https://doi.org/10.1177/0004867420924101 Australian & New Zealand Journal of Psychiatry 2020, Vol. 54(10) 970–984 DOI: 10.1177/0004867420924101 © The Royal Australian and New Zealand College of Psychiatrists 2020 Article reuse guidelines: sagepub.com/journals-permissions journals.sagepub.com/home/anp Australian & New Zealand Journal of Psychiatry, 54(10) Prevalence estimates of mental health problems in children and adolescents with intellectual disability: A systematic review and meta-analysis Nicholas Buckley 1 , Emma J Glasson 1 , Wai Chen 2,3,4,* , Amy Epstein 1 , Helen Leonard 1 , Rachel Skoss 1 , Peter Jacoby 1 , Amanda Marie Blackmore 1,5 , Ravisha Srinivasjois 6 , Jenny Bourke 1 , Richard J Sanders 7 and Jenny Downs 1,8 Abstract Background: Children and adolescents with intellectual disability are at risk of developing psychiatric symptoms and disorders; yet, the estimates reported in the literature have been inconsistent, presenting a potential barrier for service planning and delivery. Sources of variability could arise from differences in measurement instruments as well as subgroup membership by severity of intellectual disability, gender and age. This systematic review aimed to address these gaps. Method: MEDLINE and PsycINFO databases were searched from inception to 2018 and selected studies were reviewed. Studies were included if they reported point prevalence estimates of mental health symptomology or diagnoses in a gen- eral population of 6- to 21-year-old individuals with intellectual disability. The Joanna Briggs Institute Prevalence Critical Appraisal Checklist was applied to eligible papers to appraise their scientific strength. Pooled prevalence for mental health symptomology was determined using a random-effects meta-analysis. Results: A total of 19 studies were included, including 6151 children and adolescents. The pooled prevalence estimate captured by the Developmental Behaviour Checklist was 38% (95% confidence interval = [31, 46]), contrasting with 49% (95% confidence interval = [46, 51]) captured by the Child Behaviour Checklist; both rates were higher than a non-intellectual disability population. Severity of intellectual disability did not significantly influence the Developmental Behaviour Checklist risks. Insufficient data were available to conduct statistical analyses on the effects of age, gender and socioeconomic status. Of diagnosed psychiatric disorders, attention deficit/hyperactivity disorder (30%), conduct disorder (3–21%) and anxiety disorders (7–34%) were the most prevalent conditions. Conclusion: This review consists of the largest sample hitherto evaluated. In the intellectual disability population, mental health comorbidities could be better detected by a symptom phenotype than a psychiatric diagnostic phenotype. Crucially, future research needs to address the effect of measurement validity in the intellectual disability population. Estimated prevalence rates were high compared to the general population, indicating the importance of systematic screening, case detection and appropriate management. Keywords Mental health, intellectual disability, prevalence, children, adolescents 1 Telethon Kids Institute, The University of Western Australia, Perth, WA, Australia 2 Mental Health Service, Fiona Stanley Hospital, Department of Health, Perth, WA, Australia 3 Centre for Child and Adolescent Related Disorders, Graduate School of Education, The University of Western Australia, Perth, WA, Australia 4 School of Paediatrics and Child Health, The University of Western Australia, Perth, WA, Australia 5 Therapy and Other Health Services, Ability Centre, Perth, WA, Australia 6 Department of Neonatology, Joondalup Health Campus, Perth, WA, Australia 7 Sanders Consulting, Perth, WA, Australia 8 School of Physiotherapy and Exercise Science, Curtin University, Perth, WA, Australia * Current affiliations: Mental Health Service, Fiona Stanley Hospital, Department of Health, Perth, WA, Australia; School of Medicine, Notre Dame University, Fremantle, Perth, WA, Australia and College of Science, Health, Engineering and Education, Murdoch University, Perth, WA, Australia Corresponding author: Jenny Downs, Telethon Kids Institute, The University of Western Australia, PO Box 855, West Perth, WA 6872, Australia. Email: Jenny.Downs@telethonkids.org.au 924101ANP ANZJP ArticlesBuckley et al. Key Review