Abstracts

Rationale: The main goal of in patients undergoing pre-surgical intracranial recording for the treatment of intractable epilepsy is to determine the region of cortex for resection or ablation.  Here we sought to study whether we could identify the clinically-determined region of ablation using corticocortical evoked potentials (CCEPs) by developing a statistical approach to compare CCEPs to the pattern of early seizure spread. Methods: Single pulse electrical stimulation at 1 Hz in trains of 20-30 seconds was performed at multiple sites in 10 patients with refractory epilepsy undergoing stereo-EEG monitoring.  The resulting CCEPs were averaged and the amplitude was defined as the root mean squared error (RMSE) from 10 ms to 50 ms after the onset of the stimulus artifact.  The ordering of the electrodes by CCEP amplitude was correlated with the ordering of electrodes by early ictal propagation using a novel metric based on Spearman’s Footrule.  The resulting correlation coefficient was compared between electrodes inside versus outside the ablation zone, which was defined as the set of electrodes with contacts targeted for laser or radio-frequency ablation. Results: Across the 10 patients, there were 33 electrodes inside the ablation zone versus 52 electrodes outside.  The mean of the highest amplitude CCEPs inside the ablation zone was 742 mV, which was not significantly different from the mean amplitude of 901 mV outside (p=0.47, t-test).  However, the median Footrule CCEP/ictal correlation coefficient was significantly different: 0.62 inside the ablation zone compared to 0.01 outside (p=1e-6, Wilcoxon rank sum).  When the correlation coefficients were converted to p-values, the distributions of p-values were significantly different inside versus outside the ablation zone (p=6e-6, Kolmogorov-Smirnov), and the distribution of p-values outside was not significantly different from a uniform distribution (p=0.99).  Moreover, 15/33 (45%) of electrodes inside the ablation zone had correlation p-values < 0.05, significantly more than the 4/52 (8%) electrodes outside (p=9e-5, Fisher exact).   Conclusions: A measure of correlation between the ordering of electrodes by CCEP amplitude and ordering by early ictal propagation was able to distinguish between electrodes inside or outside the clinically determined ablation zone.  Future work will determine if CCEP/ictal correlation can determine whether the ictal onset has been captured, and whether congruence between CCEP/ictal correlation and the ablation zone predicts good surgical outcome. Funding: NPP was supported by the American Academy of Neurology and American Brain Foundation (CRTF).