Introduction: Epilepsy surgery can achieve seizure freedom in more than 80% of patients with intractable temporal lobe epilepsy, when optimal localization of the epileptogenic zone is performed [1]. When routine pre-operative testing provides discordant results, functional neuroimaging, such as Positron Emission Tomography (PET), can aid in localization. However, these tests are invasive, expensive, and are not easily accessible. Cerebrovascular reactivity (CVR) refers to the blood vessels’ response to a vasoactive stimulus such as carbon dioxide (CO2). We hypothesized that impaired CVR may be a novel biomarker for identifying the epileptogenic zone.

Methods: A homogenous group of unilateral mesial TLE patients (n=11) who had undergone standard pre-surgical investigations for seizure localization were studied. We utilized a novel technology that can precisely prospectively target arterial CO2 levels (Respiract™). Functional MRI images using the Blood Oxygen Level Dependent (BOLD) sequence as a surrogate for blood flow changes were obtained while iso-oxic changes in CO2 were induced. CVR imaging data for 11 healthy volunteers who had underwent the same study protocol was retrieved from an existing database at our institution, and age and gender matched to patients.

Results: We found a significant decrease in inter-ictal CVR in the epileptogenic parahippocampal gyrus (0.377+/-0.06) compared to the contralateral hemisphere (0.512+/-0.04) P=0.03 in the patient group. We also observed significantly increased inter-ictal CVR in the epileptogenic posterior cingulate cortex (0.439+/-0.09) compared to the contralateral hemisphere (0.340+/-0.06) P=0.02.These changes were not correlated with cortical thickness measurements. This inter-hemispheric variance in CVR values for these ROIs was not observed in healthy controls.

Conclusions: Our data suggests that CVR is altered both locally (epileptogenic focus) as well as in other areas of the brain that participate in epileptic activity. This finding also implies that the BOLD signal may be confounded if used for analysis of the resting state in these patients.

Patient Care: This study aims to develop a new low-cost functional imaging technique for seizure localization that would increase the availability of a localizing test at any centre with an MRI scanner. We therefore believe we can leverage existing technologies to provide low-cost functional imaging to localize epileptogenic regions.

Learning Objectives: • Assessing vascular reserve can be achieved with a reproducible, accurate, non-invasive imaging technique • Mesial temporal lobe epilepsy patients have impaired vascular reactivity that lateralizes with the epileptogenic focus • This observed inhomogeneity of the BOLD response to a vasoactive stimulus might confound findings of resting state studies in this cohort of patients.

References: 1. Mansouri, A., Fallah, A. & Valiante, T. A. Determining Surgical Candidacy in Temporal Lobe Epilepsy. Epilepsy Research and Treatment, 1–16 (2012).