PATHOPHYSIOLOGY OF HEMIMASTICATORY SPASM

Two patients aged 21 and 50 years presented with facial hemiatrophy an d unilateral spasms of the masticatory muscles. Masticatory muscle bio psy showed normal findings in both patients and facial skin biospy spe cimens only showed atrophy, although morphoea (localised facial sclero derma) had been diagnosed nine years previously in the second patient. The involuntary movements consisted of brief twitches and prolonged c ontractions clinically and electromyographically similar to those of h emifacial spasm and cramps. The jaw jerk and the silent periods were a bsent in the affected muscles. Direct stimulation of the muscle nerve and transcranial stimulation of the trigeminal root demonstrated slowi ng of conduction and after-activity due to autoexcitation. Observation s in other reported cases and these two patients suggest that hemimast icatory spasm is produced by ectopic activity secondary to focal demye lination of the trigeminal motor nerve fibres. The proposed cause of t he neuropathy is focal damage to the masticatory nerves caused by comp ression, possibly resulting from the deep tissue changes that occur in facial hemiatrophy.