Early diagnosis of Osmotic Demyelination syndrome with 18-FDG PET CT

Introduction: Osmotic Demyelination Syndrome (ODS) which compasses both Central Pontine Myelinolysis (CPM) and Extrapontine Myelinolysis (EPM) is a rare neurological complication attributed to rapid correction of hyponatremia or rapid shifts of osmolality in certain metabolic and toxic states. It is characterized by destruction of the myelin sheath and oligodendrocytes in pons, cerebellum, hippocampus, basal ganglia, brain stem and thalamus without any inflammation and with relative preservation of the axons. Early diagnosis is imperative for adequate management of patients with Osmotic Demyelination Syndrome. MRI with its special sequences is the investigation of choice for the detection of the osmotic changes in the brain but sometimes is non contributory very early in the course of disease. 18-FDG PET shows areas of focal hypermetabolism in the pons, cerebellum, brain stem and bilateral medial temporal regions as early as within 24 hours of the disease and may be useful for establishing the diagnosis in clinically suspected ODS. Objectives: We aimed to assess the clinical utility of FDG PET CT in patients with suspected Osmotic Demyelination Syndrome.

Methods: 6 consecutive patients (3 males and 3 females) with suspected Osmotic Demyelination Syndrome, were evaluated with FDG PET/CT scan of the brain. The patterns of FDG uptake were recorded and comparison with normalized data was attempted. The areas of hypo/hypermetabolism that were two standard deviations from the mean were considered as abnormal. The patients were also analyzed based on the Z score surface maps of the 3D stereotactic surface projections (SSP) image and regional Z scores were evaluated. Post treatment follow-up scans, when available were compared with the pre-treatment scans. The FDG PET CT scans were compared with the diffusion weighted MRI scans and the areas of concordance recorded.

Results: All patients had an abnormal pattern of FDG uptake, both on visual inspection and semiquantitative analysis. Variable degrees of focal hypermetabolism was noted in the pons, mid brain, thalami, basal ganglia, cerebellum, brain stem and bilateral medial temporal regions with or without associated cortical hypometabolism. Out of the 6 subjects, 3 patients had normal MRI at the time of clinical and scintigraphic diagnosis and 3 showed mildly restricted diffusion in the pons. All the subjects were positive for ODS on delayed MRI scans. On follow up, 2 out 6 showed improvement in the cerebral glucose metabolic pattern.

Conclusions: FDG PET is a sensitive non invasive imaging modality which may contribute to the early diagnosis and management of patients with clinical suspicion of ODS, especially in the setting of non contributory/normal MRI. It increases the overall diagnostic accuracy and has an impact on the clinical management in a significant proportion of patients. Key words: Osmotic Demyelination Syndrome, pons, MRI,18- FDG PET