mouth-opening was combined with paroxysms triggered by various sensory stimuli. The diagnosis of temporo-mandibular joint syndrome was considered but treatment failed to improve the symptoms. Neurological investigation two years after onset of the spasms showed by electrophysiological studies excessive co-contraction of the antagonistic jaw-closers, mainly the right masseter during attempts at jaw opening and absence of the silent period in the right masseter and anterior temporalis following jaw tap and trigeminal exteroceptive stimulation. Jaw dystonia was therefore considered and Botulinum A toxin was injected into the right masseter and temporalis which dramatically improved the patient's condition.