Interhemispheric interaction between the hand motor areas in patients with cortical myoclonus

Objective: To study interhemispheric interaction between the hand motor areas of both hemispheres through the corpus callosum in myoclonus epilepsy. Subjects: Five patients with benign myoclonus epilepsy and ten age matched normal volunteers. Methods: We studied effects of a medially directed conditioning stimulus over the right hand motor area on responses in the right first dorsal interosseous muscle to a posteriorly directed test stimulus over the left hand motor area. Results: In normal subjects, inhibition was evoked at interstimulus intervals (ISIs) of 8-20ms (late inhibition). In contrast, facilitation occurred in patients at ISIs of 4-6ms (early facilitation) with no late inhibition. Conclusions: The lack of late inhibition in the patients is consistent with the idea that cortical inhibitory interneurones are affected in myoclonus epilepsy. We propose that this releases interhemispheric facilitation from powerful surround inhibition. The consequence is a predominant early facilitation between the hemispheres in patients with myoclonus epilepsy.