The prevalence of autism and attention deficit hyperactivity disorder in the UK: a targeted systematic review

Background Autism or autism spectrum disorder (ASD, as per the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition, DSM-5) (American Psychiatric Association, 2013) and attention deficit hyperactivity disorder (ADHD) are common neurodevelopmental conditions typically identified in childhood though they may persist or be identified in adulthood. Increased awareness, better access to diagnostic services and a broadening of diagnostic criteria has been linked with rising diagnosis rates of both conditions. Historically, there has been a focus on childhood autism, which has resulted in many adults, especially those with fewer support needs, remaining undiagnosed (O'Nions et al., 2023). These conditions frequently co-occur. A high proportion of autistic children also exhibit characteristics of ADHD (22% to 83%), whereas many children with ADHD present with major autistic traits (30% to 65%) (Sokolova et al., 2017). Furthermore, approximately 50% of children living with autism in parts of the UK have co-occurring learning disability (Holtom and Lloyd-Jones, 2016). Rationale for the review Current evidence suggests that prevalence estimates for these conditions vary widely depending on the populations studies and methodologies used. Understanding the prevalence of autism only, ADHD only, and co-occurring AuADHD, specifically stratified by the presence or absence of learning disability in the UK, is essential for informing policymaking and service provision. Aim The primary aim of this evidence synthesis is to systematically map available prevalence estimates in the UK and summarise the methodologies used in their estimation. Research scope and questions The review will focus on individuals of any age living in the United Kingdom (England, Northern Ireland, Scotland and Wales) and would seek to answer the following questions: 1. What is the current prevalence of ASD, ADHD, and AuADHD with or without learning disability in individuals of any age living in the United Kingdom (England, Northern Ireland, Scotland, and Wales). 2. What is the prevalence of learning disability, defined according to the UK Department of Health and Social Care (Department of Health and Social Care 2001) criteria, within the relevant populations? 3. Which methodologies are used to determine these prevalence estimates? 4. How do reported estimates vary across demographic and geographic factors, specifically age, sex/gender, race/ethnicity, region or setting? Methods The review will be undertaken according to accepted recommendations. Literature searches To ensure comprehensive coverage, the research team will search eight bibliographic databases and eleven websites or webpages of national organisations and charities (collectively referred to as grey literature sources). The sources are listed below: Bibliographic databases MEDLINE Embase Social Sciences Citation Index (SSCI) PsycINFO Applied Social Sciences Index & Abstracts (ASSIA) Education Resources Information Centre (ERIC) Scopus Epistemonikos Websites and webpages. National Autistic Society https://www.autism.org.uk/ Autism Action Austitica Autism Alliance Ambitious about Autism NHS England Digital https://digital.nhs.uk/ Health and Care of People With Learning Disabilities dataset National Contract Direct Enhanced Service dataset Adult Psychiatric Morbidity Survey Scottish Government: The Scottish Strategy for Autism https://www.gov.scot/publications/scottish-strategy-autism/ Department of Health (Northern Ireland) https://www.health-ni.gov.uk/ ADHD UK https://adhduk.co.uk/ ADHD foundation Bibliographic and grey literature searches will be supplemented by citation searches and manual reference list review of relevant studies. Box 1 outlines the MEDLINE search strategy. This will be adapted for remaining bibliographic searches. Box 1: MEDLINE search strategy Database: Ovid MEDLINE(R) Epub Ahead of Print and In-Process, In-Data-Review & Other Non-Indexed Citations and Daily 1 autism spectrum disorder/ or asperger syndrome/ or autistic disorder/ 2 autis*.tw. 3 asd.tw. 4 Child Development Disorders, Pervasive/ 5 pervasive developmental disorder*.tw. 6 (pdd or pdd-nos).tw. 7 asperger*.tw. 8 Attention Deficit Disorder with Hyperactivity/ 9 "attention deficit and disruptive behavior disorders"/ 10 (adhd or addh or ad hd or ad??hd).tw. 11 attention deficit disorder*.tw. 12 attention deficit hyperactivity disorder*.tw. 13 attention deficit hypersensitivity disorder*.tw. 14 ((attenti* or disrupt*) adj3 (disorder* or deficit*)).tw. 15 (hyper adj1 activ*).tw. 16 hyperactiv*.tw. 17 (attention adj3 (problem* or difficult* or issue*)).tw. 18 neurodiver*.tw. 19 neurodevelop*.tw. 20 or/1-19 (249563) 21 exp epidemiologic studies/ or exp epidemiology/ 22 epidemiology.ti,ab,kf,kw. 23 prevalence/ 24 (prevalence or prevalences).ti,ab,kf,kw. 25 incidence/ 26 incidence.ti,ab,kf,kw. 27 ep.fs. 28 or/21-27 29 20 and 28 30 exp United Kingdom/ 31 (national health service* or nhs*).ti,ab,in. 32 (english not ((published or publication* or translat* or written or language* or speak* or literature or citation*) adj5 english)).ti,ab. 33 (gb or "g.b." or britain* or (british* not "british columbia") or uk or "u.k." or united kingdom* or (england* not "new england") or northern ireland* or northern irish* or scotland* or scottish* or ((wales or "south wales") not "new south wales") or welsh*).ti,ab,jw,in. 34 (bath or "bath's" or ((birmingham not alabama*) or ("birmingham's" not alabama*) or bradford or "bradford's" or brighton or "brighton's" or bristol or "bristol's" or carlisle* or "carlisle's" or (cambridge not (massachusetts* or boston* or harvard*)) or ("cambridge's" not (massachusetts* or boston* or harvard*)) or (canterbury not zealand*) or ("canterbury's" not zealand*) or chelmsford or "chelmsford's" or chester or "chester's" or chichester or "chichester's" or coventry or "coventry's" or derby or "derby's" or (durham not (carolina* or nc)) or ("durham's" not (carolina* or nc)) or ely or "ely's" or exeter or "exeter's" or gloucester or "gloucester's" or hereford or "hereford's" or hull or "hull's" or lancaster or "lancaster's" or leeds* or leicester or "leicester's" or (lincoln not nebraska*) or ("lincoln's" not nebraska*) or (liverpool not (new south wales* or nsw)) or ("liverpool's" not (new south wales* or nsw)) or ((london not (ontario* or ont or toronto*)) or ("london's" not (ontario* or ont or toronto*)) or manchester or "manchester's" or (newcastle not (new south wales* or nsw)) or ("newcastle's" not (new south wales* or nsw)) or norwich or "norwich's" or nottingham or "nottingham's" or oxford or "oxford's" or peterborough or "peterborough's" or plymouth or "plymouth's" or portsmouth or "portsmouth's" or preston or "preston's" or ripon or "ripon's" or salford or "salford's" or salisbury or "salisbury's" or sheffield or "sheffield's" or southampton or "southampton's" or st albans or stoke or "stoke's" or sunderland or "sunderland's" or truro or "truro's" or wakefield or "wakefield's" or wells or westminster or "westminster's" or winchester or "winchester's" or wolverhampton or "wolverhampton's" or (worcester not (massachusetts* or boston* or harvard*)) or ("worcester's" not (massachusetts* or boston* or harvard*)) or (york not ("new york*" or ny or ontario* or ont or toronto*)) or ("york's" not ("new york*" or ny or ontario* or ont or toronto*))))).ti,ab,in. 35 (bangor or "bangor's" or cardiff or "cardiff's" or newport or "newport's" or st asaph or "st asaph's" or st davids or swansea or "swansea's").ti,ab,in. 36 (aberdeen or "aberdeen's" or dundee or "dundee's" or edinburgh or "edinburgh's" or glasgow or "glasgow's" or inverness or (perth not australia*) or ("perth's" not australia*) or stirling or "stirling's").ti,ab,in. 37 (armagh or "armagh's" or belfast or "belfast's" or lisburn or "lisburn's" or londonderry or "londonderry's" or derry or "derry's" or newry or "newry's").ti,ab,in. 38 or/30-37 39 (exp africa/ or exp americas/ or exp antarctic regions/ or exp arctic regions/ or exp asia/ or exp australia/ or exp oceania/) not (exp United Kingdom/ or europe/) 40 38 not 39 41 29 and 40 42 limit 41 to english language 43 limit 42 to yr="2013 -Current" 44 *child development disorders, pervasive/ or *autism spectrum disorder/ or *asperger syndrome/ or autistic disorder/ 45 autis*.tw. 46 *Attention Deficit Disorder with Hyperactivity/ 47 *"Attention Deficit and Disruptive Behavior Disorders"/ 48 2 or 3 or 5 or 6 or 7 or 10 or 11 or 12 or 13 or 14 or 15 or 16 or 17 or 18 or 19 or 44 or 45 or 46 or 47 49 28 and 48 50 40 and 49 51 limit 50 to english language 52 limit 51 to yr="2013 -Current" 53 *Epidemiology/ 54 exp *epidemiologic studies/ 55 53 or 54 56 *incidence/ or *prevalence/ 57 24 or 26 or 27 or 53 or 54 or 56 58 20 and 57 59 40 and 58 60 limit 59 to english language 61 limit 60 to yr="2013 -Current" 62 43 not 61 63 48 and 57 64 40 and 63 65 limit 64 to english language 66 limit 65 to yr="2013 -Current" 67 61 not 66 Screening and selection of records All records will be managed within a reference manager software for selection of eligible sources informed by pre-specified criteria. Calibration: An initial calibration will be performed by reviewers independently examining an initial set of 100 randomly selected records to assess the appropriateness and clarity of selection criteria. Subsequently, all reviewers will meet to discuss any identified inconsistencies in applying the selection criteria. If required, revisions will be made to improve reliability of selection decisions. Study selection: This will follow a two-staged process: (i) title and abstract screening followed by (ii) detailed examination of full text articles or sources. The remaining (non-piloted) records will be examined by one reviewer and checked by a second reviewer. Arising differences will be resolved between reviewers or referred to a third researcher for consensus. Inclusion criteria: We will select peer-reviewed articles, publicly available statistical datasets, reports of charities or government publications available from 2013 onwards that report prevalence data or provide information for obtaining prevalence estimates for the following: Population: A well-defined representative population of individuals of any age living in the UK Case identification and ascertainment: A formal diagnosis of ASD and/or ADHD based on (a) any version of diagnostic criteria or codes and/ or (b) clinical assessment in line with recommended UK clinical guidelines. Outcomes: Prevalence estimates of ASD only, ADHD only, co-occuring ASD and ADHD. Proportion of individuals with learning disability (as per Department of Health criteria). We will include studies which provide sufficient information to estimate prevalence where this information is not reported. Date limit: Publications available from 2013 onwards. This date limit is applied because sources of information from 2013 are most likely to reflect current diagnostic criteria (e.g. DSM-5 and ICD-11). However, given the life-long nature of the conditions of interest, it is anticipated that an individual’s diagnosis could be based on the relevant diagnostic criteria in force at the time of assessment. Exclusion criteria: Studies will be excluded to minimise the likelihood of including data from non-representative populations. Therefore, studies conducted in higher education settings, studies including in-patients or looked-after persons and studies in people with specific health conditions other ASD and/ or ADHD will be excluded. Data extraction We will extract information from included studies and sources using a piloted standardised data extraction template. The extracted information will include study characteristics; population characteristics; data collection methods; case identification and ascertainment methods and reported prevalence data. Data extraction will be completed by one reviewer (or machine) and cross-checked by a second researcher to ensure consistency and accuracy of data. Quality assessment The methodological quality of the included studies and evidence sources will be assessed using a checklist adapted from the Joanna Briggs Institute (JBI) critical appraisal tool for prevalence data (Munn et al., 2015). Synthesis The review findings will be presented in evidence tables, narrative and graphical summaries. The overall strength of the body of evidence will be considered in the interpretation of findings. Expert input The review will incorporate feedback and guidance from two external groups: Advisory group: An advisory group of experts in the topic area will be developed to review both the methodology and findings of the review. Patients and public involvement and engagement (PPIE) group: We will also ask a patients and public involvement and engagement (PPIE) group to help identify data sources and provide feedback on terminologies and the structure of the report’s plain language summary. Outputs The findings of the review will be presented in a number of formats for different audiences. These will include the following: A report to the funder A manuscript for submission to a peer-reviewed journal. We will also work with our PPIE group to develop appropriate outputs to share the findings of the review with a wider audience.