Abstracts

Rationale: Various degree of bilateral epileptic activity is often found in scalp EEG in temporal lobe epilepsy. In some cases, implantation of subdural or depth electrodes is required on both temporal lobes to define, or confirm, the epileptogenic hemisphere before proceeding to surgical treatment. However, the indication of such procedures, with a certain degree of surgical risks, is not well defined. In this study, we reviewed our surgical cases who had undegone bilateral implantation of subdural electrodes before temporal lobe surgery, in order to define what subgroup of patients could potentially have omitted the invasive evaluation.Methods: This study included 17 consecutive patients with medically intractable temporal lobe epilepsy who underwent implantation of subdural electrodes on the both temporal lobes for pre-surgical evaluation of epilepsy. Patient database was reviewed to associate pre-operative studies, including MRI, interictal and ictal scalp EEG, and functional imaging such as SPECT, to the side of surgical treatment and post-operative seizure outcome.Results: All the 17 patients were characterized by bilateral interictal spikes on scalp EEG before surgery. When either MRI or ictal EEG was lateralized and the two findings were not contradictory (n=14), resection surgery was performed on the side of abnormality except for one case, even when the other modality showed ambiguity, with good post-operative outcome: 10 of 13 (76%) became seizure free. Functional imaging study showed abnormality in the contralateral hemisphere in 4 of those cases. However, none of them were related to worse outcome. In one exceptional case of normal MRI, surgery was performed on the side contralateral to the ictal scalp EEG, resulting in poor outcome (class IV). Among the other 3 cases, contradiction between MRI and ictal EEG was seen in 1, and both MRI and ictal EEG showed ambiguity in 2. Surgery was performed on the side of MRI abnormality with excellent outcome (class I) in the former case. For the latter cases, surgery was abandoned in one, and performed in the other after the seizure onset side was determined by the invasive EEG with good outcome (class II).Conclusions: When either MRI or ictal EEG is lateralized and the other modality shows ambiguity, temporal lobe surgery on the side of abnormality may warrant good post-operative outcome. Invasive evaluation using bilateral subdural electrodes may be omitted in those cases. The bilateral implantation is indicated with caution for the cases with normal MRI, ambiguity in both MRI and ictal EEG, or contradiction between MRI and ictal EEG.