Observational Cross-sectional Study Investigating the History of Concussion and Exposure to Contact Sports in Patients With REM Sleep Behaviour Disorder (RBD) Versus Controls (Without a Diagnosis of RBD)

Rapid eye movement (REM) sleep behaviour disorder (RBD) is a parasomnia characterised by the presence of acting-out behaviours in sleep and vivid dreams, which diagnosis requires the demonstration of loss of REM atonia or an RBD episode on a video-polysomnography (v-PSG). Importantly, patients with the isolated form of RBD are at a high risk to develop Parkinson's disease or related neurodegenerative conditions, such as dementia with Lewy bodies or multiple system atrophy (synucleinopathies). Hence, RBD is a condition frequently associated to, and often preceding, a manifest neurodegenerative disorder. It is therefore extremely relevant to identify factors that may be linked to these early neuropathological processes. This will be key for a timely diagnosis of isolated RBD, especially when neuroprotective trials become available.

There is also solid evidence showing that repeated concussions lead to an increased risk to develop a neurodegenerative disorder such as Parkinson's disease, chronic traumatic encephalopathy, all-causes of dementia, and amyotrophic lateral sclerosis.

In epidemiological studies, self-reported head injury was more frequently present in RBD vs controls. Head injury was also proposed as a possible risk factor for probable RBD in large community-based studies, although the diagnosis of RBD was not confirmed by v-PSG in these cases. Of note, a higher frequency of professional football players was demonstrated on a recent study including a large cohort of RBD patients when compared to a control group. Moreover, in a group of deceased contact sport athletes with neuropathology diagnosing traumatic chronic encephalopathy, probable RBD was present in over 30% of the sample.

We hypothesise that the presence of a history of concussions or exposure to contact sports is more frequently reported by patients with v-PSG-confirmed RBD than by sex- and age-matched controls. There is, however, a lack of further characterisation of the head-impact events and circumstances in which they have occurred. It would be relevant to know whether repeated head impacts while playing contact sports or concussions that may have not been formally diagnosed, and that may have happened in non-professional as well as in professional sport activities, may be linked with RBD. The demonstration of a higher percentage of concussions or exposure to contact sports in RBD would provide greater evidence related to neuropathological changes in the context of certain sport practices.

Given the prognostic implications of a diagnosis of RBD and the established links between head injury and neurodegenerative conditions, it seems relevant to assess the characteristics of previous concussions or exposure to head-impact sports in patients with v-PSG-confirmed RBD. This will provide further evidence to help increase the awareness of RBD (hopefully leading to a more timely diagnosis of this condition), and potentially aid the development of health and government policies, including those relating to safer sport practices.