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Sleep-related Rhythmic Movement Disorder (RMD) affects around 1% of UK pre-school children. Little is known about RMD in Down syndrome (DS). We aimed to determine: (a) the prevalence of RMD in children with DS aged 1.5–8 years, (b) phenotypic and sleep quality differences between children with DS and RMD and sex- and age-matched DS controls, and (c) night-to-night variability in rhythmic movements (RMs). Parents who previously reported RMs from a DS research registry of 202 children were contacted. If clinical history suggested RMD, home videosomnography (3 nights) was used to confirm RMs and actigraphy (5 nights) was used to assess sleep quality. Phenotype was explored by demographic, strengths and difficulties, Q-CHAT-10/social communication and life events questionnaires. Eight children had confirmed RMD. Minimal and estimated maximal prevalence were 4.10% and 15.38%, respectively. Sleep efficiency was significantly lower in RMD-cases (69.1%) versus controls (85.2%), but there were no other phenotypic differences. There was considerable intra-individual night-to-night variability in RMs. In conclusion, RMD has a high prevalence in children with DS, varies from night to night and is associated with poor sleep quality but, in this small sample, no daytime phenotypic differences were found compared to controls. Children with DS should be screened for RMD, which is amenable to treatment.

Rhythmic Movement Disorder consists of repetitive stereotypic movements, such as head banging or body rocking, that recur every second or so and may last from a few minutes to hours, usually prior to sleep onset. This review of childhood rhythmic movement disorder highlights the lack of systematic research into core aspects of the condition, relying heavily on small case series or case reports. Interpretation is further limited by almost universal failure to confirm the core the diagnostic criteria (C) of the International Classification of Sleep Disorders (III), namely that the rhythmic movements should have clinical consequences. Nonetheless, a number of themes emerge. Rhythmic movement disorder is likely to start in infancy and have a developmental course with spontaneous resolution in early childhood in many cases. Factors associated with persistence are, however, unclear. Associations with ADHD and neurodevelopmental disorders are intriguing, require further study and may shed light on the underlying cause of the condition. There is a pressing need for a systematic approach to classify rhythmic movement disorder, to allow standardization of the much needed research into the underlying aetiology and treatment of this relatively neglected sleep disorder.