Intraoperative hippocampal electrocorticography to predict the extent of hippocampal resection in temporal lobe epilepsy surgery

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Abstract
Among the variety of surgical procedures that are performed for the treatment of medically refractory mesial temporal lobe epilepsy (TLE), no consensus exists as to how much of the hippocampus should be removed. Whether all patients require a maximal hippocampal resection has not yet been determined. At the University of Washington, all TLE operations are performed in a tailored fashion, guided by electrocorticography (ECoG). The amount of hippocampal resection is determined intraoperatively by the extent of interictal epileptiform abnormalities on ECoG recorded from that structure, resulting in a hippocampal resection that is individualized for each patient. Using this approach, the authors prospectively observed 140 consecutive patients who underwent surgery for mesial TLE with pathological diagnoses of either mesial temporal sclerosis with neuronal loss (MTS group) or mild gliosis without neuronal loss (non-MTS group) to determine whether the extent of hippocampal resection correlates with outcome when a tailored approach is used. Additionally, the authors analyzed whether the presence of residual interictal epileptiform activity on ECoG following mesial temporal resection predicts poorer seizure control. With at least 18 months of clinical follow up, 67% of the 140 patients were seizure free or had only a single postoperative seizure. There was no correlation between the size of the hippocampal resection and seizure control in the group as a whole or when stratified by pathological subtype. Using an intraoperatively tailored strategy, individuals with a larger hippocampal resection (> 2.5 cm) were not more likely to have seizure-free outcomes than patients with smaller resections (p = 0.9). Additionally, both MTS and non-MTS patients, in whom postoperative ECoG detected residual epileptiform hippocampal (but not cortical or parahippocampal) interictal activity following surgical resection, had significantly worse seizure outcomes (p = 0.01 in the MTS group; p = 0.002 in the non-MTS group). Intraoperative hippocampal ECoG can predict how much hippocampus should be removed to maximize seizure-free outcome, allowing for sparing of possibly functionally important hippocampus.