Abstracts

RATIONALE: Verbal memory is often impaired in left TLE. We previously demonstrated that performance of verbal memory tasks with minimal semantic structure correlated with uptake of the glucose tracer FDG in mesial temporal structures whereas performance of semantically structured tasks correlated with uptake in the temporal neocortex. Here, we aimed to determine whether structural changes in the hippocampus (Hc) and entorhinal cortex (EC) affect verbal memory performance or modulate its relationship with FDG uptake. METHODS: Uptake of FDG measured by PET and performance on the 'hard' (arbitrarily-related) and 'easy' (semantically-related) paired associates of the Wechsler Memory Scale were studied in 33 patients with intractable left TLE. Normalised tracer uptake was obtained in spherical regions of interest (5mm radius) centred at sites within the EC and the temporal neocortex. These sites were previously shown to correlate with performance on 'hard' pairs and 'easy' pairs respectively. Hc and EC volumes were obtained using established anatomical landmarks and an asymmetry index calculated: 200+(R-L)/(R+L). Memory test scores were regressed against volume/asymmetry measurements and normalised activity. RESULTS: Memory for 'hard' pairs correlated with FDG uptake in the EC (r=0.39; p=0.027)but not with the asymmetry indices or volume of the left Hc or the left EC. Using ANCOVA, a significant main effect for FDG uptake was observed; there was no significant interaction between volume loss and tracer uptake. Memory for 'easy' pairs correlated with FDG uptake in the temporal neocortical ROI (r=0.37; p=0.036)but not with FDG uptake in the EC ROI. The asymmetry indices and volume measurements did not correlate with performance on 'easy' pairs. There was no significant interaction between volume loss and tracer uptake on ANCOVA. CONCLUSIONS: Interictal FDG uptake predicts memory performance independent of changes in volume. Our findings indicate that impairment of verbal memory cannot solely be explained by neuronal loss. Impaired FDG uptake in TLE reflects alterations in neuronal function sufficient to affect cognitive performance.