Intrahemispheric and interhemispheric spread of cerebral cortical myoclonic activity and its relevance to epilepsy

Nine patients with cortical myoclonus (due to various pathologies), in whom movement of one limb induced bilateral limb jerks, were investigated. Three of these patients also had bilateral cortical reflex myoclonus when one limb was subjected to an electrical stimulus. The relative latencies to onset of electromyogram (EMG) activity in various ipsilateral and contralateral muscles in action and reflex jerks were studied. Bilateral reflex and action jerks induced by unilateral electrical stimuli or limb movement were not synchronous. EMG activity was usually recorded in the muscles of the stimulated or moved limb, before being recorded in the homologous muscles of the contralateral limb. It is proposed that this difference in relative latency between homologous muscles represents the interhemispheric delay due to the transcallosal spread of excitation from one sensorimotor cortex to the opposite cerebral cortex. The relative latencies of muscles on the same side of the body reflected not only the differences in efferent delays from the motor cortex via spinal cord and peripheral nerves, but also delays due to the spread of myoclonic activity within the sensorimotor cortex itself. This intrahemispheric spread followed a grossly somatotopic pattern. It is suggested that this spread involves cortico-cortical pathways. The additional delay due to spread of activity from hand to leg area of the sensorimotor cortex was about 10 ms in the first active hemisphere, in both generalized reflex and action jerks. The delays due to somatotopic spread of activity in the opposite later activated hemisphere were shorter. The tendency for spread of excitation through callosal and cortico-cortical pathways is an additional pathophysiological abnormality in some patients with cortical myoclonus, and may be important in the generalization of seizures seen in these patients.