Hippocampal Atrophy Is Not a Major Determinant of Regional Hypometabolism in Temporal Lobe Epilepsy

Purpose: The pathophysiologic basis for the [18F]fluorodeoxyglucose positron‐emission tomography (FDG‐PET) temporal lobe hypometabolism in patients with hippocampal sclerosis (HS) is uncertain. We tested the hypothesis that hippocampal atrophy, which is strongly correlated with hippocampal cell loss, is largely responsible for the regional hypometabolism in HS. Methods: Regions of interest (ROIs) on FDG‐PET scanning were determined in the medial, lateral, and posterior temporal lobe, thalamus, and basal ganglia. A right/left asymmetry index for each ROI was calculated. These results were correlated with hippocampal magnetic resonance imaging (MRI) volume ratios. Results: There was no correlation between the magnitudes of the FDG‐PET asymmetry index and the MRI volume ratio for the mesial or lateral temporal regions (r =−0.09, r =−0.04). When the right/left asymmetry index was compared with the right/left hippocampal volume ratio, correlations for the mesial temporal ROI (r = 0.79, p < 0.0001) and lateral temporal ROI (r = 0.57, p < 0.0005) were found. These, however, simply indicated that both tests accurately reflect the side of the epileptogenic region. The concordance of the side of relative hypometabolism of the FDG‐PET with the side of the hippocampal atrophy was higher for the mesial temporal region (100%) than for the lateral (77.5%). Conclusions: The lack of correlation between the magnitudes of the ratios argues against hippocampal atrophy and cell loss having a central role in the FDG‐PET temporal hypometabolism.