Abstracts

Intraoperative electrocorticography (ECoG) is classically applied in epilepsy surgery but its use in mesial temporal lobe epilepsy (MTLE) surgery has been abandoned in most centers. More recently, after descriptions of high-frequency EEG patterns, a renewed interest has been created. In this study we aim to characterize the electrophysiological characteristics of the temporal lobe structures during intraoperative ECoG.
ECoG of 8 patients with unilateral MTLE was analysed. All recordings were performed under isoflurante 0.1-0.2% and N2O 50-60% anesthesia and according to the following protocol: a) pre-resection ECoG: three 8-contacts subdural strips over the lateral cortex (LC), covering the superior, medium, and inferior temporal gyrus, and two 4-contacts strips in the inferior temporal region directed towards the entorhinal cortex (EC); b) ECoG post-resection of the temporal pole: subdural strips over the LC plus one 4-contacts strip along the long axis of the hippocampus; c) ECoG post-resection of mesial temporal structures: strips exclusively over the LC, at the border of the resection. Each recording session lasted 10 minutes, and sampling rate of 1,000 Hz and bandpass of 0.3-300 Hz were applied. Spikes and high-frequency oscillations (HFO) were quantified.
In the pre-resection ECoG HFO were observed in 5/8 patients, in trains of 80-1000 ms duration, and amplitude range of 30-340 uV, over the LC in one patient, over the EC in another patient, and over both areas in the other three patients. In the remaining 3/8 patients only spikes were recorded, over the LC in one patient, over the EC in another patient, and over both areas in the third patient. After resection of the temporal pole, 6 patients had high amplitude spikes over the hippocampus, none had HFO in this location, 5 had spikes in the resection margin, and 3 out of this margin. In the ECoG post-resection of mesial structures, 4 patients remained with spikes in the border of the resection, the others had occasional spikes out of this margin, including one patient who had an electrographic seizure 5 cm from the resection margin. All five patients that had pre-resection HFO had more prolonged and irregular trains after resection.
Spikes and HFO are frequently encountered in the LC as well as in the mesial temporal cortex, when properly recorded (adequate sampling rate and filter bandpass), but in the hippocampus only isolated spikes were present. Post-resection ECoG showed persistance of these abnormalities, however the spikes tend to diminish. These findings suggest that the epileptogenic zone probably extends beyond the atrophic mesial temporal structures. In the other hand, the pathophysiological basis of the high-frequency oscillations is still debatable.
[Supported by: FAPESP, CNPq]