Objective: To compare dopaminergic imaging, cardiac sympathetic denervation, olfactory function and expression of a PD-specific metabolic cerebral pattern in patients with idiopathic REM sleep behavior disorder (iRBD).

Background: iRBD is considered the most specific prodromal stage of Parkinson’s disease (PD). The characteristic brain pathology in PD results in a pattern of abnormal cerebral glucose metabolism: the PD related pattern (PDRP). Expression of this pattern can be quantified in any FDG PET scan. Previous reports have shown that this pattern is a useful marker of disease, even in predicting phenoconversion from iRBD to PD. Other markers for imminent PD are considered poor olfaction, abnormal dopaminergic imaging (DAT SPECT), and cardiac sympathetic denervation demonstrated by MIBG SPECT.

Methods: 31 iRDB patients and 19 controls underwent FDG PET. PDRP expression values were computed using spatial covariance analysis and were z-transformed to control values. Olfactory function was tested in all iRBD patients and controls with a standardized test (Sniffin’ Sticks). 23 iRBD patients additionally underwent FP-CIT SPECT. Striatum-to-occipital ratios were calculated in each FP-CIT scan for the putamen and caudate nucleus in each hemisphere. In 17 patients, MIBG SPECT was also performed. A heart-to-mediastinum ratio was calculated in each MIBG scan.

Results: 27/31 iRBD patients had hyposmia (range 7 mild to 7 anosmic). 11/17 iRBD patients had an abnormal MIBG scan. There was a significant relationship between MIBG SPECT uptake and a sub score of the olfactory test (ρ=0.728;P=0.005). 6/23 patients had an abnormal DAT scan. The PDRP was abnormally expressed in 24/31 iRBD patients, even in some with still normal dopaminergic imaging (n=7). PDRP expression correlated to DAT uptake in the putamen (ρ=-0.51;P=0.01), but did not correlate to olfactory function (ρ=-0.33;P=0.09).

Conclusions: Our findings suggest that loss of olfactory function and cardiac sympathetic denervation occur simultaneously. PDRP expression is present before abnormalities are evident on dopamine transporter imaging. Long-term studies in iRBD patients will determine the value of these markers in predicting conversion to clinical PD.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/what-predicts-parkinsons-disease-hyposmia-mibg-spect-dat-spect-and-the-pd-specific-metabolic-pattern/