Abstracts

Rationale: Recent studies of humans and animal models of epilepsy have suggested that paroxysmal high-frequency oscillations (HFOs) with frequencies of 80 Hz or greater have a close relation to generation of seizures. It has been hypothesized that cortical sites showing faster HFOs may be particularly significant. It remains to be statistically determined whether HFOs with faster frequencies occur prior to those with slower frequencies at seizure onset. It is also uncertain how HFOs drive or are driven by seizures. In the present study, the spatial and temporal characteristics of HFOs were assessed in relation to the onset of clinical seizure manifestation. Methods: We studied 11 children with epileptic spasms undergoing extraoperative subdural ECoG recording and assessed 636 epileptic spasms. Electrocorticography (ECoG) signals were sampled from 1,308 cortical sites, and the dynamic changes of HFOs were animated on each individual s three-dimensional MR image surface. Results: Visual assessment of ictal ECoG recordings revealed that each spasm event was characterized by augmentation of HFOs (Figure 1A). Time-frequency analysis demonstrated that ictal augmentation of HFOs at 80-200Hz was most prominent and generally preceded those at 210-300Hz and at 70Hz and slower (p=0.02). Recruitment of HFOs in the Rolandic cortex preceded and persisted at the clinical onset objectively visualized as electromyographic deflection (Figure 1B). The presence or absence of ictal motor symptoms was related more to the amplitude of HFOs in the Rolandic cortex than in the seizure onset zone (p=0.001). In a substantial proportion of epileptic spasms, seizure termination began at the seizure onset zone and propagated to the surrounding areas; we referred to this observation as the ictal doughnut phenomenon (Figure 1C). Univariate analysis suggested that complete resection of the sites showing the earliest augmentation of ictal HFOs was associated with a good surgical outcome (p=0.015). Conclusions: Our study suggested that HFOs at 80-200Hz in the Rolandic area play a role in determining seizure semiology in epileptic spasms. Our study using macro-electrodes failed to demonstrate the presence of a hierarchy process where ictal HFOs at 210-300Hz drive those at 80-200Hz, but rather demonstrated that HFOs at 80-200Hz preceded those at 210-300Hz.