Abstracts

Rationale: Intraoperative electrocorticography is an essential examination in epilepsy surgery. Recently many reports have indicated that intraoperative high frequency oscillation (HFO) is useful in diagnosis of epileptogenic area. Previously we reported that, in intraoperative ECoG recordings of the medial temporal lobe epilepsy (TLE), concentrations of sevoflurane anesthesia significantly affected the number of spikes and HFOs in the parahippocampal gyrus (medial temporal epileptogenic area). On the other hand, some other reports indicated that intraoperative ECoG in the alveus of the hippocampus is employed when performing multiple hippocampal transection. However, whether intraoperative ECoG in the alveus of the hippocampus reflects epileptogenesis of the hippocampus or not remains unclear. Because the alveus is closer to the cortex of the hippocampus, we hypothesized that intraoperative ECoG in the alveus could be more useful in diagnosis of epileptogenic area than that in the parahippocampal gyrus. In this study, we sought to elucidate the utility of intraoperative ECoG in the alveus by focusing on the effects of anesthesia on spikes and HFOs. Methods: Eight patients who suffered from medication-resistant TLE which was confirmed by preoperative examinations were included in this study. They all underwent temporal lobectomy using lateral temporal polar approach and obtained good surgical outcome (Engel class I or II ). Thus, we believed that medial temporal lobe was the epileptic focus of these patients. After the inferior horn was opened, we placed subdural electrodes on the parahippocampal gyrus and the alveus of the hippocampus. Initially, 1.5% end-tidal sevoflurane concentration was maintained for 15 minutes to obtain the steady state of anesthesia. Then ECoG recordings for more than three minutes were carried out at four different concentrations of sevoflurane: 1.5%, 2.0 %, 2.5%, and 3.0 %. We analyzed the number of spikes and HFOs (ripples: 80-200Hz, fast ripples (FR): 250-600Hz) that were recorded for 60 seconds of duration (for the middle 60 seconds during the 180 minutes of each recording) at each concentration using an analysis software (EMSE, Source Signal Imaging Incorporated, USA). We analyzed the number of spikes, ripples, and FRs at each concentration in both groups in order to detect effects of sevoflurane anesthesia. Results: Table 1 shows the number of spikes, ripples and FRs for a minute in the alveus of the hippocampus (A) and the parahippocampal gyrus (B). Two-way ANOVA analysis showed that the number of spikes, ripples and FRs gradually increased along with sevoflurane concentration both in the alveus of the hippocampus and the parahippocampal gyrus. No interaction was detected between the two factors and sevoflurane concentration. In each sevoflurane concentration, the number of HFO was statistically larger in the alveus of the hippocampus than in the parahippocampal gyrus (p = 0.046 for RP, p = 0.048 for FR, Fig. 1). Conclusions: This study revealed that spikes and HFOs increased according to the concentration of sevoflurane anesthesia. Also, we demonstrated that the increasing ratio was larger in the alveus than the parahippocampal gyrus, suggesting that the alveus reflects the epileptogenesis of the hippocampus compared with the parahippocampal gyrus. Funding: No funding