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Electrical source imaging (ESI) is a well-established approach to localizing the epileptic focus in drug-resistant focal epilepsy. So far, ESI has been used primarily on interictal events. Emerging evidence suggests that ictal ESI is also feasible and potentially useful. We aimed to investigate the diagnostic accuracy of ESI on ictal events using high-density electroencephalography (EEG).

We performed ictal ESI on 14 patients (9 with temporal lobe epilepsy) admitted for presurgical evaluation who presented seizures during a long-term (≥18 h) high-density EEG recording (13 with 256 electrodes and one with 128 electrodes), and subsequently 8 of them underwent epilepsy surgery (postoperative follow-up >1 year). Artifact-free EEG epochs at ictal οnset were selected for further analysis. The predominant ictal rhythm was identified and filtered (±1 Hz around the main frequency). ESI was computed for each time point using an individual head model and a distributed linear inverse solution, and the average across source localizations was localized. For validation, results were compared with the resection area and postoperative outcome.

Ictal ESI correctly localized the epileptic seizure-onset zone in the resection area in five of six postoperatively seizure-free patients. Interictal and ictal ESI were concordant in 9 of 14 patients and partially concordant in additional 4 of 14 patients (93%). Divergent solutions were found in only one of the 14 patients (7%).

Ictal ESI is a promising localization technique in focal epilepsy.