Epileptic negative myoclonus. Subdural EEG recordings indicate a postcentral generator

We report a patient with epileptic negative myoclonus (ENM) presenting as status epilepticus of the right arm. The etiology was a cortical malformation in the contralateral postcentral gyrus. Electrical stimulation of the right median nerve revealed giant surface somatosensory evoked potentials. Subdural recordings, performed to plan epilepsy surgery, demonstrated that the epileptogenic zone was in the left postcentral gyrus. Repetitive left postcentral spikes were consistently followed by an EMG silent period in the right arm with a latency of about 20 to 30 msec. The silent period in the EMG reflected the negative myoclonus and lasted 30 to 340 msec, mostly between 100 and 200 msec. Spikes in other regions of the cortex did not elicit ENM. The amplitude and duration of the spikes correlated with the EMG silent period: the more cortex involved in the spike generation, the longer the duration of the silent period. This suggests that epileptic activation of postcentral cortex leads to an inhibition of tonic activity of motor neurons. Focal ENM may be related to a hyperexcitability of the postcentral cortex.