Abstracts

An electrodecremental pattern is a feature seen at the onset of certain epileptic seizures as well as during the post-ictal period. The exact cause of the electrodecremental response is unclear. Higher sampling rates and better resolution have revealed the presence of high frequency components during the onset of certain epileptic seizures. We report here quantitative analyses of electrodecremental patterns at seizure onset and during the post-ictal period using a complexity measure of the signal., The onset of seizures from 56 patients who were monitored in the epilepsy monitoring unit for presurgical evaluation with intracranial subdural grid arrays were examined. Only five patients had seizures visually classified as containing an electrodecrement pattern at onset. Data was obtained from 26 partial seizures (16 partial seizures and 10 partial seizures with secondary generalization). The time-frequency decomposition was obtained with the matching pursuit (MP) method. The Gabor atom density (GAD) (Jouny et al. 2003), derived from MP, provides a measure of signal complexity. For each seizure, we measured the average complexity in the pre-ictal period, the minimum during the electrodecremental onset and the minimum during the post-ictal period at the electrode closest to the focus. To normalize the difference between patients, we computed the difference between GAD levels and the pre-ictal average level used here as a baseline for each seizure. GAD levels typically increase during seizures., GAD propagation maps reveal the electrodecremental event and its regional extent. The average electrodecremental decrease at seizure onset compared to baseline is 0.060[plusmn]0.002. The post-ictal decrease is 0.26[plusmn]0.03. The electrodecremental GAD levels were only measured at the focus, but it is interesting to note that the focus itself does not always present a clear electrodecrement pattern even if the propagation maps clearly show a regional change. The matching pursuit method from which the GAD levels are derived allows for determination of prominent high frequency components during the electrodecremental period., GAD propagation maps are very useful tools to assess not only the propagation of epileptic seizures but also the coincidental occurrence of specific patterns. Post-ictal depression can be seen on propagation maps as an extensive and often dramatic decrease in complexity visible on almost all channels. The intensity of the post-ictal GAD changes as well as their spread are different from the electrodecremental pattern visible at seizure onset even in the same seizure. We hypothesize that this pattern at seizure onset may include synchronized high frequency activity whereas an electrodecremental pattern in the post-ictal period may be due to desynchronization or lack of activity. GAD analyses can provide insights into these potential mechanisms., (Supported by NIH grant NS 48222.)