Abstracts

Rationale: In mesial temporal lobe epilepsy, the intracranial depth EEG patterns have been investigated in relation to surgical prognosis and to pathological findings of hippocampal sclerosis. Studies about intracranial EEG using subdural electrodes usually have been performed regarding to lateralization of temporal lobe epilepsy foci. However, it has not been elucidated yet the relationship between ictal onset patterns of depth electrodes and those of subdural electrodes and the relationship between the range of ictal onset area and the location of intracranial electrodes. The aim of this study was to investigate ictal EEG patterns on subdural electrodes corresponded to ictal EEG on depth electrodes and to analyze whether the position of medial subdural electrodes and depth electrodes has influence on the range of the onset patterns. Methods: We reviewed total 120 seizures recorded by bilateral hippocampal depth electrodes and medial and lateral temporal subdural electrodes in 20 medically intractable mesial temporal lobe epilepsy patients who underwent anterior temporal lobectomy. All patients had hippocampal sclerosis confirmed pathologically, and had been seizure free in more than two years after operation. The ictal depth EEG patterns categorized into the following four types; low voltage fast activity (LVFA), fast strain (FS), rhythmic activity (RA), and electrodecremental pattern (ED). We identified the ictal subdural EEG patterns corresponded to the ictal depth EEG patterns. The positions of depth and subdural electrodes were measured by using the brain CT image after intracranial electrodes insertion surgery. We examined whether there was positional difference of the medial subdural electrodes between patients group with only ictal depth EEG and patients group with synchronized ictal EEG on both electrodes. Results: We observed that LVFA on depth electrodes, the most common intracranial ictal EEG pattern, was well corresponded to LVFA or ED pattern on subdural electrodes, FS corresponded to LVFA on subdural electrodes and RA and ED patterns as each RA and ED patterns on subdural ictal EEG. The 3-dimensional distance of medial subdural electrodes from the hippocampus in patients group with only ictal onset on depth EEG was significantly farther than that in patients group with synchronized ictal EEG onset on both electrodes. Conclusions: We suggest that the ictal subdural EEG patterns were well corresponded to the ictal depth EEG patterns in the mesial temporal lobe epilepsy with hippocampal sclerosis. The distance of medial subdural electrodes from the hippocampus influences on the range of ictal onset area defined by intracranial electrodes such as depth electrodes and subdural electrodes.