Abstracts

Rationale: Patients with temporal lobe epilepsy (TLE) and normal MRI scans present a challenge to surgical therapy because neocortical and mesial TLE can be indistinguishable. In these patients, chronic implantation of intracranial electrodes is often employed to guide temporal lobe resection. Such a two-stage surgery carries increased morbidity, length of stay and cost compared with a standardized anteromedial resection (SAMR). Recent studies indicate that interictal spikes may be useful in localizing ictal onsets in TLE. We hypothesized that we could use acute intraoperative ECoG to identify a subset of patients with MRI negative mesial epileptogenicity who could proceed directly to SAMR, potentially bypass chronic implantation of electrodes, and that these patients would have a better outcome than other MRI-negative TLE patients. Methods: We examined a series of patients with TLE and a normal MRI who underwent acute ECoG prior to chronic implantation of electrodes and recording of ictal onsets. Based on 5 minutes of low anesthesia acute ECoG, interictal spikes were classified to be either mesial (M), lateral (L) or mesial/lateral (ML). We then correlated the results of the acute ECoG with the ultimate ictal onset zone following chronic implantation. Seizure onsets were also classified as either M , L , or ML . We further evaluated the correlation of ictal onsets with other modalities including PET, scalp-EEG, and WADA testing. Outcome was assessed with Kaplan-Meier analysis of Engel grading system. Results: Sixteen patients fit criteria for inclusion. Eight were male (age range 20-47). Mean post-operative follow-up was 45.2 months (range 9-86 months). Of the 16 patients, nine had Engel I outcomes, two had Engel II outcomes, four had Engel III outcomes, and one had an Engel IV outcomes. Localization from scalp EEG and PET hypometabolism correlated with ictal onsets from chronic ECoG in 69% and 64% of patients, respectively. WADA memory scores correlated with onsets in 47% of patients. Acute intraoperative ECoG correlated with seizure onsets on chronic intracranial recordings in all sixteen (100%) patients. All 8 patients with M pattern acute ECoG went on to have a SAMR, and 6 (75%) experienced Engel I outcomes, whereas in the L and ML subgroups, only 3 of 8 patients (38%) had Engel I outcomes. There were 2 complications in 32 surgeries (6%), both associated with grid placement with no long-term sequelae. Conclusions: Acute intraoperative ECoG may be useful in identifying a subset of patients with MRI-negative TLE who will benefit from SAMR, without requiring chronic implantation of electrodes. These patients have uniquely mesial interictal spikes on acute ECoG and will usually go on to have an excellent post-operative seizure-free outcome. Patients with any neocortical spikes or absence of mesial spikes will benefit from chronic implantation with electrodes, but may have a worse outcome.