Abstracts

Rationale: In focal epilepsy, spontaneous high frequency oscillations (HFOs: 80-500 Hz) occur in intracranial EEG recordings. HFOs seem a marker of epileptogenicity and can be useful to delineate the epileptogenic region. If they could be evaluated per-operatively, this could improve epilepsy surgery. However, the clinical use is now compromised because artefact-free recordings in sleep, followed by time-consuming analyses, are needed. HFOs occur in brain regions that are sensitive to produce after discharges at electrical stimulation for function localization. With single pulse stimulation it has been demonstrated that spike-like delayed responses can be evoked, which are partly pathological. If pathological HFOs could be evoked likewise, this might improve the feasibility of using them for clinical purposes. We studied whether it is possible to evoke HFOs by single pulse stimulation. Methods: A 32 year old female patient with longstanding temporal lobe epilepsy was evaluated for surgery with subdural electrocorticography. 64 channels were recorded at 2048 Hz sample rate and evaluated by stimulating each consecutive electrode pair with ten single pulses of one millisecond duration, with inter-pulse intervals of five seconds. The bipolar channels were visually evaluated for evoked HFOs after the EEG was high-pass filtered by comparing one second before the stimulus to one second after the stimulus. Responses were divided into early and delayed responses and into ripples (80-250 Hz) and fast ripples (250-500 Hz). Two channels were compared specifically: a seizure onset zone channel and a non-seizure onset zone channel. Results: Early and delayed HFO responses could be evoked, both ripples and fast ripples. Ripple, fast ripple and spike responses all occurred significantly more often in the presumed seizure onset zone channel than in the non-seizure onset zone channel (paired t-test). However, spike responses also occurred in the presumed non-seizure onset zone channel, while HFOs did not. Conclusions: HFOs can be evoked by single pulse electrical stimulation and might be more specific for the seizure onset zone than single pulse evoked spike responses. Single pulse may offer an easy method to evoke and evaluate pathological HFOs in clinical practice.