Usefulness of interictal spike source localization in temporal lobe epilepsy: electrocorticographic study

The success of epilepsy surgery depends on delineation of the suspected epileptogenic zone. The gold standard to delineate it is to use the ictal onset zone from an electrocorticography (ECoG). Although interictal spikes are also associated with the epileptogenic zone, their clinical significance has been under-evaluated. The aim of this study was to evaluate the source localization of interictal spikes in terms of the association with epileptogenic zone in surgical temporal lobe epilepsy patients. The proposition is that the resection volume in patients with favorable outcomes includes the epileptogenic zone. The association with the epileptogenic zone was assessed as follows: (1) how many of the interictal spike sources are within the resection volume in patients with favorable outcomes and (2) how many of the interictal spike sources are outside the resection volume in patients with unfavorable outcomes. Thirty-eight temporal lobe epilepsy (TLE) patients who underwent both ECoG monitoring and epilepsy surgery were recruited and their 10min of ECoG recordings were analyzed. Six tumor-related TLE patients were excluded in the analysis. Of the remaining 32 patients, 20 patients achieved favorable surgical outcomes (Engel I and II), while the surgical outcomes of 12 patients were unfavorable (Engel III and IV). In each patient, interictal spike sources were localized using sLORETA and co-registered into a reconstructed brain model. The correspondence rate with the resection volume was estimated by counting the percentage of interictal spike sources in the resection volume. The correspondence rate in patients with favorable outcomes was 72.8±22.1, which was significantly higher than that (41.2±28.8) of the patients with unfavorable outcomes (p=0.002). Nine out of twelve patients (75%) with unfavorable outcomes had multiple interictal spike source clusters both interior and exterior to the resection volume, while 4 of the 20 patients with favorable outcomes (20%) had such multiple clusters (p=0.021). In conclusion, interictal spike sources are highly associated with the epileptogenic zone. ECoG interictal spike source localization could help in the delineation of the potential resection volume.