EEG dipole source localization of interictal spikes in non-lesional TLE with and without hippocampal sclerosis

Purpose: A subgroup of patients with non-lesional temporal lobe epilepsy (NLTLE) have no evidence of hippocampal sclerosis (HS) on MRI or on histopathology. It is controversial whether this represents a different clinicopathological syndrome from NLTLE with HS, or whether both subgroups represent different ends of the spectrum of mesial TLE. Here the EEG source localization dipoles were compared between NLTLE patients with HS (HS+) and without HS (HS-), and the relationship with post-surgical outcome was investigated. Methods: EEG dipole source localization of interictal epileptiform spikes recorded during prolonged video-EEG monitoring was performed from 22 consecutive HS+ and 12 HS- NLTLE patients. EEG was acquired using 29 scalp electrodes, including an inferior temporal row. Up to 13 spikes per patient were averaged and sources localized using a boundary element model based on the patients volumetric MRI. Results: 21/34 patients (62%) had dipoles for the interictal spikes localized to the epileptogenic temporal lobe. The site of the intratemporal localization did not differ significantly between the HS+ and HS- patients, with the dipoles localizing to the mesial temporal region in 27% of HS+ and 25% of HS- patients. There was no significant relationship between the localization and orientation of the dipoles and the surgical outcome. Conclusion: The dipoles for interictal spikes do not differ between HS+ and HS- patients, suggesting that these subgroups of NLTLE patients do not have distinct cerebral generators.