Cortical myoclonus in Janz syndrome.

OBJECTIVE

To evaluate the characteristics of EEG paroxysms and the relationship between EEG spikes and ictal myoclonic jerks in patients with juvenile myoclonic epilepsy (JME).

METHODS

Six patients with a typical form of JME entered the study and underwent computerized polygraphic recordings. In each patient, the inter-peak spike interval was measured on repeated EEG bursts, and jerk-locked back averaging was performed on ictal epochs using a time window including the 100 ms before and the 100-200 ms after the point at which the jerk-related EMG potential diverged from baseline.

RESULTS

In all cases, the myoclonic jerks were associated with polyspike waves (PSW) complexes. The frequency of repeated spikes within the PSW complex ranged from 16 to 27 Hz. Jerk-locked averaging revealed a positive-negative EEG transient with maximal amplitude on the frontal leads, which preceded the myoclonic jerk by 10.25+/-0.96 ms. A delay of 9.50+/-1.73 ms was measured between the jerk-locked positive peak detected on the frontal EEG leads of the two hemispheres; a comparable time lag was observed between the onset of myoclonic jerks in the two deltoid muscles.

CONCLUSIONS

Our data suggest that the ultimate mechanism responsible for ictal myoclonic jerks in JME is largely similar to that sustaining cortical myoclonus in more severe pathological conditions such as progressive myoclonus epilepsies, despite the different pathogenic substrate and triggering mechanisms.