Psychosis in Autism and Intellectual Developmental Disorders with High Support Need. Validation of a new diagnostic tool “Systematic Psychopathological Assessment for Persons with Intellectual and Developmental Disabilities - Psychotic Disorders” (SPAIDD-P).

The purpose of this study was to validate the psychometric properties of a new instrument, the SPAIDD-P, for diagnosing schizophrenia spectrum disorders in adults with Intellectual Developmental Disability (IDD) and High Support Need- Autism Spectrum Disorder (HSN-ASD). The latest version of Systematic Psychopathological Assessment for Persons with Intellectual and Developmental Disabilities - Psychotic Disorders-P (SPAIDD-P 2.0), was developed based on Diagnostic and Statistical Manual of Mental Disorders, 5th Edition, Text Revision (DSM-5-TR) and International Classification of Diseases, 11th Revision (ICD-11) criteria. Item wording and descriptions of clinical features were enhanced with adaptations for individuals with IDD, drawing from the Diagnostic Manual–Intellectual Disability, 2nd Edition (DM-ID 2) and DC-LD, as well as insights from a systematic review of the literature on the unique and atypical manifestations of schizophrenia spectrum disorders (SSD) in this population and from an international panel of experts. The instrument is a dichotomous response questionnaire (present/absent) designed to be administered to informants or caregivers familiar with the persons being assessed. Like all instruments in the SPAIDD system, the SPAIDD-P focuses on detecting behavioural changes—both in quantity and quality—relative to the person's baseline, as well as other observable symptoms over time that significantly impact personal functioning. The theoretical framework is grounded in the concept of behavioural equivalence of subjective symptoms, which is particularly useful for diagnosing psychiatric comorbidities in persons with IDD and/or ASD and severe communication impairments. The SPAIDD-P demonstrated good psychometric properties and factor analysis of the items revealed that behavioural equivalents clustered with other observable symptoms characteristic of psychotic disorders, divisible into productive, cognitive and negative symptoms. These findings endorse the integration of structured behavioural equivalents as a valuable complement to the clinical psychiatric diagnostic process. For persons with IDD and/or HSN-ASD who have suspected comorbid SSD, we recommend structuring multidisciplinary assessment protocols that involve various professionals and informants. Utilizing appropriate psychodiagnostic tools will enable the detection of behavioural changes from baseline and facilitate the identification of their potential behavioural equivalence and associated symptoms. Further investigations will contribute to a more detailed understanding of the properties of the SPAIDD-P and, by extension, the phenomenology of psychotic disorders in persons with cognitive and communication disabilities.