Abstracts

Rationale: Mesial temporal lobe epilepsy (mTLE) is the most common form of epilepsy in adults. Medical therapy cannot provide adequate seizure control in approximately one third of patients, and the majority of these patients are candidates for epilepsy surgery. Seizure freedom can be achieved in more than 80% of patients when the epileptogenic zone is accurately localized [1]. When routine tests cannot achieve this task, or provide discordant localization, functional neuroimaging tests that detect ictal and inter-ictal changes in perfusion and metabolism are used. Inter-ictal FluoroDeoxyGlucose Positron Emission Tomography (FDG-PET), detects areas of hypometabolism that can identify the epileptogenic zone with a sensitivity and specificity of approximately 80% [2]. Cerebrovascular reactivity (CVR) refers to the response of blood vessels to vasoactive metabolites, such as carbon dioxide and can be measured by combining precise controlled CO₂ stimuli with measures of the MRI BOLD response. We hypothesized that localized areas of inter-ictal hypometabolism in mTLE are associated with vascular dysfunction that can be detected with CVR imaging. Methods: A homogenous group of unilateral mTLE patients (n=5) with prior standard pre- operative investigations for seizure localization were studied. A computer-controlled gas blender (RespirAct™) was used to precisely target normoxic end-tidal CO₂ (PETCO₂) gas changes. The experimental protocol consisted of two manipulations of PETCO₂ while fMRI images were acquired. CVR values were computed voxel-by-voxel for the whole brain as the best-fit regression for the BOLD signal vs. (PETCO₂) to generate CVR maps. For specific regions of interest, patient CVR values were compared to age-matched healthy volunteers who had undergone the same protocol (n=10) using kolmogorov smirnov tests. Inter-hemispheric variance in CVR values was assessed with two-way ANOVAs. Results: CVR was significantly increased in the epileptogenic anterior temporal pole compared to controls (p<0.04), and compared to the contralateral non-epileptogenic temporal pole. CVR was also increased in the amygdala compared to controls and the contralateral non-epileptogenic hemisphere. Interestingly, no significant differences in CVR values between patients and controls were found in the epileptogenic hippocampus or the ipsilateral middle temporal gyrus. Conclusions: The observed increase in CVR in unilateral mTLE patients ipsilateral to the epileptogenic side might represent a pathophysiological increase in vascular capacity required to support the sustained increase in neuronal activity during a seizure. CVR imaging thus might represent a non-invasive test similar to FDG-PET for lateralization of the epileptogenic zone, without the requirement of radioactive tracers. References: 1) Mansouri A, Fallah A, Valiante TA. Determining Surgical Candidacy in Temporal Lobe Epilepsy. Epilepsy Research and Treatment, doi:10.1155/2012/706917 2) La Fougère C, Rominger A, Förster S, et al. PET and SPECT in epilepsy: A Critical Review. Epilepsy & Behavior 2009; 15: 50-56