Summary:Purpose: To determine the value of scalp epileptiform EEG data and subdural interictal spikes in localizing temporal epileptogenesis among patients requiring invasive recordings. For this delineation, we related such factors to site of subdural seizure origin in 27 consecutive patients.Methods: Patients with temporal lobe epilepsy whose noninvasive lateralizing data were inconclusive and therefore required subdural electroencephalography were studied. All patients had (a) 24-h scalp telemetered EEGs, (b) adequate bitemporal subdural placements with an inferomesial line extending from a posterior burr hole anteriorly to <2.5 cm from anterior uncus and a lateral line reaching within 2.5 cm of the temporal tip, and (c) Ն2 subdurally recorded seizures.Results: Three hundred one (96%) of 314 subdurally recorded clinical seizures involving all 27 patients arose from a discrete focus; 266 (85%) arose from mesial temporal regions, which was the origin of the majority of seizures in 24 (89%) patients. The majority of subdural seizures arose ipsilateral to the majority of scalp EEG spikes in 22 (81%) of 27, and most subdural seizures of 15 (75%) of 20 arose ipsilateral to scalp seizures. Lateralization of interictal subdural spikes correlated with that of subdural seizures in 74-92% of patients, depending on the method of spike compilation; for example, most subdural seizures arose from the same lobe of most consistent principal temporal spikes in 92% of patients. These indices of epileptogenesis also appeared more commonly on the side of effective (Ն90% improvement) temporal lobectomy than contralaterally in the following proportions: most consistent principal subdural spikes, 86% of patients ipsilateral vs. 9% contralateral; scalp-recorded clinical seizures, 55% vs. 18%; scalp EEG spikes, 45% vs. 9%.Conclusions: Even among patients whose scalp data are sufficiently complex to require invasive recording for clarification, lateralization of temporal scalp interictal and ictal epileptiform activity and subdural interictal spikes should be included when assessing the side of temporal epileptogenesis. Key Words: Temporal epileptogenesis-Scalp data-Subdural data.Incredibly, virtually all writings on temporal lobe subdural electroencephalography have focused on its value relative to depth recordings (1-3). This preoccupation has limited both the amount of essential subdural electrode coverage and the extent of data analysis. As locations of scalp EEG abnormalities and those of subdural interictal findings may contribute to management decisions, their relative capabilities in identifying the principal epileptogenic zones needs evaluation. Surgical decisions hinge on the congruency of data indicating sites of epileptogenesis. Subdural recordings are terminated when the likelihood of any further contribution to congruency appears minimal. As the origin(s) of clinically typical seizures constitutes a cornerstone of this assessment, the clinician must decide when an adequate sample of seizures has been obtained. The value o...