TABLE 1

Description of studies

First author, year [ref.]	Design	Subjects n	SDB definition with respect to predictor	SDB	Indices of SDB severity	Age years	Objective
Anand, 2021 [16]	Prospective cross-sectional	53	OSA: AHI ≥1 events·h⁻¹	96	Mean±sd AHI 8.96±1.8 events·h⁻¹	Mean (range) 5.9 (3–11.8)	1) Assess the effect of quality of sleep on development and behaviour of children with trisomy 21 2) Assess the effect of sleep apnoea on development and behaviour of children with trisomy 21
Austeng, 2014 [17]	Cross-sectional	29	OSA: OAI/AHI >1.5 events·h⁻¹	AHI >1.5 events·h⁻¹ 97% OAI >1.5 events·h⁻¹ 69%	Mean±sd AHI 10±8.8 events·h⁻¹	Mean 8	1) Measure prevalence and medical follow-up of OSA in young children with Down syndrome 2) Identify the association between OSA and age, BMI and airway surgery
Banjar, 2013 [18]	Prospective cohort	23	OSA: AHI >1 events·h⁻¹	82.6%	Mean AHI 12.3 events·h⁻¹ Mean OAI 4.73 events·h⁻¹	Not reported	Identify sleep abnormalities in children with Down syndrome
Basil, 2016 [19]	Retrospective cohort	303	OSAS: AHI ≥2 events·h⁻¹	74%	Mild OSAS: 38.9% Moderate OSAS: 32.8% Severe OSAS: 28.2%	Mean±sd (range) 10.6±4.06 (2–18)	1) Investigate whether children with Down syndrome are at increased risk of obesity 2) Explore OSAS, which is associated with obesity in children with Down syndrome
Breslin, 2014 [20]	Prospective cohort	38	OSAS: AHI >1.5 events·h⁻¹	61%	Mean±sd AHI 5.79±9.86 events·h⁻¹	Mean±sd (range) 9.6±.8 (7–12)	1) Investigate the association between OSAS and cognition in children with Down syndrome 2) Investigate the effect of OSAS on sleep physiology in children with Down syndrome
Brooks, 2015 [21]	Prospective cohort	25	OSA: AHI ≥5 events·h⁻¹	40%	Mean±sd AHI 13.42±15.89 events·h⁻¹	Mean±sd (range) 10.14±3.39 (7.2–18.7)	1) Investigate the effect of sleep and SDB on neuropsychological functioning of children with Down syndrome 2) Determine whether treatment of SDB improves cognitive functioning
Chamseddin, 2019 [22]	Qualitative study with retrospective data collection	106	OSA: AHI ≥1 events·h⁻¹	90%	Mean±sd AHI 16.7±6.25 events·h⁻¹	Mean±sd 7.3±4	1) To evaluate demographic, clinical and polysomnographic features of children with Down syndrome with clinical suspicion of OSA 2) Identify factors that predict OSA in children with Down syndrome
Durhan, 2019 [23]	Cross-sectional	18	OSA: AHI ≥1 events·h⁻¹	61.1%	Mean AHI 3.23 events·h⁻¹	Median (IQR) 6.6 (4.4–10.5)	Determine the effect of OSA on periodontal and dental health in children with Down syndrome
Dyken, 2003 [9]	Prospective cohort	19	Sleep apnoea: apnoea index >1 event·h⁻¹ and S_aO₂ low point <92% (with baseline S_aO₂ >92%)	79%	Median (IQR) AHI 6.0 (3–8) events·h⁻¹ Median (IQR) apnoea index 3.0 (2–5) events·h⁻¹	Mean±sd 9.1±4.7	1) Investigate OSA in young Down syndrome patients using PSG 2) Identify the effects of therapy on OSA in young Down syndrome patients
Elsharkawi, 2017 [24]	Cross-sectional	101	OSA: AHI ≥1 events·h⁻¹	42.6%	Mild OSA: 28% Moderate OSA: 2% Severe OSA: 13%	Mean±sd 9.1±4.0	1) Compare urinary biomarkers of children with Down syndrome to neurotypically healthy controls 2) Determine whether urinary biomarkers could predict a diagnosis of OSA
Friedman, 2018 [25]	Cross-sectional	113	OSA: AHI ≥2 events·h⁻¹	Not reported	Mean AHI 13.76 events·h⁻¹	Mean 5.89	1) Assess parents' accuracy in reporting their children's breathing patterns 2) Assess risk factors associated with abnormal sleep study
Jayaratne, 2017 [26]	Qualitative study with prospective data collection	63	OSA: AHI >1 events·h⁻¹	44.2%	Not reported	Mean±sd (range) 7.49±4.86 (3.1–24.4)	1) Characterise the facial morphology of children with Down syndrome 2) Compare facial anthropometric characteristics of Down syndrome patients with published norms 3) Compare facial anthropometric characteristics of Down syndrome patients with and without OSA to predict OSA status in patients with Down syndrome
Lee, 2020 [27]	Cross-sectional	30	OSA: AHI ≥1 events·h⁻¹	80%	Median (IQR) AHI 5.2 (1.7–15.7) events·h⁻¹	Median (IQR) 11.3 (9.4–15.6)	1) Measure the prevalence of OSA in children with Down syndrome 2) Identify the role of OSA and sleep structure in affecting cognitive performance
Maris, 2016 [28]	Cross-sectional	54	OSA: AHI2 events·h⁻¹	57.1%	Median (IQR) AHI 7.25 (5.7–9.8) events·h⁻¹	Median (range) 7.5 (4–18)	1) Identify prevalence of sleep problems in children with Down syndrome as measured by Children's Sleep Habits Questionnaires 2) Compare prevalence of sleep problems in children with Down syndrome and controls 3) Investigate association between sleep problems and OSA
Maris, 2016 [29]	Cross-sectional	122	OSA: AHI 2 events·h⁻¹	66.4%	Median (IQR) AHI 8.2 (4.3–16.7) events·h⁻¹	Median (IQR) 5.0 (2.8–10.5)	1) Measure prevalence of OSA in children with Down syndrome 2) Identify factors that are associated with disease severity
de Miguel-Díez, 2003 [4]	Prospective cohort	108	OSA: AHI 3 events·h⁻¹	54.6%	Mean±sd AHI 6.1±6.7 events·h⁻¹	Mean±sd (range) 7.9±4.5 (1–18)	1) Measure the prevalence of SDB in children with Down syndrome 2) Identify factors that predispose children with Down syndrome to SDB
Naime, 2021 [30]	Retrospective cohort	158	CSA: CAI >2 events·h⁻¹ OSAS: AHI >2 events·h⁻¹	79.1% OSAS 12% CSA	Median (IQR) CAI 0.20 (0.0–0.83) events·h⁻¹ Median (IQR) AHI 5.95 (2.40–16.52) events·h⁻¹	Median (IQR) 4.8 (0.04–18.3) Median (IQR) 4.8 (2.31–9.02)	1) Identify clinical predictors of central breathing problems in children with Down syndrome 2) The role of sex and OSA status on presence of central breathing problems
Nehme, 2017 [8]	Retrospective cohort	119	OSA: AHI >5 events·h⁻¹ Hypoventilation: carbon dioxide 50 mmHg for ≥25% of total sleep time	42.9%	Median (IQR) AHI 3.6 (1.6–11.2) events·h⁻¹	Median (IQR) (range) 6.6 (4.4–10.5) (0.05–16.8)	Determine clinical predictors of SDB in children with Down syndrome
Posada, 2019 [31]	Qualitative study with retrospective data collection	53	OSA: AHI >2 events·h⁻¹	50.9%	Median (IQR) AHI: 0–23 months 7.0 (1.5–20.2) events·h⁻¹ 24–84 months 8.3 (1.9–30.0) events·h⁻¹ 85–156 months 8.4 (2.5–19.0) events·h⁻¹ >156 months 13.0 (4.9–29.2) events·h⁻¹	Median (IQR) 3.4 (1.6–8.8)	Measure the incidence of sleep-related breathing disorders in children with Down syndrome living at high altitude
Richard, 2020 [32]	Retrospective case–control study	56	Nocturnal hypoventilation: >25% of total sleep time spent with a P_tcCO₂ >50 mmHg	OSA 85.7% Nocturnal hypoventilation 17.9%	Median (IQR) AHI 5 (3–10.2) events·h⁻¹ Median (IQR) mean P_tcCO₂ 44 (43–46.5) mmHg	Median (IQR) 4.9 (2.0–7.8)	Compare P_tcCO₂ and pulse oximetry (S_pO₂) in children with Down syndrome and in control children with clinical signs of OSA
Rosen, 2020 [33]	Qualitative study with retrospective data collection	418	OSA: AHI >1 events·h⁻¹	42.1%	Mean AHI 3.6 events·h⁻¹ Median AHI 0.6 events·h⁻¹	Mean (range) 7.27 (2–17) Median 6	1) Identify the prevalence of increased periodic limb movements of sleep in children with Down syndrome 2) Determine the correlation of periodic limb movements of sleep with OSA and levels of ferritin
Shires, 2010 [34]	Retrospective cohort	52	OSA: AHI >1.0 events·h⁻¹	63.5%	Mean AHI 18.7 events·h⁻¹	Mean±sd 9.3±4.5	Identify the effect of body mass on the incidence of OSA in children with Down syndrome
Skotko, 2017 [35]	Prospective cohort	102	OSA: AHI >1 events·h⁻¹	44.1%	Not reported	Median (range) 5.6 (3–24.4)	To create a predictive model to help screen for OSA in children with Down syndrome
Waters, 2020 [36]	Qualitative study with retrospective data collection	152	OSA: AHI >2 events·h⁻¹	85.5%	Mean±sd AHI 13.1±22.4 events·h⁻¹	First PSG: mean±sd 5.0±4.3 Last PSG: mean±sd 8.2±5.1	Identify the spectrum of OSA in children with Down syndrome

SDB: sleep disordered breathing; OSA: obstructive sleep apnoea; AHI: apnoea–hypopnoea index; OAI: obstructive apnoea index; BMI: body mass index; OSAS: OSA syndrome; IQR: interquartile range; S_aO₂: arterial oxygen saturation; PSG: polysomnography; CSA: central sleep apnoea; CAI: central apnoea index; P_tcCO₂: transcutaneous partial pressure of carbon dioxide; S_pO₂: oxygen saturation measured by pulse oximetry.