Abstracts

Rationale: Slow negative shifts ( ictal DC shifts ) occurring during the ictal onset in neocortical epilepsy of human beings were first described in 1996. It has been reported that evaluation of ictal DC shifts could help in localizing the epileptogenic area. High Frequency Oscillations (HFO) includes ripples (80-200 Hz) and fast ripples (200-500 Hz). It recently has been reported that HFO, especially fast ripples, occur almost exclusively in the epileptogenic zone in both neocortical and mesial temporal epilepsy. In this study we evaluated the hypothesis that ictal DC potentials are an expression of fast neuronal firing (HFO). To assess this hypothesis, we analyzed the interrelationship of ictal DC shifts, HFOs and the traditional EEG seizure onset in 7 patients with mesial temporal epilepsy. Methods: 7 patients with medically refractory mesial temporal lobe epilepsy underwent depth intracranial monitoring. 26 intracranial ictal EEGs recorded at 1000 Hz sampling rate (1 to 8 recordings per patient) were studied. For each recorded seizure the EEG onset was studied with the following 3 recording settings: 1.Traditional EEG: Time constant (TC) = 0.03 seconds; Display: 15 seconds per page; High Frequency Filters (HFF): 120 Hz; Sensitivity: 100 uV. 2. HFOs: TC = 0.001 seconds; Display: 5 seconds per page; HFF: 300 Hz; Sensitivity: 15 uV. 3. Ictal DC shifts: TC = 10 seconds; Display: 300 seconds per page; HFF: 30 Hz; Sensitivity: 100 uV. Results: DC shifts occurred in 92% (24/26) of the clinical seizures. For each patient at least one seizure showed a DC shift. DC shifts occurred 4-20 before the traditional EEG onset in 2/24 seizures. In 5/24 seizures the DC shifts occurred 3-15 after the traditional EEG seizure onset. In the remaining 17/24 seizures the DC shift was observed at the time of traditional EEG seizure onset. Negative and positive DC shifts can be seen and frequently show phase reversals along the longitudinal axis of the hippocampus/amygdala complex. HFOs occurred in 85% (22/26) of the clinical seizures and 10-40 after traditional EEG seizure onset. In 19 out of 22 seizures, the frequency of the HFOs increased as seizures progressed (frequency increased from 80 Hz to 300 Hz). In all cases the HFO occurred superimposed on traditional epileptiform discharges (ictal or interictal). In all cases the maximum amplitude of the DC shifts and also of the HFOs occurred at the electrodes of traditional EEG seizure onset. Conclusions: DC shifts and HFOs can be recorded using depth electrodes and are observed in all patients and most of the clinical seizures. The information from both DC shifts and HFOs complements and strengthens the traditional EEG findings in confirming the location of the epileptogenic area. Traditional EEG seizure onsets, DC shifts, and HFOs do not occur simultaneously indicating that they are an expression of different neurophysiological phenomena occurring during an epileptic seizure onset.