PATHOGENESIS OF CORTICAL MYOCLONUS STUDIED BY MAGNETOENCEPHALOGRAPHY

Myoclonus-associated cortical activities were studied by simultaneous recording of a magnetoencephalogram and an electroencephalogram in 6 p atients with cortical myoclonus due to various causes. Cortical activi ties were averaged, with respect to the precise onset of the myoclonic jerk, to evaluate the myoclonus-associated cortical magnetic fields, The estimated generator of their earliest peak was localized at the co ntralateral precentral gyrus in all patients. As judged from the direc tion of the electrical current, surface positive activity preceding th e electromyographic discharge was detected in 3 patients with cortical reflex myoclonus and in 1 patient with possible corticobasal degenera tion. In contrast, in the remaining 2 patients (Lennox-Gastaut syndrom e and Alzheimer's disease), magnetic fields time-locked to the myoclon ic jerk were associated with surface negative activity at the precentr al cortex. The present study, applying for the first time an off-line jerk-locked back-averaging analysis to magnetoencephalography, demonst rated the important role of the precentral cortex in generating sponta neous myoclonus. It is most likely that the differing polarity of the electromagnetic activity reflects the differing activation patterns wi thin the cortical laminar structure in the precentral area, underlying the generation of various types of myoclonus.