The conversion rate and estimated risk of neurodegenerative diseases vary with idiopathic rapid eye movement sleep behavior disorder (IRBD).

This retrospective cohort study examined 273 patients (213 men, 60 women) with polysomnographic‐confirmed IRBD (192 and 81 patients in the Sleep Center [SC] cohort and Neurological Center [NC] cohort, respectively) who were followed longitudinally. The date of diagnosis was determined as the onset of an overt neurological syndrome. The conversion rate was calculated; the risk of developing an overt neurological syndrome was estimated using the Kaplan‐Meier method.

The estimated onset risk for a neurodegenerative syndrome from the time of IRBD diagnosis when the SC and NC cohorts were combined was 11.9%, 20.3%, 33.2%, and 51.4% at three, five, seven, and ten years, respectively. The phenoconversion rate (21.7% with a mean follow‐up period from the time of IRBD diagnosis of 3.9 ± 3.0 years) was lower than in prior studies, but the conversion risk increased progressively as the follow‐up period increased. The majority of patients developed Lewy body disease, while multiple system atrophy was rare. The risk of developing Lewy body disease differed significantly between the SC and NC cohorts (P = 0.02).

In this first study of a large Asian IRBD population, the estimated conversion risk leading to diagnosis differed between the two cohorts, which could be attributed to different evaluation results depending on the observed population due to a referral bias and follow‐up duration. Researchers should be aware of potential selection bias in their clinical studies.