Abstracts

Rationale: Patients with mesial temporal lobe epilepsy (MTLE) may present ictal onset contralateral to the unilateral hippocampal sclerosis (HS) during prolonged monitoring with scalp-sphenoidal electrodes, which has been attributed to the so-called “burned-out hippocampus.” However, depth electrode monitoring has confirmed false lateralization of the non-invasive evaluation. It has been suggested that in this situation the hippocampus is so badly damaged that recruitment of enough neocortical neurons to produce visible scalp discharges is not accomplished until propagation of the seizure to the contralateral temporal lobe. Up to now quantitative assessment to determine how damaged/sclerotic the hippocampus is to be called “burned-out” is lacking. The objective of this study was to evaluate the degree of HS in these patients using quantitative MRI.Methods: Pre-surgical patients diagnosed with MTLE who presented false lateralization during video-EEG monitoring were re-submitted to it using semi-invasive foramen ovale electrodes. Hippocampal volume and degree of asymmetry between the atrophic and contralateral hippocampi were measured in 1.5 T MRI using the same protocol for all patients according to previously published protocols (Lin K, et al. Epilepsia. 2007 Apr 13; [Epub ahead of print]). We identified four patients harboring the so-called “burned-out hippocampus.” Surgical outcome was graded according to Engel’s classification. Furthermore, a comparative analysis was performed between the group of four patients (BO group), another group with 24 matched controls (patients with MTLE not “burned-out” - MTLE group) and a healthy control group with 30 volunteers, members of the medical staff.Results: The mean volume of the atrophic hippocampus in BO group was 1111.77mm3 (SD 206.56) while contralateral hippocampus measured 2327.26mm3 (SD 305.63), with degree of asymmetry of 70.52% (SD 17.05%). With a mean post-surgery follow-up period of 20 months (5-43 months) two patients were Engel I and one, Engel III. The fourth patient is still awaiting surgical treatment. The mean volume of the atrophic hippocampus of the MTLE group was 1081.82mm3 (SD 388.21) and of the contralateral 1891.35mm3 (SD 206.82) with asymmetry index of 58.12% (SD 29.15%). Statistical analysis did not demonstrate any difference in the volume of the atrophic hippocampus and degree of asymmetry between BO and MTLE groups (p > 0.05). Larger contralateral hippocampi were observed in BO group when compared to MTLE group. Their values reached marginally statistical significance (p = 0.06). In the healthy control group, the mean hippocampal volume was 2078.21mm3 (SD 268.05).Conclusions: Quantitative assessment by MRI did not demonstrate volumetric evidences of the so-called “burned-out hippocampus.” Pathophysiologic mechanisms involved in its pathogenesis need further elucidation.