'MRI-negative PET-positive' temporal lobe epilepsy (TLE) and mesial TLE differ with quantitative MRI and PET: a case control study

Abstract

Background: 'MRI negative PET positive temporal lobe epilepsy' represents a substantial minority of temporal lobe epilepsy (TLE). Clinicopathological and qualitative imaging differences from mesial temporal lobe epilepsy are reported. We aimed to compare TLE with hippocampal sclerosis (HS+ve) and non lesional TLE without HS (HS-ve) on MRI, with respect to quantitative FDG-PET and MRI measures.

Methods: 30 consecutive HS-ve patients with well-lateralised EEG were compared with 30 age- and sex-matched HS+ve patients with well-lateralised EEG. Cerebral, cortical lobar and hippocampal volumetric and co-registered FDG-PET metabolic analyses were performed.

Results: There was no difference in whole brain, cerebral or cerebral cortical volumes. Both groups showed marginally smaller cerebral volumes ipsilateral to epileptogenic side (HS-ve 0.99, p = 0.02, HS+ve 0.98, p < 0.001). In HS+ve, the ratio of epileptogenic cerebrum to whole brain volume was less (p = 0.02); the ratio of epileptogenic cerebral cortex to whole brain in the HS+ve group approached significance (p = 0.06). Relative volume deficits were seen in HS+ve in insular and temporal lobes. Both groups showed marked ipsilateral hypometabolism (p < 0.001), most marked in temporal cortex. Mean hypointensity was more marked in epileptogenic-to-contralateral hippocampus in HS+ve (ratio: 0.86 vs 0.95, p < 0.001). The mean FDG-PET ratio of ipsilateral to contralateral cerebral cortex however was low in both groups (ratio: HS-ve 0.97, p < 0.0001; HS+ve 0.98, p = 0.003), and more marked in HS-ve across all lobes except insula.

Conclusion: Overall, HS+ve patients showed more hippocampal, but also marginally more ipsilateral cerebral and cerebrocortical atrophy, greater ipsilateral hippocampal hypometabolism but similar ipsilateral cerebral cortical hypometabolism, confirming structural and functional differences between these groups.

Figure 1

Volumetric MRI and coregistered FDG-PET: (a) MRI Extracted whole brain is segmented into (b) MRI right and left cerebral hemispheres, in turn thresholded into (c) MRI right and left cerebral cortices. Coregistration of the (d) FDG-PET whole brain allows accurate anatomical definition on the metabolic studies of, for example, (e) FDG-PET right and left cerebral hemispheres and (f) FDG-PET right and left cerebral cortices.

Figure 2

FDG-PET left temporal cortex with coregistered hippocampal tracing, coronal section.

Figure 3

MRI volumetric comparisons: HS-ve versus HS+ve. Case-control pairs are presented in order of ascending age. Ratios presented relate to epileptogenic/contralateral MRI volumes: hippocampus. Mean and standard deviation figures are given in Table 2.

Figure 4

MRI volumetric comparisons: HS-ve versus HS+ve. Case-control pairs are presented in order of ascending age. Ratios presented relate to epileptogenic/contralateral MRI volumes: cerebrum. Mean and standard deviation figures are given in Table 2.

Figure 5

MRI volumetric comparisons: HS-ve versus HS+ve. Case-control pairs are presented in order of ascending age. Ratios presented relate to epileptogenic/contralateral MRI volumes: cerebral cortex. Mean and standard deviation figures are given in Table 2.