Diffuse occipital hypometabolism on [18F]-FDG PET scans in patients with idiopathic REM sleep behavior disorder: Prodromal dementia with Lewy bodies?

Background: Previous longitudinal studies have revealed that specific patterns on [F-18]-fluoro-D-glucose (FDG) positron emission tomography (PET) scans in patients with amnesic mild cognitive impairment can predict Alzheimer's disease (AD). However, the significance of particular patterns on [F-18]-FDG PET scans in prodromal patients with dementia with Lewy bodies (DLB) remains unclear. Methods: Based on the prevailing evidence that rapid eye movement (REM) sleep behavior disorder (RBD) often precedes the onset of DLB, [F-18]-FDG PET scans of nine non-demented patients reporting recurrent nocturnal dream-enactment behavior in our memory clinic were compared with the normative database using three-dimensional stereotactic surface projection (3D-SSP) images. All patients underwent clinical and neuropsychological examinations as well as cardiac [I-123]-metaiodobenzylguanidine ([I-123]-MIBG) scintigraphy. Results: Four patients were found to have diffuse areas of reduced cerebral metabolic rate of glucose (CMRgIc), predominantly in the occipital lobe, which is the preferentially affected region in DLB patients. In constrast, five patients showed no such occipital hypometabolism; instead, these five patients showed hypometabolism in the left anterior cingulate gyrus (Broadmann area (BA) 24), right frontal lobe (BA 32) and right anterior temporal lobe (BA 38), which are the preferentially affected regions in Parkinson's disease rather than DLB. The extent of the reduction in CMRgIc in the left occipital lobe was correlated with scores on the Bender Gestalt Test, which reflects visuospatial ability, but not with global cognitive measures. All patients showed reduced cardiac [I-123]-MIBG levels, consistent with underlying Lewy body disease. Conclusion: These variations in [F-18]-FDG PET scans raise the possibility that the specific pattern of CMRgIc reduction may predict developing DLB in patients with idiopathic RBD. Further follow-up studies are needed, particularly on patients with diffuse occipital hypometabolism.