Abstracts

Rationale: Loss of consciousness during seizures is a major source of injury and of decreased quality of life in patients with epilepsy. Previous studies suggest that loss of consciousness (LOC) during complex partial seizures (CPS) might be related to the presence of sleep-like rhythms in extra-temporal cortex. However, the exact pattern of activity present during secondarily generalized tonic-clonic seizures (GTC) – which are accompanied by deeper LOC - remains unknown. Here we compared intracranial EEG activity during CPS and GTC, with a particular emphasis on quantifying high frequency activity in the high-gamma range (80-150 Hz), and beta (15-25 HZ)/delta (1-4 Hz) ratio, which may help to detect respectively actively seizing cortex and sleep-like cortical activity.  Methods: 11 CPS and 10 GTC from 10 mesial temporal lobe epilepsy patients were included. After rejecting noisy epochs and channels, we computed power spectrum for each channel during baseline (60 seconds before seizure onset) and ictal period (from seizure onset to seizure offset). Power spectrum data were then pooled within frontal, temporal and parietal regions. Changes in EEG power between baseline and ictal periods were assessed in delta to high-gamma frequencies (1-4 Hz to 80-150 Hz ranges). T-tests compared baseline and ictal periods within GTC and CPS, and between GTC and CPS. P values were corrected for multiple comparisons using false discovery rate. Results: When comparing baseline to ictal activity during CPS, we found a significant high-gamma band increase only in temporal lobe regions (4-fold increase, p < 0.005). Interestingly, beta/delta ratio also significantly increased in temporal regions (11-fold increase, p < 0.001) during CPS while it was reduced in frontal and parietal regions. When comparing baseline to ictal activity during GTC, we found a significant increase in high-gamma band for all brain regions (15-17 fold increase, p < 0.005). A significant increase in beta/delta ratio was also found during GTC in frontal and parietal regions (6-9 fold increase, p < 0.001). A direct comparison between GTC and CPS showed significantly higher high-gamma power and beta/delta ratio during GTC in extra-temporal regions. Conclusions: During CPS, high-gamma activity and increased beta/delta ratio were restricted to the temporal lobe, while the rest of the cortex showed sleep-like activity. In contrast, GTC were accompanied by a widespread increase in high-gamma activity and beta/delta ratio in temporal, frontal and parietal cortex. The widespread increase in broadband oscillatory activity in extra-temporal regions during GTC, possibly reflecting widespread cortical hyper-synchrony, might be accompanied by deeper LOC than the sleep-like activity observed in CPS. Funding: NINDS 1R03NS096379 to M.B. and Tiny Blue Dot Foundation to G.T.