Cortical activity-associated negative myoclonus.

A patient with myoclonus epilepsy had 2 types of negative myoclonus as well as positive myoclonus at rest. One type followed a large EMG discharge at the end of continuous contraction and the other did not. Both types of negative myoclonus and positive myoclonus were preceded by EEG activity, predominantly distributed in the contralateral central region. It is suggested that an abnormal activity around the motor cortex suppresses a voluntary contraction as well as induces myoclonus. Clonazepam was markedly effective on both positive and negative myoclonus.