Objective: Idiopathic rapid eye movement sleep behavior disorder (iRBD) is a well-known risk factor for Parkinson’s disease, and provides an opportunity to test biomarkers in the prodromal stage. Recently, expression of the Parkinson’s disease related metabolic pattern (PDRP) was used to predict conversion from iRBD to Parkinson’s disease. We compared PDRP expression in iRBD to striatal dopamine transporter (DAT) binding and olfaction.
Methods: PDRP expression z-scores were computed in 18F-FDG PET brain scans of 30 iRBD patients and 19 controls. PDRP z-scores were compared to z-scores in Parkinson’s disease (n=14) and dementia with Lewy bodies (n=14). Based on prior research, a cut-off z-score of 1.8 was used to indicate 100% specificity for Parkinson’s disease.
In 21/30 iRBD patients, DAT imaging with 123I-FP-CIT SPECT was performed. The relationship between pattern expression, DAT binding and olfactory function (Sniffin’ Sticks test) was studied.
Results: PDRP expression was higher in iRBD compared to controls (P=0.003), but lower compared to Parkinson’s disease. Seventeen iRBD patients (57%) had a z-score≥1.8. PDRP z-scores correlated to putamen DAT binding (r=-0.61, P=0.005). Loss of striatal DAT binding was observed in 9/21 patients (43%). Interestingly, of the 12 iRBD patients with a normally rated 123I-FP-CIT SPECT scan, 5 (42%) expressed the PDRP (z-score≥1.8). Olfactory dysfunction was observed in 23/26 iRBD subjects. There was no significant relationship between olfaction and PDRP z-scores or DAT binding.
Interpretation: The PDRP is a suitable biomarker for neurodegeneration in prodromal Parkinson’s disease, and its expression may be detected before the nigrostriatal pathway is affected.