Abstracts

Rationale: Intraoperative electrocorticography (ECoG) is one of the most important examinations for detecting epileptic focus. However, unlike ictal ECoG in chronic monitoring, interictal ECoG is employed during intraoperative study. We occasionally encountered electric seizures (ES) during intraoperative ECoG recordings. We analyzed the characteristics and clinical utility of intraoperative ES in patients with temporal lobe epilepsy (TLE).

Methods: In this study, we performed ECoG recording under sevoflurane anesthesia in 43 TLE patients who underwent temporal epilepsy surgery and 18 TLE patients who underwent intracranial electrodes (ICE) placement between March 2015 and May 2024. In the 43 patients, a trapezoid-shade subdural electrode with eight contacts is inserted into the mesiobasal temporal lobe. A grid electrode with 12 contacts is placed in the lateral temporal lobe. After the craniotomy and opening of the dura, a 1.5% end-tidal sevoflurane concentration was maintained for 15 min to obtain a steady state of anesthesia. Subsequently, we recorded ECoG while incrementally increasing the concentration of sevoflurane. Consecutive recordings that were longer than three minutes were carried out at four different concentrations of sevoflurane: 1.5%, 2.0%, 2.5%, and 3.0%. During the 5-minute interval between recordings, sevoflurane concentration levels were stabilized. Eighteen patients underwent ICE placement in the bilateral mesial temporal lobe. ECoG was recorded immediately after ICE placement under the aforementioned anesthetic conditions . We analyzed the characteristics of intraoperative ES in 43 TLE patients and then examined its clinical usefulness by comparing intraoperative ES and clinical seizure in 18 TLE patients who underwent ICE placement. ES was defined as epileptiform discharges >2.5Hz for at least 10 seconds.


Results: 1) ES was observed in 9 (21%) among 43 patients. The number of ES increased as the concentration of sevoflurane increased. At 1.5% sevoflurane, ES was recorded in one (2%) patient, three (7%) at 2.0%, five (12%) at 2.5%, and eight (19%) at 3.0%, respectively. 2) Except for one patient with focal cortical dysplasia in the lateral temporal lobe, ES was only recorded in the mesial temporal lobe and did not propagate to the lateral temporal lobe. 3) Three (17%) of 18 patients showed ES immediately after ICE placement on the same focus side as detected by chronic monitoring. ES was observed confined in the mesial temporal lobe and did not propagate the neocortex, while two among the three patients showed secondarily generalized seizures during chronic monitoring.


Conclusions: Although the rate of ES observation during intraoperative ECoG recording was low, it appeared primarily confined in the mesial temporal lobe and the same focus side detected by chronic monitoring. Therefore, intraoperative ES might be a helpful biomarker for accurate and safe temporal lobe epilepsy surgery.


Funding: This work was supported by JSPS KAKENHI Grant Number 23K08560.