Intraoperative Monitoring of ECoG for Detection of Desynchronizing Effects After Corpus Callosotomy
Authors: Maehara Taketoshi , Shimizu Hiroyuki , Nakayama Hideto , Kaito Nobuyoshi 
Published: August 2009

Purpose: Corpus callosotomy (CC) was recently recognized as an effective treatment for disabling generalized seizures, especially falling attacks. Previous reports suggested that desynchronization of bilateral epileptic discharges plays an important role in the surgical effects of CC. This implies that intraoperative electrocorticography (ECoG) of bilateral epileptic discharges can serve as a useful indicator for deciding the extents of callosal sections. We established an ECoG‐monitoring method under sevoflurane anesthesia, in which stable detection of desynchronization is feasible for a short time.

Methods: Between October 1996 and August 1997, 10 patients underwent total CC because they had extremely frequent disabling seizures and EEG abnormalities involving the entire hemispheres. The seven male and three female patients ranged in age from 5 to 22 years with a mean of 12 years. Anesthesia was induced by inhalation of 5% sevoflurane, and the trachea was intubated with vecuronium. Anesthesia was maintained with 2.5% sevoflurane in oxygen. The end‐tidal sevoflurane concentration was continued at 2.5% and end‐tidal CO2 was maintained at 30 mm Hg. After craniotomy, two pairs of strip electrodes with four contacts were inserted bilaterally over the superior frontal gyrus in the anterior and posterior directions. The contacts of the anterior electrodes covered Brodmann's areas 8, 9, and 10, and those of the posterior electrodes covered Brodmann's areas 4 and 6 and the anterior parietal lobe. The ECoG was recorded at 5 times during the surgical procedures: before CC and after the completion of 1/2, 2/3, 4/5, and total section of the callosum. The anterior 1/2 callosotomy included the rostrum, the genu, and half of the body of the callosum. The 2/3 section extended from the rostrum to the end of the body, and the 4/5 section reached to the isthmus just before the splenium. The number of spikes synchronized in the bilateral anterior electrodes and the number of spikes synchronized in all of the four electrodes were counted by visual inspection for 3 min at each extent of the callosal section. We investigated the degree of desynchronization in relation to the extent of the callosal section.

Results: Before CC, the number of bilaterally synchronized spikes were recorded at a rate of 25–134 (53 ± 30)/3 min in the anterior electrodes and 13–132 (36 ± 35)/3 min in the four electrodes. Bilateral synchronized spikes in the anterior frontal lobe were reduced to 49% of the pre‐CC rate at 1/2 section, 31% at 2/3, 22% at 4/5 and 1% at total section. The effect of desynchronization with a >90% reduction of synchronized spikes in the anterior electrodes was finally achieved after total callosotomy in seven (70%) of the, 10 patients.

Conclusions: In the frontal lobe electrodes, the ECoG under 2.5% sevoflurane anesthesia showed a gradual desynchronization of bilateral spikes, proportional to the extension of the callosal section. Sufficient desynchronization of bilateral frontal spikes was not obtained in seven of the 10 patients until the corpus callosum was totally sectioned. Therefore, total CC may be necessary in many of the cases with diffusely synchronized epileptic discharges to achieve a satisfactory result. This study additionally suggested that ECoG under sevoflurane anesthesia can be a useful intraoperative indicator to determine the optimal extent of the callosal section.