Pathophysiology 16 (2009) 39–42 Interhemispheric propagation time and temporal lobe epileptogenicity Martin Weinand a,∗ , Benjamin Serxner a , David Labiner b , Geoffrey Ahern b a Division of Neurosurgery, Department of Surgery, University of Arizona College of Medicine, Tucson, AZ, United States b Department of Neurology, University of Arizona College of Medicine, Tucson, AZ, United States Received 14 October 2008; received in revised form 5 December 2008; accepted 16 December 2008 Abstract Long-term subdural electroencephalographic (EEG) recording was performed in a series of patients with medically intractable complex partial seizures to test the hypothesis that ictal interhemispheric propagation time (IHPT) is correlated with temporal lobe epileptogenicity. In 41 patients, the duration from initial subdural EEG seizure onset to the first appearance of subdural EEG epileptic activity in the contralateral hemisphere (IHPT) was measured in seconds and analyzed for a quantitative relationship to temporal lobe seizure interval (frequency -1 ), in hours. A statistically significant, nonlinear correlation between IHPT and seizure interval was found (Arctan y = -0.009x 2 + 0.598x + 75.187, y = IHPT, in seconds, x = seizure interval, in hours, r = 0.326, d.f. = 39, t = 2.15, p < 0.05). The results suggest that, for seizure intervals less than 33h, increasing IHPT is associated with increasing seizure interval (i.e. decreasing epileptogenicity). Conversely, for seizure intervals greater than 33 h, decreasing IHPT is associated with increasing seizure interval. Because the relationship between IHPT and seizure interval is a trigonometric (i.e. arctangent) function of a second degree polynomial, small changes in IHPT are associated with substantial changes in seizure interval. These findings suggest that temporal lobe epileptogenicity is a complex, nonlinear function of the electrocorticographic EEG time factor (i.e. IHPT) involved in the transmission of ictal epileptic activity from the seizure focus to the contralateral hemisphere. The results suggest that, on an electrophysiologic basis, patients with temporal lobe epilepsy represent two distinct populations based on seizure interval. The results should improve the understanding of the electrocorticographic pathophysiology of temporal lobe epilepsy. The development of cortical neuromodulation strategies designed to suppress temporal lobe seizures should consider this complex relationship between temporal lobe interhemispheric propagation time and epileptogenicity. Published by Elsevier Ireland Ltd. Keywords: Temporal lobe epilepsy; Subdural EEG; Interhemispheric propagation time; Epileptogenicity 1. Introduction Among patients with epilepsy, temporal lobe seizures are the most difficult to control medically. Temporal lobec- tomy, including amygdalohippocampectomy, is the most commonly performed operation for medically intractable seizures [1]. Research which improves the understanding of the pathophysiology of temporal lobe epilepsy should improve the design of medical and surgical strategies aimed at curing this devastating disease. Previous research has delineated the ictal electrocortico- graphic EEG correlates of temporal lobe epileptogenicity ∗ Corresponding author at: P.O. Box 245070, 1501 N. Campbell Ave., Tucson, AZ 85724-5070, United States. Tel.: +1 520 626 5003; fax: +1 520 626 8313. E-mail address: Mweinand@u.arizona.edu (M. Weinand). [2,3]. However, among subdural EEG time factors, inter- hemispheric propagation time (IHPT) has only been shown to correlate with seizure frequency (or interval) as part of a multivariate model [2]. No univariate association between IHPT and temporal lobe epileptogenicity has, heretofore, been discovered. For patients in whom the seizure focus must be localized using invasive techniques, it has been customary to tailor surgical therapy for temporal lobe epilepsy based on ictal seizure focus localization within the epileptic temporal lobe [1]. Similarly, the delineation of a quantitative relationship between IHPT and an objective measure of temporal lobe epileptogenicity (i.e. seizure interval or frequency) should improve development of therapeutic neuromodulation tech- niques tailored for each specific patient based, at least in part, on the relationship between IHPT and seizure interval for the individual patient. 0928-4680/$ – see front matter. Published by Elsevier Ireland Ltd. doi:10.1016/j.pathophys.2008.12.003