Reduced Cardiac 123I-MIBG Scintigraphy and Olfactory Impairment in Idiopathic REM Sleep Behavior Disorder

Cardiac 123I-MIBG scintigraphy (cMIBG) and an olfactory function test using the Odor Stick Identification Test for Japanese (OSIT-J) were performed for 46 patients with idiopathic REM behavior disorder (iRBD). The H/M ratio on cMIBG of iRBD patients was classified according to the Hoehn-Yahr's (H-Y) stages of Parkinson's disease (PD) as follows: early images, normal range, 11(23.9%); H-YI, 3 (6.5%); H-YII, 5 (10.9%); ≥ H-YIII, 27 (58.7%); and delayed images, normal range, 9 (19.6%); H-YI, 3 (6.5%); H-YII, 1 (2.2%); ≥ H-YIII, 33 (71.7%). The OSIT-J scores decreased in 76.2% of iRBD patients, and 20.5% of Normal subjects. The OSIT-J scores in iRBD patients were strongly correlated with the H/M ratio on early and delayed cMIBG images (p<0.001, Pearson correlation coefficient). PD progresses from H-Y stage I to V, with the H/M ratio on cMIBG decreasing as the H-Y stage progresses.The H/M ratio in iRBD patients decreased to PD H-Y III/IV or dementia with Lewy bodies (DLB) levels in 71.7% of patients.Thus, in many cases, iRBDs is a precursor of DLBs but not of PDs. Although olfactory dysfunction has no disease specificity, it is a simple and useful screening test that can be applied before cMIBG for patients with iRBD.