Abstracts

Rationale: Intraoperative electrocorticography(ECoG) is an important examination in epilepsy surgery, however, there are still debates about its usefulness. One of the reasons is that types and concentrations of anesthesia deeply affect intraoperative ECoG recordings including spikes, slow waves, etc. Since spikes on ECoG are inspected visually to decide epileptogenicity, types and concentrations of anesthesia need to be considered carefully. On the other hand, many reports have recently indicated that intraoperative high-frequency oscillation (HFO) could be a more useful biomarker than spike. Therefore, we need to elucidate effects of anesthesia on HFO in order to discuss usefulness of intraoperative ECoG. Methods: Subjects were seven patients (aged 6 to 38 years old) who suffered from medication-resistant temporal lobe epilepsy (TLE) caused by unilateral hippocampal sclerosis which was confirmed by preoperative MRI. They all underwent antero-medial temporal lobectomy and seizure-free outcome was achieved. Thus, we believed that medial temporal lobe was the epileptic focus of these patients. After dural opening, we placed two types of subdural electrodes. One trapezoid-shaped electrode with eight contacts was placed on the basal temporal lobe and the parahippocampal gyrus where we considered being the focus area. Another was a grid electrode with 12 contacts placed in the lateral temporal lobe of non-focus area. Initially, 1.5% end-tidal sevoflurane concentration was maintained for 15 min to obtain a steady state of anesthesia. Then more than three min ECoG recordings were carried out at four different concentrations of sevoflurane: 1.5%, 2.0 %, 2.5%, and 3.0 %. We analyzed spikes and HFO (ripples: 80-200Hz, fast ripples (FR): 250-600Hz) recorded within 60 seconds duration (during one to two minute of each recording) at each concentration using an analysis software (EMSE, Source Signal Imaging Incorporated, USA). Spikes or HFO were divided into 2 groups, A and B, according to their distribution. Group A, those observed in medial temporal lobe (focus area) with or without lateral temporal lobe; Group B, those observed in non-focus area confined to the lateral temporal lobe. We analyzed number of spikes, ripples, and FR at each concentration in both groups in order to detect effects and appropriate concentration of sevoflurane anesthesia. Results: Group A) In medial temporal lobe (focus area), the average numbers of spikes, ripples, FR were gradually increased along with sevoflurane concentration. The data were shown below (Fig1).spike : 9.3±9.3 at 1.5%, 13.9± 11.8 at 2.0%, 27.6±17.0 at 2.5%, 41.9±23.5 at 3.0%.ripple : 10.9±10.2 at 1.5%,15.7± 12.4 at 2.0%,29.0±17.9 at 2.5%, 43.6±27.6 at 3.0%.FR : 10.4±9.0 at 1.5%, 16.0± 14.0 at 2.0%, 26.3±20.6 at 2.5%, 46.6±39.9 at 3.0%.Group B) In lateral temporal lobe (non-focus area), spikes were observed in 6, ripples were in 5, FR were in 4 patients, respectively.  The numbers of ripples and FR were steeply increased at 3.0 %. The data were shown below (Fig2).spike : 0.3±0.8 at 1.5%, 1.7± 2.3 at 2.0%, 4.5±6.2 at 2.5%, 4.0±3.6 at 3.0%. ripple : 0.8±1.3 at 1.5%, 1.4± 1.9 at 2.0%, 2.2±3.3 at 2.5%, 5.8±6.1 at 3.0%.FR : 0.8±1.0 at 1.5%, 1.0± 1.4 at 2.0%, 2.3±2.6 at 2.5%, 5.8±6.4 at 3.0%. Conclusions:  This study demonstrated that, in focus area, both ripples and FR increased depending on the concentration of sevoflurane anesthesia as well as spikes. Also, we found out that intraoperative ECoG recording at sevoflurane concentration of 3.0 % had a potential risk of false induction of ripples and FR in non-focus area. Funding: None