Subdural electrode analysis in focal cortical dysplasia

Objective: Patients undergoing epilepsy surgery for focal cortical dysplasia (FCD) guided by subdural EEG generally have a poor surgical outcome. Our objective was to identify predictors of postoperative seizure recurrence in this patient cohort. Methods: We retrospectively reviewed 48 consecutive surgeries guided by subdural electrode recordings between 1990 and 2004 in patients with a pathologic diagnosis of isolated FCD. Using survival analysis, we analyzed results of the noninvasive evaluation, MRI, subdural interictal and ictal EEG patterns, extent of resection, proximity to eloquent cortex, and postoperative EEG. Results: After a median follow-up of 2.7 years, 45% of patients were completely seizure-free. Most seizures recurred in the first 6 months or between years 2 and 3 after surgery. On univariate analysis, seizure recurrence was associated with bilateral EEG abnormalities, multiple semiologic seizure types, and incomplete resection of the ictal onset zone. The absence of an MRI lesion did not affect outcome, nor did proximity to eloquent cortex. Interictal paroxysmal fast and runs of repetitive spikes correlated with the ictal onset zone, whereas isolated spikes did not. The 6-month EEG predicted ultimate surgical failure in patients seizure-free at that stage. An ictal spread pattern from the edge of the subdural grids was an independent predictor of seizure recurrence on multivariate analysis. Conclusions: We have identified specific predictive factors that may guide the surgical evaluation of patients with focal cortical dysplasia and intractable epilepsy requiring subdural EEG monitoring. Successful surgical results can be obtained utilizing subdural EEG in carefully selected patients.