Relevance of substantia nigra hyperechogenicity and reduced odor identification in idiopathic REM sleep behavior disorder

BackgroundSubstantia nigra (SN) hyperechogenicity determined by transcranial sonography (TCS) and olfactory dysfunction are common findings in Parkinson disease (PD), which may reveal a prodromal synucleinopathy in idiopathic REM sleep behavior disorder (iRBD).MethodsTCS and the Odor Stick Identification Test for Japanese (OSIT-J) were performed in 34 consecutive patients with iRBD (67.9 ± 6.1 years), 17 consecutive patients with PD (66.4 ± 6.7 years), and 21 control group subjects (64.4 ± 5.8 years).ResultsThere was a significantly increased area of echogenicity in the SN in the iRBD group (0.20 ± 0.13 cm²) and PD group (0.22 ± 0.11 cm²) compared with the control group (0.06 ± 0.06 cm²). We found pathological SN hyperechogenicity (?0.20 cm²) in 41.2% of the iRBD group, 52.6% of the PD group, and 9.5% of the control group. Further, there were abnormal findings of both pathological SN hyperechogenicity (?0.20 cm²) and functional anosmia or hyposmia in 4 (11.8%) or 9 (26.5%) of the iRBD group subjects, respectively, and 7 (57.9%) or 2 (11.8%) of the PD group subjects, respectively.ConclusionPathological SN hyperechogenic abnormality and functional anosmia in iRBD may be a disease state in the transition to a neurodegenerative disease.