Abstracts

Rationale: 2-[18F]fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) can often help identify epileptic foci when the MRI is unremarkable. PET hypometabolism concordant with the side of surgery has been identified in as many as 77% of MRI-negative TLE cases, with a positive predictive value for good outcome (defined as Engel I or II) in 80%.1 The prevalence and surgical outcomes of bilateral temporal hypometabolism or temporal hypometabolism contralateral to the resection side are unknown. We aimed to correlate the patterns of PET hypometabolism with both the intracranial EEG (ICEEG) sublobar onset zones, as well as with surgical outcomes in this group. Methods: Sixty-five MRI-negative TLE patients with pre-surgical FDG-PET and 1.5 / 3 Tesla MRI scans, who had undergone epilepsy surgery between 1999 and 2014, were identified retrospectively from the surgical databases of the University of California San Francisco Medical Center and the Montreal Neurological Institute. Demographics, electro-clinical information as well as surgical outcomes at 1 year were collected. The FDG-PET scans were blindly reviewed and scored independently by 2 epileptologists (YLT, RCK) trained in PET-imaging. The following regions of PET hypometabolism were tabulated for both temporal lobes: mesial, lateral, infero-basal, and temporal pole. Extratemporal hypometabolism was also marked if seen. Cohen's kappa was >0.9 and any discrepancy was resolved following a repeat review and discussion. Results: Clinical demographics, as well as the spatial patterns of PET hypometabolism and their correlation to seizure onset zone on ICEEG and surgical outcomes are summarized (Tables 1 and 2). Overall, PET hypometabolism was concordant to the resection side in 46/65 (70.8%) patients, of which 82.6% achieved good (Engel I or II) surgical outcomes. Bilateral temporal hypometabolism was present in an additional 8/65 (12.3%) patients; 87.5% achieved good surgical outcomes. Contralateral temporal hypometabolism, present in 9/65 (13.8%) patients, was significantly associated with bad surgical outcomes (77.8% Engel III/IV; Fisher exact test p < 0.01). Extratemporal hypometabolism was present in 6/65 (9.2%) patients; 50% had good surgical outcomes. Of the 39/65 patients who underwent ICEEG monitoring, 23 (59.0%) had seizure onset from the mesial temporal lobe structures, 3 (7.7%) from lateral temporal lobe, and 12 (30.8%) diffusely from both mesial and lateral temporal regions. There was no correlation between PET pattern and sublobar seizure onset zone within the temporal lobe. Conclusions: Neither extratemporal nor bilateral temporal hypometabolism in MRI-negative TLE should preclude epilepsy surgery, as good surgical outcomes can be obtained in up to 87.5% of such patients. In contrast, the presence of temporal hypometabolism predominantly contralateral to the intended resection site is a negative predictor for post-operative seizure freedom. Larger datasets will be valuable in validating these results. Funding: Nil