Abstracts

Rationale:
Topographical changes in brain activity leading to loss of consciousness (LOC) during seizures are not fully understood. Previous intracranial EEG (iEEG) studies suggested the presence of sleep-like slow wave activity (SWA) during seizure-induced LOC. In this study, we utilized the full spatial coverage of high-density (HD) EEG to compare topographical distribution of EEG oscillatory activity during wake, partial seizures with preserved consciousness (focal aware, FA) and with impaired consciousness (focal with impaired awareness, FIA).
Method:
Focal epilepsy patients were recruited from the University of Wisconsin-Madison Epilepsy Monitoring Unit. Ictal HD-EEG recordings were categorized based on seizure type: FIA (19 seizures from 7 patients, mean age 43 ± SD 11, 4 females; focus location in bilateral temporal: n =2; R temporal: n = 4; and R occipital: n = 1) and FA (6 seizures from 3 patients, mean age 38 ± SD 9, 1 female; focus location in R temporal: n = 2; and R occipital: n = 1). HD-EEG data were filtered between 1-25 Hz, and manual rejection of noisy epochs and bad channels was performed. Then, adaptive mixture independent component analysis (AMICA, EEGlab) was performed to ensure clean seizure epoch selection that could undergo ICA decomposition, and only clean components were kept in the analysis. EEG SWA power (i.e. delta band (1-4 Hz)) from clean epochs of wakefulness hours away from seizure period and clean ictal periods were compared. Random effects analysis compared SWA between wake and FA, and between wake and FIA in the same subjects using paired t-tests; then group comparison also directly contrasted FA and FIA. Random effects analyses were performed using Statistical Parametric Mapping software and thresholded at p< 0.05 corrected for multiple comparisons using family-wise error rate.
Results:
Compared to wakefulness, ictal activity in both FA and FIA was characterized by a widespread, bilateral increase in EEG SWA. However, the topography of changes differs between the two seizure types: difference between FA and wake was maximum in frontal channels, while difference in SWA between FIA and wake was maximal in parieto-occipital channels. A direct comparison between FIA and FA revealed significantly higher SWA in left frontal channels.
Conclusion:
Our HD-EEG results confirm previous iEEG findings reporting the presence of sleep-like activity in extra-temporal cortex during FA, but also show that widespread SWA increases may also occur during FA. Interestingly, SWA appeared maximally frontal during FA while it was maximal in posterior cortex during FIA. Left-sided differences between FA and FIA may relate to mostly right-sided foci in FA patients and should be confirmed in future studies. Overall, our results are in line with findings during both NREM sleep and REM sleep suggesting that increased SWA in posterior parieto-occipital regions is most predictive of LOC.
Funding:
:The Tiny Blue Dot Foundation & Lily's Fund for Epilepsy Research