Masseter inhibitory reflex in amyotrophic lateral sclerosis

OBJECTIVES: To elucidate underlying abnormalities of the masseter motor system in amyotrophic lateral sclerosis (ALS). METHODS: We recorded biphasic EMG suppressions of the masseter muscle (silent or inhibitory periods 1 and 2) after electrical stimulation of the mental nerve during maximum clenching of the teeth in 16 ALS patients and 15 healthy control subjects. We also examined the recovery cycle of the silent periods by delivering paired stimuli with an interstimulus interval of 250 ms. RESULTS: Silent period 1 in the patients was not significantly different from that in control subjects, whereas silent period 2 (as measured by its area on the graph) was increased in the patients, especially with exaggerated jaw jerk. Only two patients showed absence of silent period 2. The recovery cycle of silent period 2 in the subgroup of patients with exaggerated jaw jerk was significantly lower than the control value. CONCLUSIONS: The abnormalities of the masseter inhibitory reflex in ALS could be ascribable to involvement of the corticobulbar or corticoreticular tract projecting to the inhibitory interneurons mediating silent period 2.