Abstracts

Rationale: Magnetic resonance imaging (MRI) cannot always depict the laterality of epileptic hippocampus in patients with temporal lobe epilepsy (TLE). In such cases, epileptic focus is diagnosed based on the extent of interictal epileptic discharges (IED) and the onset of seizure discharges (SD) using intracranial electrodes. Surgical seizure outcome in patients with MRI-negative TLE is poorer than that in patients with hippocampal sclerosis. In the present study, we retrospectively investigated factors affecting surgical prognosis in such MRI-negative TLE. Methods: We enrolled 15 patients with MRI-negative TLE in this study. Those patients were surgically treated and have been followed up more than one year since surgery. Patients were divided into the following three groups based on the laterality of IED and onset of SD from the video EEG monitoring using intracranial implanted electrodes: 1) IEDs were consistently recorded in unilateral temporal lobe and SDs were emerged from the same side as IEDs (unilateral group, n=6). 2) IEDs were mainly recorded in unilateral temporal lobe, however SDs were emerged from both side of temporal lobe (reverse group, n=3). 3) IEDs were recorded in both side and SDs were also emerged from both side (Bilateral group, n=6). We analyzed the significance of IED and SD for epileptic focus determination from the surgical result of those groups. Results: We performed the selective amygdalohippocampectomy for the patient whose epileptic focus was located in the non-dominant hemisphere, and the hippocampal transection for the patient with dominant side epileptic focus. In the unilateral group, surgery was carried out the same side of IED and SD, and seizures resolved postoperatively in 5 out of the 6 patients. In the reverse group, surgery was performed on the IED side, but complete resolution of seizures was not observed in any of the 3 patients. In the bilateral group, surgery was performed on the side of habitual seizure onset. Seizure resolution was achieved in 4 out of the 6 patients, moreover seizure frequency decreased in the other 2 patients. Conclusions: These findings show that seizure outcome is predictably favorable when surgery is performed the side of constant IED and SD. Outcome is also good when surgery was performed on the side of habitual seizure even when IED was recorded bilaterally. Epileptic focus detection based on origination and spread of habitual seizures is believed to be important for MRI negative TLE patients.