Abstracts

Rationale: Mechanisms of loss of consciousness (LOC) during epileptic seizures are not yet fully understood. In the present study we used high density (HD-)EEG to compare topographical changes in EEG oscillatory activity during simple partial seizures (SPS, without loss of consciousness) and complex partial seizures (CPS, where consciousness is impaired) in order to identify neurophysiological signatures of seizure-induced LOC. Methods: Patients with focal epilepsy were recruited at the Epilepsy Monitoring Unit of University of Wisconsin-Madison and divided into two groups depending on seizure type: CPS (n = 7, mean age 40 ± SD 13, 3 females; focus location in bilateral temporal: n =1, R temporal: n = 3; L temporal n = 2; R occipital: n = 1) and SPS (n = 5, mean age 45 ± SD 12, 3 females; focus locations in R temporal: n = 2; L temporal: n = 1; R occipital: n = 1; L fronto-temporal: n = 1). HD-EEG data during 15 CPS and 9 SPS was filtered between 0.3 and 50 Hz, and noisy epochs and bad channels were visually rejected. EEG power spectrums in the delta, theta, alpha, sigma and beta bands (1-4; 4-8; 8-12; 12-15 and 15-25 Hz) as well as beta/delta power ratio were compared between baseline (60 second before seizure onset) and ictal periods (from seizure onset to seizure offset). Quantitative power changes between baseline and ictal period were also compared between CPS and SPS. Random effects analyses were performed and results were thresholded at corrected p < 0.05. Results: Compared to baseline, ictal activity in both SPS and CPS was characterized by a widespread, bilateral increase in EEG power from delta to beta frequencies. In addition, CPS were characterized by a sleep-like decrease in beta/delta ratio in posterior midline regions, which was not found during SPS ictal activity. A direct comparison between CPS and SPS revealed significantly higher theta power in posterior midline scalp regions compared to baseline during CPS ictal activity.  Conclusions: While comparison to baseline EEG reveals widespread changes in all EEG frequencies during both SPS and CPS ictal activity, a higher increase in theta frequency and a decrease in beta/delta ratio were selectively observed in posterior midline scalp areas during CPS. This result is in line with recent studies identifying low-frequency activity in posterior parieto-occipital regions as most predictive for LOC during both NREM sleep and REM sleep. These results also confirm previous iEEG findings reporting the presence of sleep-like activity in extra-temporal cortex during CPS.  Funding: NIH NINDS 1R03NS096379