Abstracts

Rationale: To investigate the usefulness of broad-band EEG recorded from implanted subdural electrodes in pre-surgical evaluation of intractable focal epilepsy. Methods: The study included 9 consecutive adult patients with intractable focal epilepsy who underwent pre-surgical evaluation with chronically implanted subdural electrodes (5 frontal lobe epilepsy, 3 temporal lobe epilepsy, 1 occipital lobe epilepsy). Intracranial EEG was recorded with 1000Hz sampling rate. Ictal video-EEG was successfully recorded, and surgical treatment was performed in all cases. Focal rhythmic activities with a discrete frequency above 100Hz at the time around ictal onset were defined as “ictal high frequency oscillation (iHFO)”. The presence of this pattern and the relationship to the epileptogenic zone were described. Results: The iHFO was seen only in two cases with supplementary motor area seizures. The activity was characterized by 120Hz low-amplitude rhythm sustained for 3 to 5 seconds after the ictal onset. The iHFO was recorded from a quite limited area (one to two electrodes) over the seizure onset zone in the supplementary motor area. In one patient with subdual electrodes implanted over bilateral medial fronto-parietal cortices, the iHFO was recorded only from the epileptogenic hemisphere. Cortical resection including those areas rendered both patients seizure free. Histo-pathological study revealed focal cortical dysplasia. Conclusions: Broad-band EEG with conventional subdural electrodes is useful in detecting ictal HFO. Our observation suggests that (1) the iHFO is not a common finding in subdural electrodes placed over the neocortex, (2) but is probably related to the functionality as well as epileptic abnormality of the underlying cortex. Further investigation is necessary to define clinical utility of the method.