Intracranially recorded ictal direct current shifts may precede high frequency oscillations in human epilepsy

Objective We assessed the temporal–spatial characteristics of ictal direct current (DC) shifts (or infraslow activity) and high frequency oscillations (HFOs) in 16 patients with intractable focal epilepsy. Methods The underlying etiology consisted of cortical dysplasia, glioma, hippocampal sclerosis, and low-grade neuroepithelial tumor in nine, four, two, and one patients, respectively. The median number of analyzed seizure events was 8.0 per patient (range: 2–10). Chronic electrocorticographic recording was performed with (1) a band-pass filter of 0.016–600 Hz (or 0.016–300 Hz) and a sampling rate of 2000 Hz (or 1000 Hz). Results Ictal DC shifts and a sustained form of ictal HFOs were observed in 75.0% and 50.0% of the patients, and 71.3% and 46.3% of the analyzed seizures. Visual assessment revealed that the onset of ictal DC shifts preceded that of ictal HFOs with statistical significance in 5/7 patients. The spatial extent of ictal DC shifts or HFOs was smaller than that of the conventionally defined seizure onset zone in 9/12 patients. Conclusion Both ictal DC shifts and HFOs might represent the core of tissue generating seizures. Significance The early occurrence of ictal DC shifts warrants further studies to determine the role of glia (possibly mediating ictal DC shifts) in seizure generation.