Temporal epileptogenesis: Localizing value of scalp and subdural interictal and ictal EEG data

Purpose: To determine the value of scalp epileptiform EEG data and subdural interictal spikes in localizing temporal epileptogenesis among patients re quiring invasive recordings. For this delineation, we related such factors to site of subdural seizure origin in 27 consecutive patients. Methods: Patients with temporal robe epilepsy whose noninvasive lateralizin g data were inconclusive and therefore required subdural electroencephalogr aphy were studied. All patients had (a) 24-h scalp telemetered EEGs, (b) ad equate bitemporal subdural placements with an inferomesial line extending f rom a posterior burr hole anteriorly to <2.5 cm from anterior uncus and a l ateral line reaching wi;hin 2.5 cm of the temporal tip, and (c) <greater th an or equal to>2 subdurally recorded seizures. Results: Three hundred one (96%) of 314 subdurally recorded clinical seizur es involving all 27 patients arose from a discrete focus; 266 (85%) arose f rom mesial temporal regions, which was the origin of the majority of seizur es in 24 (89%) patients. The majority of subdural seizures arose ipsilatera l 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. Lateralizat ion of interictal subdural spikes correlated with that of subdural seizures in 74-92% of patients, depending on the method of spike compilation; for e xample, most subdural seizures arose from the same lobe of most consistent principal temporal spikes in 92% of patients. These indices of epileptogene sis also appeared more commonly on the side of effective (greater than or e qual to 90% improvement) temporal lobectomy than contralaterally in the fol lowing proportions: most consistent principal subdural spikes, 86% of patie nts 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 s pikes should be included when assessing the side of temporal epileptogenesi s.