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 correlate 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.