Subdural Recording of Ictal DC Shifts in Neocortical Seizures in Humans

PURPOSE: Invasive ictal EEG recording is often necessary to delineate epileptogenic areas in patients with intractable partial epilepsy, but even intracranial ictal recordings often reveal ill-defined onset zones in neocortical epilepsy. We studied the physiologic significance of ictal direct current (DC) potentials recorded intracranially in human epilepsy. METHODS: We made intracranial ictal EEG recordings in three patients with intractable partial seizures arising from frontal, lateral temporal, and parietal neocortical areas by using closely spaced subdural electrodes (platinum in two patients and stainless steel in one patient) with both standard (1.5 Hz) and open (0.016 Hz) low-frequency filter (LFF) settings. RESULTS: The initial ictal pattern was localized to two to nine subdural electrodes and characterized by very low voltage and high-frequency rhythmic activity ("electrodecremental pattern"). A slow-rising negative potential (DC potential) was seen in a slightly more restricted area (two to six electrodes) and occurred 1-10 s before the initial ictal EEG discharges in two patients. CONCLUSIONS: These results agree with those of previous studies of ictal DC shifts in animals and suggest that ictal DC shifts may be helpful in delineating the epileptogenic area more precisely in human epilepsy.