S17-3. Clinical significance of ictal direct current shifts: Is it made by glia?

To assess clinical usefulness of ictal direct current (DC) shifts and also high frequency oscillations (HFO) for delineating epileptogenic zone in human partial epilepsy, we analyzed subdurally recorded EEG in 16 patients, and correlated the findings with pathology. 4, 9, 1 and 2 patients had the diagnosis of glioma, focal cortical dysplasia, low grade neuroepithelial tumor and hippocampal sclerosis, respectively. As the results, ictal DC shifts occurred in 75%, and ictal HFO in 44% of patients. 2 patients with glioma accompanied with focal cortical dysplasia showed ictal DC shifts and ictal HFO with good reproducibility. Ictal DC shifts occurred earlier than or as early as ictal HFO. Both of ictal DC shifts and HFO or either occurred in more restricted areas than conventional ictal changes. We conclude that both ictal DC shifts and HFO finally complementarily contributed to delineate core epileptogenic zone. Earlier occurrence of ictal DC shifts than HFO suggests an active role of glia in seizure generation. A tendency of ictal DC shifts and HFO to occur more in glioma with focal cortical dysplasia may reflect decreased threshold for abnormal depolarization of pyramidal neurons due to glial dysfunction.