Evaluation of patterns of altered FDG Metabolism in ictal brain 18F-FDG PET/CT in patients with refractory epilepsy.

Introduction: To evaluate the abnormal brain metabolism in ictal 18F-FDG PET/CT and to describe the patterns of brain metabolism and associated common etiologies.

Methods: We retrospectively analysed scans, reports and treatment records of patients with refractory epilepsy over the last 82 months, who underwent brain 18F-FDG PET/CT in our department. Patients with documented epilepsia partialis continua or who had documented seizure episode in the post 18F-FDG injection period were included in the analysis. Z-score maps of brain 18F-FDG-PET/CT were analysed in Cortex ID (GE Healthcare). Brain region with focal hypermetabolism and mean Z-scores with magnitudes ≥2.00 were interpreted as significant. In addition visual analysis of altered metabolism of cortices, subcortical grey matter regions and cerebellar hemispheres were also done by two nuclear medicine physicians.

Results: Scans of twenty two patients (M:F=9:13) with median age of 8years (range1-50 years) were finally analysed and 14/22(63%) had epilepsia partialis continua. Majority of the patients (n=16) were less than 18 years of age. Final diagnosis was Rasmussen’s encephalitis in 12 patients, FCD in 3 and mitochondrial disease in 1. 6 patients had no final etiological diagnosis with 3 suspected of having Rasmussen’s encephalitis. Ictal PET detected focal hypermetabolism consistent with electroencephalographic (EEG) recording in all patients. Involvement was confined to single lobe in 13 patients (frontal-10, temporal-2, occipital-1) and was extending to single adjacent lobe in 5 and 2 adjacent lobes in 4. Lobe most commonly involved is frontal (17), followed by temporal (10), parietal(5) and occipital(3). Non-contiguous focal hypermetabolism of the precuneus and visual cortices were present only in 2 and 1 patients respectively.Hypermetabolism of the ipsilateral basal ganglia, ipsilateral thalamus, ipsilateral cerebral peduncles/brain stem were present in 13/22(59%), 15/22(68%) and 7/22(32%) of the scans. Hypermetabolism of the contralateral cerebellar hemisphere was present in 12/22(54%) patients. Ictal PET showed epileptogenic zone as focal hypermetabolism in patients with Rasmussen’s encephalitis and associated hypometabolism involving multiple lobes.

Conclusions: Ictal PET is useful in localization of the epileptogenic zone in patients with epilepsia partialis continua. Large area of hypermetabolismseen involving multiple lobes could be due to variable uptake or seizure spread in the brain FDG uptake period. Similar to hyperperfusion of the basal ganglia and cerebellum seen in ictal brain perfusion study, hypermetabolism of the basal ganglia, thalamus and cerebral peduncle/brain stem and contralateral cerebellar hemispheres are the findings which can help in differentiating the ictal PET from other causes of hypermetabolism.