Different neurophysiologic patterns of myoclonus characterize Lennox-Gastaut syndrome and myoclonic astatic epilepsy

Purpose: To study the neurophysiologic characteristics of epileptic myoclonus in patients with Lennox-Gastaut syndrome (LGS) and myoclonic astatic epilepsy (MAE). Methods: Three patients with symptomatic LGS (mean age, 15 years +/- 4) and three with cryptogenic MAE (mean age, 9 years 3) were studied. Temporal relationships between electroencephalographic (EEG) and electromyographic activity were studied by analyzing latencies of EEG activity related to the onset of single myoclonic jerks, by using burst-locked EEG averaging where necessary. Results: LGS: neurophysiologic analysis indicated that jerks and the accompanying premyoclonic spikes showed latency differences between sides (mean+/-SD, 18+/-5 ms for both) with a constant leading side in each patient. The premyoclonic spike latency was 20+/-10 ms (mean+/-SD). Topographic voltage mapping of the premyoclonic spike peak showed a unilateral frontal distribution. MAE: muscles from both sides were activated synchronously, and the EEG con-elate was a generalized spike-wave, in which the negative peak of the spike preceded the generalized jerks by 30+/-2 ms (mean+/-SD). Topographic voltage mapping of the premyoclonic spike peak showed a diffuse distribution of the electrical field, predominating over the anterior regions, but not lateralized. Conclusions: These neurophysiologic findings indicate that epileptic myoclonus in LGS originates from a stable generator in the frontal cortex, to spread to contralateral and ipsilateral cortical areas, whereas myoclonus in MAE appears to be a primary generalized epileptic phenomenon.