Abstracts

Rationale: High frequency oscillations (HFOs) encompassing ripples (80–200 Hz) and fast ripples (250–500 Hz) have been linked to the mechanism of seizure generation. Cerebral region with early demonstration of fast ripple may label the seizure-onset zone. In contrast, increase of ripple activity may indicate seizure onset. There are animal models to estimate the occurrence of ripple and fast ripple from preictal to postictal stages. However, whether the onset HFO patterns from animal experiments are consistent with human seizures is not known. And the interaction of fast ripples and ripples during a seizure are not understood well. In this research, we investigated temporal features of ripple and fast ripple for two most common onset intracranial EEG patterns from patients with drug-resistant epilepsy. Methods: We collected Intracranial EEG (iEEG) data from patients with staged approach to epilepsy surgery. For the study demand, we selected two most common ictal onset pattern: low-voltage fast activity (LVF) onset and low frequency high amplitude repetitive spiking (hypersynchronous, HYP) onset for analysis. Signals derived from subdural grid and depth electrodes (Ad-Tech Medical Instrument Corporation, Racine, WI, USA) were sampled by 4 kHz (EBNeuro S.p.A., Italy). The iEEG signals from the seizure onset channel were first low-pass, filtered at 500 Hz and then downsampled to 2 kHz to prevent aliasing. We studied the temporal evolution of HFOs over time by measuring the envelope correlation coefficient between ripple (80-200 Hz) and fast ripple (250-500 Hz) for each seizure. The preictal (30 s) and ictal period of the selected seizures was normalized into 100 bins, respectively. We then compared the envelope correlation of LVF and HYP using Wilcoxon rank-sum test followed by Bonferroni–Holm corrections to correct for multiple comparisons. The level of significance was set to p < 0.05. Results: Sixteen LVF and six HYP onset iEEG from 8 patients were analyzed. In the early seizure onset phase of HYP, the envelope correlation between ripple and fast ripple was significantly increased compared with LVF. However, the envelope correlation between ripple and fast ripple of LVF was significantly higher than HYP during the later phases. Conclusions: LVF is known to associate with neocortical epilepsy and HYP is associated with mesial temporal lobe epilepsy. Clear identification of both ictal onset patterns is associated with better surgical outcome. Regarding to the mechanisms, these two different HFO patterns indicate different involvement of GABAergic and glutamatergic transmission for different pathology at seizure onset. Funding: This work was financially supported by the Center for Neuromodulation Medical Electronics Systems from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education (MOE) in Taiwan and partly supported by the Ministry of Science and Technology (MOST) of Taiwan under Grant MOST 106-2221-E-009-068.