AimWe investigated preclinical abnormalities as revealed by transcranial sonography (TCS) in patients with idiopathic rapid eye movement sleep behavior disorder (iRBD) compared with those revealed in patients with multiple system atrophy (MSA) or Parkinson's disease (PD) and in normal controls. MethodsTwenty-two patients with iRBD, 21 patients with MSA, 22 patients with PD, and 21 normal controls were included in this study. All participants underwent one night of video-polysomnography monitoring, and the sleep parameters were analyzed using Polysmith software and by visual analysis. TCS was performed following a standardized procedure. The echogenicity of the substantia nigra and basal ganglia were evaluated. ResultsA greater proportion of PD patients were found to have substantia nigra hyperechogenicity (86.4%) when compared to iRBD patients (31.8%), MSA patients (23.8%), and normal controls (4.8%) (P < 0.001). Fourteen MSA patients (66.7%) and 11 iRBD patients (50.0%) had hyperechogenicity in the basal ganglia, whereas hyperechogenicity in the basal ganglia was less frequent in PD patients (18.2%) and normal controls (9.5%) (P < 0.001). Poor sleep efficiency, less stage II sleep time, and more periodic leg movements were found in MSA and PD patients, whereas iRBD patients had almost normal sleep. ConclusionSome iRBD patients had basal ganglia hyperechogenicity that was similar to that observed in MSA, which may represent another possible convert direction. The present study further confirmed iRBD as a prodromal stage of synucleinopathy. TCS could detect subclinical changes and thus might provide useful markers for identifying individuals at increased risk for developing a synucleinopathy.