Autism spectrum disorder in Williams syndrome: a case report highlighting diagnostic complexity

Williams syndrome (WS) is a rare neurogenetic condition classically associated with ‘hypersociability’, relative verbal strengths, and marked visuospatial weakness. Despite this phenotype, a subset of children with WS exhibit clinically significant autism spectrum disorder (ASD) features, creating diagnostic ambiguity and risk for under-recognition of co-occurring ASD (‘secondary autism’). We present a case of a 5-year-old boy with genetically confirmed WS and profound developmental delays who was referred for concerns regarding limited reciprocal interaction, sensory sensitivities, and restricted/repetitive behaviors. A comprehensive, multi-method evaluation was completed, including cognitive and adaptive testing (WPPSI–IV; ABAS–3), caregiver interview (ADI–R), and direct autism assessment (ADOS-2, Module 1), alongside standardized behavioral observation. The child demonstrated extremely low cognitive functioning and severely impaired adaptive skills, meeting criteria for intellectual disability. Across ASD-specific measures and clinical observation, he showed clear deficits in social reciprocity and joint attention, minimal functional communication, and prominent repetitive/sensory behaviors, supporting a co-occurring ASD diagnosis. This case underscores the heterogeneity of WS and highlights the clinical importance of systematic ASD assessment in children with known genetic syndromes, as accurate dual diagnosis prevents diagnostic overshadowing and meaningfully informs intervention planning, educational supports, and access to autism-specific services.