Summary:Purpose: To assess the additional value of subtraction ictal single-photon emission computed tomography (SPECT) coregistered to MRI (SISCOM) for localization of the epileptogenic zone in patients with drug-resistant epilepsy scheduled for invasive video-EEG (VEEG) before epilepsy surgery by a descriptive study from clinical practice.Methods: Forty-nine consecutive epilepsy patients between January 2000 and March 2006 were included. Thirty-six of the 49 patients were offered surgery, and 34 underwent resective surgery during the study period. Localizing and outcome data are presented from 31 patients with a follow-up period of ≥12 months. Successful ictal SPECT was performed in 26 patients, and SISCOM showed significant hyperperfusions with 3.5 SD above reference. Twenty patients had SISCOM-guided electrode placement, invasive monitoring, and 1-year postsurgical followup data. Two independent epileptologists evaluated whether SISCOM results (a) altered the hypothesis and extended the strategy for electrode placement at invasive recording, or (b) were confirmatory of other localizing data and did not alter the strategy. We defined that SISCOM had an impact on seizure outcome if the seizure-onset zone was seen in electrodes overlying a brain region with a significant hyperperfusion. When SISCOM was concordant with ictal onset in the extended electrodes, SISCOM was considered a prerequisite for the outcome at postoperative follow-up.Results: SISCOM findings altered and extended the strategy for electrode placement at invasive recording in 15 patients (group A). SISCOM was a prerequisite for seizure outcome in all six patients with favorable outcomes. Nine patients had poor results from surgery in this group; SISCOM was concordant with invasive VEEG in six patients, and discordant with invasive VEEG in three patients. SISCOM findings were confirmatory with other localizing data and did not alter the strategy at invasive recording in five patients (group B). Two patients had favorable surgical outcomes. In this group, three patients had poor results; SISCOM and other localizing findings were concordant with invasive VEEG in one patient and discordant with invasive VEEG in two patients.Conclusions: SISCOM is valuable for the identification of the epileptogenic zone in patients with drug-resistant epilepsy scheduled for invasive VEEG. SISCOM analysis was either a prerequisite for favorable result or concordant with other localizing methods in all patients with favorable seizure outcome at 1 year of follow-up [40%; confidence interval (CI), 19-64). Key Words: Epilepsy-Ictal SPECT-SISCOMSurgical outcome.Epilepsy surgery is successful in ∼70-80% of operatedon patients. The likelihood of seizure freedom is higher if an epileptogenic lesion has been identified on MRI during the presurgical workup (Engel et al., 1993). During the last decade, enhanced diagnostic sensitivity by advances in neuroimaging has reduced the proportion of patients with normal MRIs and has led to a decrease in the need for invasive video-EEG (...