Electroencephalographic correlates of myoclonus.

Fifty-five consecutive cases of myoclonus owing to various etiologies were studied by conventional EEG-EMG polygraphic recordings and/or jerk-locked or back averaging. The technique of back-averaging was shown to be useful not only for detecting EEG correlates of myoclonus that are not recognizable on the routine polygraph but also for investigating the temporal and topographic relationship between the EEG activities and myoclonus. Thirteen of 17 cases of PME and related disorders, in whom back-averaging and SEP were studied, were shown to have both a myoclonus-related cortical spike over the contralateral central area, preceding the myoclonus of an upper extremity by 6 to 22 msec, and a giant SEP accompanied by an enhanced C reflex. In these cases of "cortical reflex myoclonus," the myoclonus-related spike was similar to the P25-N33 components of the giant SEP in its wave form, scalp topography, temporal relationship to myoclonus or to C reflex, succeeding cortical excitability, and drug effect. All of this suggests participation of common physiological mechanisms in those two activities. In two cases of PME, in which myoclonus involved bilateral proximal muscles synchronously, the myoclonus-related spike was maximal near the vertex, and there was no giant SEP. The significance of this subgroup remains undetermined. In six cases of the PME group, back-averaging was inapplicable because of rare occurrence of myoclonus, but they showed a typical giant SEP accompanied by an enhanced C reflex. In CJD, back-averaging demonstrated a sharp wave or PSD over the contralateral hemisphere, preceding the myoclonus by 50 to 85 msec. This form of myoclonus seems to be subcortical in origin. In essential myoclonus and oculopalatal-somatic myoclonus, there was neither myoclonus-related cortical spike nor giant SEP. Electrical stimulation of the peripheral nerve at variable intervals after the myoclonus onset (jerk-locked-SEP paradigm) was shown to be useful for investigating the influence of myoclonus on cortical excitability.