Mar 18, 2026

Public workspaceADHD Symptom Presentation and Functional Impairment in People with Intellectual Disability: A Scoping Review

DOI
https://dx.doi.org/10.17504/protocols.io.e6nvww4kwvmk/v1
  • Alfie Thain1,
  • Marios Adamou2,
  • Mahesh Odiyoor3,
  • Ken Courtenay4,
  • Steven Marwaha5,
  • Therese van Amelsvoort6,
  • Rohit Shankar7,8,
  • Petra Hurks9,
  • Bhathika Perera10
  • 1Division of Psychiatry, University College London, London, UK;
  • 2South West Yorkshire Partnership NHS Foundation Trust, UK;
  • 3CANDDID, Cheshire and Wirral Partnership NHS Foundation Trust, UK;
  • 4Barnet, Enfield and Haringey Mental Health NHS Trust, London, UK;
  • 5Institute of Mental Health, University of Birmingham, UK;
  • 6Department of Psychiatry and Neuropsychology, Maastricht University, Netherlands;
  • 7Peninsula Medical School, Faculty of Health, University of Plymouth, UK;
  • 8CIDER, Cornwall Partnership NHS Foundation Trust, Truro, UK;
  • 9Department of Neuropsychology and Psychopharmacology, Maastricht University, Netherlands;
  • 10Division of Psychiatry, University College London, London, UK.
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DOI: https://dx.doi.org/10.17504/protocols.io.e6nvww4kwvmk/v1
Protocol Citation: Alfie Thain, Marios Adamou, Mahesh Odiyoor, Ken Courtenay, Steven Marwaha, Therese van Amelsvoort, Rohit Shankar, Petra Hurks, Bhathika Perera 2026. ADHD Symptom Presentation and Functional Impairment in People with Intellectual Disability: A Scoping Review. protocols.io https://dx.doi.org/10.17504/protocols.io.e6nvww4kwvmk/v1
License: This is an open access protocol distributed under the terms of the Creative Commons Attribution License,  which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited
Protocol status: Working
We use this protocol and it's working
Created: March 17, 2026
Last Modified: March 18, 2026
Protocol Integer ID: 313462
Keywords: functional impairment rating scale for adhd, adhd symptom presentation, adhd symptom, adhd, significant functional impairment, functional impairment in individual, functional impairment in people, functional impairment, functional impairment rating scale, intellectual disability, associated functional impairment, hyperactivity disorder, associated functional impairment in this group, neurodevelopmental condition, existing assessment approach, assessment approach, assessment tool, dedicated assessment instrument, hyperactivity, standard diagnostic framework, scoping review methodology, symptom presentation, scoping review, scr reporting guidance
Funders Acknowledgements:
Bailey Thomas Charitable Fund
Abstract
Background: Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental condition characterised by persistent inattention and/or hyperactivity-impulsivity associated with significant functional impairment. Although prevalence is estimated at approximately 5-8% in children and 2-5% in adults, rates are consistently higher among people with intellectual disability (ID), with prevalence estimates reported as high as 30%. Standard diagnostic frameworks and assessment tools were developed primarily in populations without ID, therefore do not adequately capture symptom presentation or associated functional impairment in this group. There has been no synthesis of how ADHD symptoms and functional impairments are conceptualised and operationalised in people with ID, raising questions about the validity of existing assessment approaches.
Aim: This scoping review aims to map and synthesise existing evidence on the presentation of ADHD symptoms and associated functional impairment in individuals with ID, with a view to informing the development of a dedicated assessment instrument.
Methods: The review will follow established scoping review methodology informed by Arksey and O’Malley, Levac and the Joanna Briggs Institute guidance. Electronic searches of MEDLINE, Embase, PsycINFO and CINAHL will be conducted to identify relevant studies. Empirical studies and relevant peer-reviewed conceptual publications from 1994 onwards will be eligible. Data will be charted using a structured framework and summarised descriptively using tabular and narrative synthesis in accordance with PRISMA-ScR reporting guidance.
Dissemination: Findings will clarify how ADHD symptoms and associated functional impairments have been described and assessed in people with ID in the current literature.  Results will inform a Delphi consensus process to support development of a dedicated Symptoms and Functional Impairment Rating Scale for ADHD in this population.
Troubleshooting
Background
Attention-deficit/hyperactivity disorder (ADHD) is a common neurodevelopmental condition characterised by persistent patterns of inattention and/or hyperactivity-impulsivity that interfere with a significant functional impairment (1,2).  Prevalence estimates suggest rates of approximately 5-8% in children and adolescents and 2-5% in adults, although substantial variation exists depending on diagnostic criteria and assessment methods (3–5).
 
Intellectual disability (ID) is defined by significant limitations in intellectual functioning and adaptive behaviour, expressed in conceptual, social, and practical adaptive skills, with onset during the developmental period (2,6). ID affects approximately 1% of the population, although published estimates typically range from 1-3% depending on case definition, age group, and ascertainment method (7,8). 
 
ID is a heterogeneous condition frequently associated with neurodevelopmental and physical comorbidities, including autism spectrum disorder (ASD) (10-40%), epilepsy (~22%), and elevated rates of psychiatric disorders (~34-41%), which may further complicate assessment and diagnostic clarity (9–12). In a UK national audit of adults with co-occurring ID and ADHD, 73% had a diagnosis of ASD (13).  ADHD is consistently reported to be more common among people with ID, with prevalence estimates typically ranging from 6% to 30% (14–19). Despite this elevated prevalence, ADHD remains under-recognised and inconsistently assessed in people with ID (14,20). This may reflect diagnostic overshadowing, overlapping neurodevelopmental symptom profiles, communication difficulties, and challenges in distinguishing attentional symptoms from cognitive impairment.
 
Neurodevelopmental disorders, including ID, ADHD, and ASD, share overlapping genetic, cognitive, and neurobiological susceptibilities that may contribute to their frequent co-occurrence. Genome-wide association studies have identified multiple genetic variants associated with ADHD that are enriched among genes involved in early brain development and shared across neurodevelopmental and psychiatric conditions (21,22). Experimental evidence also suggests that genes implicated in ID may contribute to ADHD risk, indicating overlap in underlying pathways (23). In addition, prospective studies have shown that deficits in executive functions such as inhibitory control, working memory, and attention regulation are common precursors of ADHD traits across development and frequently co-occur with broader neurodevelopmental impairments (24,25), which may help explain the elevated prevalence of ADHD observed in people with ID.
 
Accurate identification of ADHD relies on the presence of characteristic symptoms alongside evidence of associated functional impairment across multiple settings (2). People with ID have historically been under-represented in research, including psychiatric and neurodevelopmental research, and many clinical trials and validation studies have excluded individuals with significant cognitive impairment (26–28). Consequently, core ADHD diagnostic criteria and commonly used assessment instruments were not specifically designed for people with ID given their different cognitive, communicative, and adaptive profiles  (1,2). Several DSM-5 criteria rely on developmental expectations, literacy, and insight that may not apply to individuals with moderate-to-profound ID. Communication difficulties and high rates of comorbidity may further obscure symptom identification and complicate differentiation from global cognitive impairment. 
 
As a result, symptom descriptions, examples, and thresholds may not translate directly to individuals with ID, particularly those with more severe cognitive or communication impairments. In a UK study of 37 psychiatrists rating case vignettes of adults with ID, the application of DSM-5 criteria alone demonstrated markedly lower sensitivity for ADHD (0.23) compared with clinician judgement (0.82; p< 0.001), highlighting potential limitations in the direct application of standard diagnostic criteria in this population (29). Although adapted diagnostic interviews for adults with ID, such as the DIVA-5-ID, have been proposed, they remain limited in validation and applicability across the spectrum of intellectual and communicative functioning (30). ADHD symptoms may overlap with co-occurring neurodevelopmental disorders or be manifested through behaviour that challenges (14,20,31). 
 
Clinical guidance and expert consensus documents highlight that strict application of standard ADHD diagnostic criteria may lead to underdiagnosis or diagnostic uncertainty in people with ID (14,32). In this population, ADHD-related difficulties may be identified primarily through observable behaviours and informant reports rather than subjective self-report, particularly where verbal communication or self-report capacity is limited (17,33). Functional impairment related to ADHD may manifest as difficulties in daily living skills, emotional dysregulation, limited social participation, or increased reliance on carers, rather than difficulties in educational or occupational settings typically referenced in diagnostic criteria (20). Consequently, clinical judgement and proxy information from carers and support staff are often emphasised in assessment. Yet there remains limited evidence on how ADHD symptoms and functional impairment are best defined and measured in people with ID (34). 
 
Failure to accurately identify ADHD in people with ID may contribute to unsuitable management strategies, unmet support needs, and increased risk of behavioural difficulties or mental illness comorbidity (35–38). The phenomenon of diagnostic overshadowing, whereby mental health problems are attributed solely to ID rather than a co-occurring condition, may further complicate this problem. The co-occurrence of ID and ADHD has been described as a “double deficit” in cognitive and adaptive functioning, increasing the likelihood that ADHD-related impairments are minimised or misattributed (14,37). In practice, this may contribute to reliance on non-specific behavioural management strategies or psychotropic medication, rather than ADHD specific evidence based interventions (39). 
 
These challenges have important implications for instrument development. Assessment tools for people with ID must distinguish ADHD-related attentional dysregulation from global cognitive impairment, incorporate behaviourally anchored and informant-based items, and capture functional impairment in domains relevant to adaptive functioning rather than relying solely on educational or occupational benchmarks.
 
Much of the published literature has focused on prevalence estimates and pharmacological management, with comparatively less attention to ADHD symptoms and associated functional impairments in people with ID (17,29). Research on ADHD in people with ID spans a wide range of study designs and settings, including clinic-based samples, population screening studies, medication trials, and service audits. Across these studies, there is substantial variability in how ADHD is defined, assessed, and reported (14,20,40). Expert-led publications and consensus reports have therefore played a central role in shaping clinical understanding and practice in this area (14).
 
There is growing recognition of the need for assessment approaches that place functional impairment at the centre of ADHD diagnosis and management in people with ID (20,33). Functional impairment is a core diagnostic requirement for ADHD and is often the most clinically meaningful indicator (1,2). In people with ID, where symptom expression may be atypical or difficult to interpret, functional impact may provide a more reliable indicator of ADHD-related difficulty than symptom count alone (14). To our knowledge, no synthesis has systematically mapped how ADHD symptoms and associated functional impairments are described and conceptualised across the literature on ID.
 
The findings of this scoping review will inform a subsequent Delphi consensus process to identify and prioritise key ADHD symptoms and associated functional impairments in people with ID. This will contribute to the development of a dedicated Symptoms and Functional Impairment Rating Scale tailored to this population, addressing current limitations in assessment approaches.
Aim: The aim of this scoping review is to map and synthesise existing evidence on how ADHD symptoms and associated functional impairment are described, measured, and conceptualised in people with ID. 
Methods
Study design
A scoping review approach was considered appropriate for this study given the heterogeneity in how ADHD symptoms and associated functional impairments are described, defined, and measured in people with ID, as well as the limited and fragmented nature of the existing evidence base (14,20). Scoping reviews are designed to map the extent and characteristics of evidence within a field, particularly where concepts are variably defined, or research is emerging (41–43).
 
This review will follow the methodological framework for scoping reviews described by Arksey and O’Malley and further refined by Levac et al., and will be conducted in accordance with guidance from the Joanna Briggs Institute (41,42,44).
 
This scoping review will follow the six-stage methodological framework proposed by Arksey and O’Malley (41): (1) identifying the research question; (2) identifying relevant studies; (3) selecting studies; (4) charting the data; (5) collating, summarising, and reporting the results; and (6) consultation with stakeholders. The conduct of the review will also be informed by subsequent methodological guidance from Levac et al. and the Joanna Briggs Institute (42,44).
 
The reporting of this scoping review will follow the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR) (45). This protocol will be registered on Protocols.io before the review commences.
Stage 1: Identifying the research question
The research question for this scoping review was developed using the Population-Concept-Context (PCC) framework recommended by the JBI (44). What is known about the presentation of ADHD symptoms and associated functional impairment in people with ID? The population of interest is people with ID. The concept of interest is the presentation, description, and measurement of ADHD symptoms and their associated functional impairments. The context is broad and includes clinical, community, educational, forensic, and research settings, drawing on internationally published literature. 
 
To address the overarching review question, a set of sub-questions was developed based on the aims of the review, existing literature on ADHD symptom domains and functional impairment in people with ID, and clinical perspectives on current gaps in assessment and diagnosis.
 
Sub-questions
• How are ADHD symptoms described and measured in people with ID?
• Which domains of functional impairment associated with ADHD are reported in people with ID, e.g. executive functioning, adaptive behaviour, daily living, social or educational functioning?
• What assessment tools, rating scales, or conceptual frameworks are used to assess ADHD symptoms and functional impact in people with ID, and what characteristics of these tools are reported in the literature?
• Are adaptations made to standard ADHD diagnostic criteria or assessment measures when used with people with ID, and how are these adaptations described in the literature?
• Do reported ADHD symptom presentations or functional impairments differ according to the severity of ID?
• How are ADHD symptoms distinguished from, or reported to overlap with, ASD and other neurodevelopmental conditions in people with ID?
• What gaps and inconsistencies exist in the current evidence base regarding the measurement, conceptualisation, and clinical assessment of ADHD in people with ID?
• Do reported ADHD symptom presentations or associated functional impairments differ by age or developmental stage in people with ID?
• In which clinical, educational, or research settings have ADHD assessment tools been applied in people with ID?
Stage 2: Identifying relevant studies
Electronic database searches across MEDLINE, PsycINFO, Embase and CINAHL will be conducted by the primary reviewer (AT) to identify peer-reviewed literature relevant to the review question.  These databases were selected to capture evidence across clinical, psychological, and allied health disciplines (46–48).
 
Given that ADHD in people with ID is an under-researched and methodologically heterogeneous area, a broad search strategy will be employed (41,42). Searches will combine controlled vocabulary terms, e.g. MeSH and equivalent thesaurus terms, with free-text keywords to maximise sensitivity and capture variations in terminology. The search strategy will be developed and iteratively refined through pilot searches and team discussion, in line with established scoping review methodology. Full search strategies, including key terms and combinations for each database, will be documented and reported in accordance with PRISMA-ScR guidance (45). Additional records will be identified through backward reference list checking and forward citation searching of included studies.
Stage 3: Study selection
Following completion of the database searches, all identified records will be imported into reference management software Mendeley, and duplicates will be removed. Additional records will be identified through forward and backward citation searching of included studies, with any additional publications added to the reference database. The final set of records will then be exported to Rayyan for screening and study selection. Before formal screening, the eligibility criteria will be piloted on a random sample of records to ensure consistent interpretation and application of the inclusion and exclusion criteria. Any conflicts will be discussed within the review team, and the criteria will be refined where necessary.
 
Titles and abstracts will be screened against the predefined inclusion and exclusion criteria. One reviewer will screen all records, with two additional reviewers independently screening a subset of records (approximately 20% each) to ensure consistency in the application of the eligibility criteria. Reviewers will be blinded to each other’s decisions. Inter-rater agreement will be assessed, and discrepancies will be discussed and resolved through consensus.
 
Full texts of potentially eligible studies will then be retrieved and assessed against the inclusion and exclusion criteria. Full-text screening will be conducted independently by two reviewers, with any disagreements resolved through discussion and consultation with another member of the review team where necessary. The study selection process will be documented using a PRISMA flow diagram, detailing the number of records identified, screened, included, and excluded at each stage, along with reasons for exclusion at the full-text stage (41,42,45).

Inclusion criteria
Studies will be included if they meet all the following criteria:
• Peer-reviewed empirical research studies using qualitative, quantitative, or mixed-methods designs examining ADHD symptoms and/or associated functional impairment in people with ID.
• Include participants with intellectual disability (across any level of severity) or borderline intellectual functioning, either as the primary study population or as a clearly identifiable subgroup with separately reported data.
• Report on ADHD diagnosis and/or ADHD-related symptom domains e.g. inattention, hyperactivity, impulsivity or associated functional impairment in people with ID.
• Any age group (children, adolescents, or adults).
• Conducted in any setting, including clinical, educational, community, residential, or research contexts.
• Published in English
• Published from 1 January 1994 onwards
 
Exclusion criteria
Studies will be excluded if they meet any of the following criteria:
• Studies that do not include participants with ID as the primary study population or as a clearly identifiable subgroup.
• Are not peer-reviewed e.g. blogs, non-academic opinion pieces
• Editorials or commentaries without substantive conceptual or clinical contribution
• Focus exclusively on treatment effectiveness or intervention outcomes without reporting ADHD symptom presentation or functional impairment at baseline.
• Examine ADHD solely as a secondary or incidental diagnosis without reporting on symptoms or functional impact in people with ID.
• Are not published in English
• Were published prior to 1 January 1994
 
Studies published from 1994 onwards were included to capture literature developed within contemporary diagnostic frameworks for ADHD and ID, while ensuring comprehensive coverage of a relatively limited and heterogeneous evidence base (49).
Stage 4: Charting the data
Data will be extracted (charted) from all sources of evidence included in the scoping review using a structured data charting framework (see Download Supplementary File 1 - Data charting framework (Table 1).docxSupplementary File 1 - Data charting framework (Table 1).docx15KB ). Data charting will be guided by the review questions and the Population-Concept-Context framework. Extracted data will include bibliographic information, study characteristics, population details, author-reported study limitations, and key information relating to the description, measurement, and conceptualisation of ADHD symptoms and associated functional impairment in people with ID. Both quantitative and qualitative data relevant to the review objectives will be charted.
 
The data charting form will be piloted on a subset of included studies and refined iteratively as familiarity with the literature develops, in line with established scoping review methodology (43,44). One reviewer will chart data from all included studies, with a second reviewer independently charting data from a subset of studies to check accuracy and consistency. Any discrepancies will be resolved through discussion, with involvement of a third reviewer where necessary. 
 
The methodological quality of all included empirical studies will be appraised using the Mixed Methods Appraisal Tool (MMAT) (50). Quality appraisal will be undertaken to support interpretation of the evidence and to provide context regarding the strength and limitations of the existing literature, and will not be used as a criterion for study exclusion.
 
The results of data charting will be summarised descriptively using tables and narrative synthesis to map the extent, nature, and characteristics of the available evidence.
Stage 5: Collating, summarising, and reporting the results
Findings will be collated and summarised descriptively in relation to the review questions (43).
 
Extracted data will be presented in tabular form, with an accompanying narrative synthesis structured around the review questions and the PCC framework. The synthesis will describe patterns, variations, and gaps in how ADHD symptoms and functional impairment are conceptualised, described, and measured across studies. These findings will also be used to identify candidate symptom and functional impairment domains, as well as measurement approaches, relevant to the development of a dedicated ADHD assessment instrument for people with ID. During synthesis, symptom domains and functional impairment domains will be grouped into conceptually coherent categories. Where possible, findings will be synthesised to illustrate how core ADHD symptom domains are described and manifested in people with ID, highlighting areas of convergence with standard ADHD diagnostic frameworks as well as potential differences or atypical presentations. Where studies describe novel or atypical presentations not captured by standard ADHD domains, these will be documented and, where appropriate, grouped into categories. Where applicable, results will be grouped by relevant characteristics such as age group, level of ID, presence of co-occurring neurodevelopmental conditions, and setting. 
Stage 6: Consultation and patient and public involvement
Consultation and patient and public involvement and engagement (PPIE) will be undertaken following completion of the evidence synthesis to enhance the relevance, interpretation, and applicability of the scoping review findings. Consultation is included as an optional but valuable stage of scoping review methodology, consistent with established guidance (41,42,44).
 
A PPIE group will be established, comprising approximately four representatives, including two people with ID and two family members or carers. PPIE representatives will provide input on the interpretation of findings, their relevance to lived experience, and priorities for future research, including the subsequent Delphi consensus process.
 
In addition, consultation with relevant professionals with expertise in ID and ADHD e.g. psychiatrists, psychologists, and clinicians working in ID services will be undertaken to contextualise findings, identify gaps between research and clinical practice, and inform the translation of results. Where reported, contextual characteristics such as geographic setting, service context, and cultural considerations will also be described. This consultation will involve reaching out to relevant clinical and research networks to obtain perspectives from professionals working across different services and disciplines. A fixed number of consultees will not be specified in advance; rather, the aim is to obtain a range of perspectives from clinicians with relevant expertise.
 
PPIE contributors will be offered a range of opportunities to participate, tailored to individual needs e.g. in-person, virtual, or written input. Consultation activities will inform interpretation and reporting of results but will not constitute a formal component of data extraction or synthesis. PPIE involvement will be reported in accordance with the Guidance for Reporting Involvement of Patients and the Public 2 (GRIPP2) reporting checklist (51).
Ethics and dissemination
Ethical approval is not required for this scoping review, as it involves secondary analysis of published literature and does not involve the collection of primary data from human participants. 
 
Findings will inform the design of a subsequent Delphi consensus study to prioritise ADHD symptoms and functional impairments for inclusion in a dedicated assessment tool. Results will be disseminated through publication in a peer-reviewed journal and presentation at relevant academic and clinical conferences. An accessible summary of findings will be developed to support dissemination to clinicians, people with ID, and their families and carers.
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