Temporal lobe epilepsy in patients with nonlesional MRI and normal memory: an SEEG study

OBJECT Temporal lobe epilepsy (TLE) in the absence of MRI abnormalities and memory deficits is often presumed to have an extramesial or even extratemporal source. In this paper the authors report the results of a comprehensive stereoelectroencephalography (SEEG) analysis in patients with TLE with normal MRI images and memory scores. METHODS Eighteen patients with medically refractory epilepsy who also had unremarkable MR images and normal verbal and visual memory scores on neuropsychological testing were included in the study. All patients had seizure semiology and video electroencephalography (EEG) findings suggestive of TLE. A standardized SEEG investigation was performed for each patient with electrodes implanted into the mesial and lateral temporal lobe, temporal tip, posterior temporal neocortex, orbitomesiobasal frontal lobe, posterior cingulate gyrus, and insula. This information was used to plan subsequent surgical management. RESULTS Interictal SEEG abnormalities were observed in the mesial temporal structures in 17 patients (94%) and in the temporal tip in 6 (33%). Seizure onset was exclusively from mesial structures in 13 (72%), exclusively from lateral temporal cortex and/or temporal tip structures in 2 (11%), and independently from mesial and neocortical foci in 3 (17%). No seizure activity was observed arising from any extratemporal location. All patients underwent surgical intervention targeting the temporal lobe and tailored to the SEEG findings, and all experienced significant improvement in seizure frequency with a postoperative follow-up observation period of at least 1 year. CONCLUSIONS This study demonstrates 3 important findings: 1) normal memory does not preclude mesial temporal seizure onset; 2) onset of seizures exclusively from mesial temporal structures without early neocortical involvement is common, even in the absence of memory deficits; and 3) extratemporal seizure onset is rare when video EEG and semiology are consistent with focal TLE.