Distinguishing subtypes of temporal lobe epilepsy with background hippocampal activity

Purpose. Two subtypes of temporal lobe epilepsy (TLE) can be defined throug h clinical observations and analysis of hippocampal tissue resected during surgical procedures for intractable TLE: (a) mesial temporal sclerosis (MTS ), which is characterized by extensive changes to the hippocampus and good surgical outcome; and (b) paradoxical temporal robe epilepsy (PTLE), which is characterized by minimal cell loss and comparatively poorer surgical out come. Patients in both subtypes have seizures that appear to begin in the m edial temporal lobe, but documented differences in substrate and outcome be tween these subtypes has defined a need to distinguish MTS and PTLE patient s before surgery. This report describes a retrospective study to investigat e the feasibility of doing so during intracranial monitoring. Methods: Background EEG epochs, 5 min in duration, were recorded from the a nterior hippocampus in 14 (10 MTS and four PTLE) patients with consistent l ocalization of seizure onset to medial temporal structures. The power spect ral density (PSD) of the EEG epochs was calculated by a Fourier spectral es timator, and the total signal power and power of the delta, theta, alpha, b eta, and gamma frequency bands were submitted to group-to-group comparison. Results: Spectral peaks were observed in the delta band in all PSD estimate s and in the theta band in nine of 14 (seven MTS, two PTLE) estimates. The MTS and PTLE subtypes could be distinguished by the total signal power and delta band power. These power measurements were greater in the PTLE subtype . Conclusions: Both delta and theta spectral components are present in hippoc ampal background EEGs recorded from patients with TLE. The results indicate that group differences exist in spectral measures of background hippocampa l signals recorded from MTS and PTLE subtypes. This suggests both that subs trate differences in cellular composition and connectivity are reflected in hippocampal background EEGs and that spectral measurements of these signal s may hold promise for tests to identify the group membership of individual patients.