Abstracts

Hypersynchronous and low voltage fast seizure onsets are the most common types of seizure onsets in depth EEG recordings of temporal lobe epilepsy (TLE) patients. Previously, we showed that low voltage fast seizure onsets are accompanied by an initial slow wave (ISW). In order to assess the spatial-temporal distribution of the ISW, we performed a comparison of surface versus depth profiles of these seizure onsets. For seizure onset localization purposes, 10 patients (9M, 1F) with refractory TLE were bilaterally implanted with depth electrodes in the amygdala and hippocampus. Additionally, each patient was implanted with subdural grid electrodes on different cortical areas, depending on previous noninvasive findings. EEG seizure onsets (n=40) were analyzed visually using wide band filter settings (0.1 to 70 Hz). Averaging of ISW and EEG interictal spikes was performed to verify the reliability of the ISW and associated seizure onset patterns. An ISW preceding seizure onset activity was found in seven out of ten patients corresponding to 72.5% of all seizures. Its duration varied between 0.5 and 3 seconds (1.5 [plusmn]0.6). In five patients an ISW was present in all recorded seizures (23/23 seizures), in two patients an ISW was detected in some seizures (6/10 seizures) and in three patients no seizure contained an ISW (0/7 seizures). Averaging procedures revealed clear phase reversal of ISW between depth electrode recordings and subdural neocortical electrodes. ISW for each patient had a consistent spatial-temporal pattern, which could be distinguished from interictal EEG discharges. ISW had a widespread spatial distribution and were visible on both amygdalohippocampal structures and on temporal and frontal neocortical areas. Focal or regional rhythmic seizure activity followed the ISW, starting with low amplitude on the ISW descending part and increasing in amplitude as the seizure progressed. Analysis of low frequency EEG components provides additional information regarding seizure onset localization which is not discernable when recording with conventional bandwidth (1-70 Hz). ISW occupy a relatively large area of the brain and are more widespread than the following onset of focal rhythmic seizure activity. The distinct spatial-temporal pattern of the ISW suggests that seizures with an ISW onset are generated by different mechanisms than other types of seizures, such as seizures with a hypersynchronous onset. (Supported by Fund for Scientific Research (FWO)-Flanders grant B/02514 and National Institutes of Health Grants NS-02808 and NS-33310)