High-Gain Analysis of Postictal sEEG Suppression Identifies Otherwise Invisible Multifrequency Activity, Including Persisting Ictal Discharges

Purpose: – Recent studies have challenged the assumption that brain activity is absent or only slow postictally, with reports of higher frequency activity. However, there are conflicting reports as to whether such activity is present under significant postictal suppression. To address this question, we performed a high-gain review combined with spectrographic analysis of postictal stereo-EEG suppression.Methods: – The postictal stereo-EEG of six focal to bilateral tonic–clonic seizures was reviewed both at standard gain (50–100 μV/mm) and at ultra-high gain (2–5 μV/mm). Time-frequency spectrographic analysis was performed of one channel in the seizure onset zone during the periictal period. Power of the gamma frequency band was quantified in the first 15 seconds postictally and compared with a preictal baseline.Results: – (1) Review at ultra-high gain identified an otherwise invisible evolving mixed-frequency background including high-frequency oscillatory activity, continuing ictal activity in one seizure, and early sharp activity (83.33%). (2) Activity was present despite significant suppression (18.46 μV in the seizure onset zone channel). (3) Time-frequency spectrogram revealed a steep drop of EEG power postictally, including gamma power, yet power was not completely absent.Conclusions: – (1) Although the stereo-EEG appeared suppressed postictally, the combination of direct review at ultra-high gain and spectrographic analysis identified otherwise invisible low-power activity. (2) We found intermittent high-frequency oscillatory activity, early postictal sharp activity, as well as the first report of persistent ictal discharges under apparent suppression. (3) Unmasking this activity could help explain postictal seizure-like behaviors and would challenge how the onset of the postictal state is declared.