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REM sleep behaviour disorder (RBD) is a parasomnia characterised by the loss of muscle atonia in REM sleep, leading to dream enactment behaviours with or without vocalisations. ‘Idiopathic’, or perhaps more accurately isolated, RBD occurs in the absence of an underlying neurological disorder and has a prevalence of 0.38% to 1.15% in the general population aged over 60 years.1 2 It is recognised that RBD frequently represents an early prodromal marker of an emerging synucleinopathy (Parkinson’s disease, dementia with Lewy bodies or multiple system atrophy), with over 70% of individuals developing one of these diseases over a 12-year follow-up.3 However, RBD does not occur exclusively with synucleinopathies. We set out to perform a systematic review to describe the spectrum of other neurological disorders associated with RBD.
We searched PubMed on 1st December 2019 to identify all previous reports of polysomnography-confirmed RBD cases occurring in association with neurological conditions (online supplementary figure). The search terms ‘rapid eye movement sleep behaviour disorder’ OR ‘rapid eye movement sleep behavior disorder’ OR ‘RBD’ were used. Only full-text articles published in the English language were considered. Pathological diagnoses were prioritised if this information was available; where mixed pathologies were reported, cases were excluded if one was a synucleinopathy. Papers were excluded if the patients had been reported previously. Cases of subclinical RBD (defined as confirmed REM sleep without atonia in the absence of RBD symptoms), cognitive impairment or neurological presentations of uncertain aetiology, or RBD occurring in healthy individuals (usually children) were excluded. We did not include RBD cases associated with primary sleep disorders such as narcolepsy (which is known to be associated with RBD) or obstructive sleep apnoea (which can co-exist with RBD or directly lead to RBD-like movements during apnoeic episodes). RBD cases occurring while taking antidepressant medications were excluded, even if the patient had another neurological condition, since it remains unknown whether this reflects a reversible side effect or an unmasking of RBD in a patient subgroup that are at increased risk of developing a defined neurodegenerative syndrome. Although RBD has been associated with psychiatric disorders, these cases were excluded since they often occurred in the context of other neurological conditions and/or antidepressants.
We found 237 patients with polysomnography-confirmed RBD occurring in association with neurological conditions not caused by alpha-synuclein pathology (table 1). Structural brain lesions, typically in the brainstem, accounted for 19% of cases (n=45). While ischaemic lesions were the predominant aetiology, other lesions were also reported (eg, inflammatory, neoplastic, vascular malformations). The importance of the brainstem location, as opposed to the underlying disease mechanism, was further highlighted by co-occurrence of RBD and Arnold-Chiari malformations in 29 patients (12%).
While RBD is typically regarded as an early feature of synucleinopathies, some patients had other degenerative brain diseases, specifically Alzheimer’s pathology (n=29, 12%) or tauopathy (n=22, 9%). We considered whether some of these cases may have been misdiagnosed synucleinopathy cases, however several of them were pathological studies which makes this less likely. Autoimmune encephalitis was observed in 22 patients (9%), which is not surprising since these conditions are known to sometimes disrupt the sleep-wake state. Epilepsy (n=19, 8%), spinocerebellar ataxia (particularly SCA-3 (n=20, 8%)), traumatic brain injury (n=11, 5%) and Wilson’s disease (n=6, 3%) were among the other causes observed.
Given the prevalence of RBD in the general population, we acknowledge that some of the reported associations may have been a coincidence and some of the patients might have gone on to develop a synucleinopathy over time. We also accept that there may be some misclassification of conditions that were diagnosed clinically in the absence of histopathology. Furthermore, our search strategy may have underestimated the overall number of cases due to the inclusion of only polysomnography-confirmed RBD cases, as well as the number of RBD cases associated with psychiatric conditions due to exclusion of these individuals (as a result of the potentially confounding influence of antidepressant medications).
Our findings indicate that a spectrum of neurological pathologies, many affecting the brainstem, can be associated with polysomnography-confirmed RBD, and these should be considered when the history or examination is not indicative of a synucleinopathy. It seems likely that synucleinopathies lead to RBD by damaging these same brainstem regions as a result of them being particularly vulnerable to synuclein-mediated neurodegeneration. Normal REM sleep atonia is believed to be under the control of the subcoeruleus in the brainstem (which sends inhibitory projections to spinal motor neurons),4 with disrupted glutamate transmission in this region being particularly implicated in RBD.5
Contributors LHMK performed the literature review and wrote the paper. DPB conceived the idea for the systematic review, performed the literature review and wrote the paper.
Funding DPB is funded by a Wellcome Clinical Research Career Development Fellowship.
Competing interests None declared.
Patient consent for publication Not required.
Provenance and peer review Not commissioned; externally peer reviewed.
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