Abstracts

Rationale: Subdural grid evaluation (SDE) in refractory focal epilepsy aims to precisely define the ictal onset zone and map eloquent cortex. SDE normally follows initial noninvasive evaluation that includes seizure monitoring and structural and metabolic imaging. In a small but significant proportion of children, SDE shows multifocal or diffuse, rather than focal, seizure onset. Resective epilepsy surgery is denied, or is unsuccessful, in the majority of such cases. We investigated whether the noninvasive data could be abstracted to predict subsequent SDE electrographic outcome (focal versus multifocal/diffuse ictal onset). Methods: We retrospectively reviewed charts of 66 children with refractory focal epilepsy undergoing SDE at Cleveland Clinic over a 7-year period. A semiquantitative score summarizing the localizing value and concordance between selected noninvasive investigations (interictal and ictal EEG; PET and/or SPECT), as well Bayesian predictors of individual investigations and their combinations, were adapted from Kalamangalam et al 2009, J Clin Neurophysiol 26(5):333-41 to the subset of patients with nonlesional cranial MRI. The score assigned a unit value to each investigation yielding a localizing result and rewarded inter-investigation concordance. The sensitivity and specificity of individual investigations and their combinations yielded Bayesian likelihood ratios (LRs) related to subsequent focality of SDE ictal onset. Results: Forty (60.6%) patients had a single MRI brain lesion, 7 (10.6%) had bilateral or diffuse MRI changes, and 19 (28.8%) were nonlesional. SDE ictal onset was nonfocal in 4 patients in the first group (10%) and in 2 patients (28.5%) in the second group. One patient in the third (nonlesional) group was excluded due to incomplete data. In the remainder (n=18), SDE ictal onset was multifocal or diffuse in n1=5 (27.8%) and focal in n2=13 (72.2%). Considered as groups, patients in the n1 cohort had significantly lower scores than patients in n2 (Wilcoxon rank sum test, p < 0.05). Bayesian predictors were highest for concordance between the interictal and ictal scalp EEG (LR=3.8). Considered separately, interictal and ictal EEG were of equivalent predictive value (LR=2.3 and 2.1 respectively). Metabolic imaging was the least useful modality (LR=1). Conclusions: (i) Diffuse or multifocal ictal onsets on SDE are almost three times as likely in nonlesional patients as in those with a single, definite MRI brain lesion. (ii) The noninvasive data of children with nonlesional brain MRI may be summarized by a score that rewards localizing information and intermodality concordance: low-scoring patients are more likely to exhibit diffuse or multifocal ictal onset on subsequent SDE. (iii) Bayesian likelihood ratios predictive of ictal focality on SDE are most favorable for concordant scalp interictal-ictal EEG combinations. Metabolic imaging adds no additional information. (iv) These data-analytic methods may find use in the selection of nonlesional pediatric presurgical candidates offered SDE.