Abstracts

Rationale: Near-DC potentials, or infraslow potentials, have been reported to accompany seizure onsets both on scalp and intracranial EEG recordings in several prior studies. We sought to determine the prevalence of infraslow potentials at ictal onset in standard scalp video-EEG recordings from a large series of patients who were evaluated for epilepsy surgery. Methods: We reviewed scalp video-EEG data from 111 patients who underwent subsequent intracranial EEG studies as part of their comprehensive presurgical work-up. The scalp recordings were acquired using a Nihon-Kohden EEG system at a sampling rate of 200 Hz. The low frequency cut-off in these recordings was 0.016 Hz in 98/111 (88.2%) and 0.08 Hz in the remaining. Each seizure was reviewed with conventional EEG-band settings (0.53 to 70Hz) and then with the low cut-off decreased to the lowest available setting. We studied only events that were deemed to be seizures independently by two epileptologists based on the presence of ictal EEG correlates in the conventional EEG bands, or the presence of stereotypic semiology. At least 1 minute of the EEG prior to seizure onset was viewed at a time-scale of 60 seconds/page when looking for IISPs. Scalp voltage topography of IISPs was reviewed to seek consistent patterns and exclude low frequency artifacts. Results: A total of 772 seizures from 111 patients were studied (6.9 seizures/patient). We found definite ictal infraslow potentials (IISPs) at seizure onset with voltage topographies that were duplicated on at least 2 seizures from the same patient in 26/111 (23.4%) patients. In 16/111 (14.4%) of the patients possible infra slow potentials were observed but cerebral origin of these potentials was uncertain due to lack of duplication of voltage topographies between seizures, or capture of only a single seizure. No IISPs were noted in the remaining (62.1%) of patients. Conclusions: Definite infraslow potentials at ictal onset are present in scalp EEG data recorded using standard Video-EEG equipment in nearly a quarter of patients with refractory partial epilepsy. The prevalence of definite IISPs in our dataset was lower than those reported from intracranial EEGs. This may derive from the greater abundance of low-frequency artifacts in scalp EEG, or the smaller sample size of prior studies. The utility of scalp-recorded ictal infraslow potentials in localizing seizure onset zones remains to be further explored.