Abstracts

Rationale: Patients with intractable partial epilepsy and normal conventional MRI are a challenging group with a high rate of surgical failure. In such patients, advanced imaging techniques may provide additional localizing or prognostic information for surgical outcome. Recent MRI software developments allow the automated analysis of brain structural features, such as the measurement of cortical thickness. Studies in temporal lobe epilepsy demonstrated that cortical thinning in distinct regions, even outside the epileptic focus, can predict poor surgical outcome (Bernhardt et al., Neurology, 2010;74:1776-84). In the present study, we evaluated if cortical thickness measures are associated with surgical outcome in patients with non-lesional neocortical epilepsy. Methods: Twenty-one young patients (mean age: 9.9 years; range: 2.4-19.7 years) with epilepsy of neocortical origin underwent two-stage epilepsy surgery with chronic subdural EEG monitoring at the Children's Hospital of Michigan (Detroit). Clinical MRI was normal, and intracranial EEG grid placements were guided by electro-clinical and PET findings. Regional cortical thickness was measured on volumetric MRI using the Freesurfer software. After quality control of cortical segmentation, asymmetries (ipsilateral vs. contralateral to the resection) of mean hemispheric cortical thickness as well as lobar cortical thickness (in frontal, parietal, temporal and occipital lobes) were calculated. These asymmetries were then compared between patients who were seizure-free vs. those who continued to have seizures in the whole group, in patients with only gliosis on histopathology (but no microscopic cortical malformation), and also according to the lobes involved in the resections. In addition, cortical thickness asymmetries were correlated with age at surgery and duration of epilepsy. Results: At 1-year follow-up, 14 patients (67%) were seizure-free. Mean hemispheric and frontal lobe cortical thickness showed no/minimal asymmetry in seizure-free patients but lower values ipsilateral to the resection in those with recurrent seizures (p=0.02). Similar differences were found in patients with resections involving the frontal (n=13; p≤0.01) and parietal lobe (n=13; p=0.03 for mean hemispheric; p=0.01 for frontal cortical asymmetries), while a trend (p=0.056) was seen for frontal cortical thickness in patients whose resection involved the temporal lobe (n=12). In patients with only gliosis on histopathology (n=14, 8 seizure-free), symmetric mean hemispheric (P=0.023), parietal (p=0.049) and frontal cortical thickness (p=0.058) were associated with seizure freedom. Cortical thickness asymmetries showed no significant correlation with age and duration of epilepsy. Conclusions: In young patients with neocortical epilepsy and normal conventional MRI, lower cortical thickness in the epileptic hemisphere, particularly in the frontal lobe, may predict unfavorable surgical outcome. This predictive value appears to be present regardless of etiology and is strongest in patients whose epileptic regions involve the frontal and/or parietal cortex.