A Comprehensive Evaluation of the International Classification of Diseases, Eleventh Revision Behavioural Indicators for Disorders of Intellectual Development

Abstract

This dissertation presents a comprehensive exploration of the International Classification of Diseases 11th revision (ICD-11) Clinical Descriptions and Diagnostic Requirements (CDDR) and embedded Behavioral Indicators (BIs) for disorders of intellectual development with or without co-occurring autism and their implementation in clinical settings. Reliable and valid diagnostic assessment measures are needed to improve access to care for individuals with disorders of intellectual development because they often have specific needs. Current intellectual and adaptive behaviour assessment measures require specialised services and training to administer that are costly, often rendering them unavailable or inaccessible in various contexts, such as in some low- and middle-income countries (LAMICs) and underserved populations. In the two studies presented in this dissertation, I aimed to investigate the reliability, validity, clinical utility, and implementation of the BIs used as a clinical support tool for disorders of intellectual development, particularly in settings when standardised assessment measures are not available or feasible. In Study 1, we assessed the reliability, validity, and clinical utility of these BIs, shedding light on their effectiveness as a support tool for evaluating severity levels for individuals with disorders of intellectual development. We recruited a total of 206 children and adolescents (5-18 years old) with a suspected or established diagnosis of disorders of intellectual development from four sites across three countries [Sri-Lanka (n = 57), Italy (n = 60) and two sites in India (n = 89)]. We assessed inter-rater reliability, concurrent validity using the Leiter-3 for intellectual functioning and the Vineland Adaptive Behaviour Scales-II for adaptive behaviours as standardised measures, and clinical utility of the CDDR and BIs. The BIs demonstrated excellent inter-rater reliability (intra-class correlations ranging from 0.91 to 0.97) and good to excellent concurrent validity (intra-class correlations ranging from 0.66 to 0.82) across sites. Compared to standardised measures, the BIs had more diagnostic overlap between intellectual and adaptive functioning. The BIs were rated as quick and easy to use and applicable across severities; clear and understandable with adequate to too much level of detail and specificity to describe disorders of intellectual development; and useful for intervention selection, prognosis assessments, communication with other health care professionals, and education efforts. The results of Study 1 support use of the BIs within the ICD-11 CDDR to assist with the accurate identification of severity ratings for individuals with disorders of intellectual development. In Study 2, we examined the implementation of the BIs in real-world clinical settings and their application in cases of disorders of intellectual development with or without co-occurring autism, emphasising the importance of tailored clinical support tools for complex neurodevelopmental conditions. We included 116 children and adolescents (5-18 years old) with a suspected or established diagnosis of disorders of intellectual development were included across four sites [Italy (n = 18), Sri Lanka (n = 19) and two sites in India (n = 79)]. A principal component analysis was conducted to evaluate the application of the ICD-11 guidance for combining severity levels. Findings demonstrated that individuals with disorders of intellectual development and co-occurring autism tended to have more severe support needs compared to individuals without autism, and that the BIs classified more individuals with mild severity compared to standard measures, across all diagnostic presentations. Despite the need for additional time and consideration for individuals with co-occurring conditions, the BIs were deemed clinically useful. The findings suggest that the ICD-11 BIs can be effectively applied and implemented internationally in clinical settings for a broad range of presentations of individuals with neurodevelopmental disorders. Overall, these studies explore the significance of utilising the ICD-11 CDDR and BIs as a clinical support tool in clinical practice for assessing individuals with disorders of intellectual development and co-occurring conditions such as autism. The findings highlight the utility of the BIs in providing reliable and valid severity assessments across diverse cultural and clinical settings, emphasising the need for tailored diagnostic tools for complex neurodevelopmental disorders. By demonstrating the feasibility and clinical applicability of the BIs, this research contributes to enhancing diagnostic accuracy, intervention planning, and overall care for individuals with intellectual development disorders, ultimately aiming to improve outcomes and quality of life for this population.