Low entropy of interictal gamma oscillations is a biomarker of the seizure onset zone in focal cortical dysplasia type II

Objective: Dynamic changes in the regularity of interictal gamma oscillations (GOs, 30-70 Hz) on intracranial electroencephalography (EEG) reflect focal ictogenesis with epileptogenic neuronal synchronization in focal cortical dysplasia (FCD). We investigated whether the regularity of interictal GOs is a biomarker of the seizure onset zone (SOZ) using multistate entropy analysis. Methods: We quantified the regularity of interictal GOs using intracranial EEG data from 1164 electrodes in 13 patients with FCD who were seizure-free postoperatively. The regularity of interictal GOs was quantified as entropy values. Low entropy represents high regularity. We standardized entropy values using Z values for each SOZ, resection area (RA), and the region outside the RA. The cutoff Z values, sensitivity, and specificity for detecting each area were calculated using area under the receiver operating characteristics curves (AUCs). Results: Low Z values represent higher regularity of GOs. The cutoff Z value of <=-2.09 for the SOZ had a sensitivity of 100% and specificity of 97.1% (AUC = 0.992 0.002). The cutoff Z value of <=- 0.12 for the RA had a sensitivity of 54.2% and specificity of 73.8% (AUC = 0.673 +/- 0.019). The cutoff Z value of >=-0.11 for the region outside the RA had a sensitivity of 73.8% and specificity of 54.2% (AUC = 0.673 +/- 0.019). Conclusions: Low entropy of interictal GOs was a reliable biomarker for the SOZ. Maintained high entropy of interictal GOs may be an auxiliary biomarker for nonepileptogenic regions. Significance: Low entropy of interictal GOs may be a biomarker for the SOZ in FCD type II. (C) 2019 Elsevier Inc. All rights reserved.