Comparative Analysis of Pupillometry in Idiopathic Rapid Eye Movement Sleep Behavior Disorder and Parkinson’s Disease

Background and Objective Pupillary light reflex (PLR) abnormalities have been reported in patients with Parkinson’s disease (PD). However, few studies have been conducted on the abnormality of PLR in patients with idiopathic rapid eye movement sleep behavior disorder (iRBD), which is a prodromal stage of α-synucleinopathy. We herein quantitatively analyzed the PLR of iRBD using an automated pupillometer, and compared the results with those of PD.Methods In this cross-sectional study, we prospectively enrolled 27 patients with polysomnography-confirmed iRBD, and 23 patients with PD. Pupillometry was performed three times in each eye, alternating left and right. We compared seven pupillometric parameters between the iRBD and PD patients.Results Maximum and minimum pupil diameters were significantly larger in PD patients than in iRBD patients. However, the other pupillometric parameters, such as mean constriction velocity, maximum constriction velocity, reflex amplitude, latency, and mean dilation velocity, did not differ between the two groups. Among iRBD patients, the pupillometric parameters were not correlated with any clinical characteristics related to autonomic dysfunction or neurodegeneration.Conclusions We found that the pupillary constriction and dilation in response to light of iRBD were not different from those of PD. These findings suggest that autonomic pupillary dysfunction already existed in the prodromal stage of α-synucleinopathy to a degree comparable to that in PD. Larger pupil diameters in PD than in iRBD may reflect the pharmacological effect of dopaminergic medications. Future studies are needed to elucidate the association between the PLR abnormalities and the risk of phenoconversion in iRBD.