A 3-year-old boy is brought to the child psychiatry clinic by his parents with concerns about delayed speech and social withdrawal. On examination, he shows repetitive hand movements, poor eye contact, and resistance to changes in routine. He was diagnosed with autism spectrum disorder at age 2.5 years. Which of the following is the most common comorbid neurodevelopmental condition associated with autism spectrum disorder?

Explanation

## Most Common Comorbidity in Autism Spectrum Disorder **Key Point:** Intellectual disability (ID) is the most frequent comorbid condition in autism spectrum disorder, occurring in approximately 30–50% of individuals with ASD, depending on the diagnostic criteria and population studied. ### Epidemiology of Comorbidities in ASD | Comorbid Condition | Prevalence in ASD | Clinical Significance | |---|---|---| | Intellectual disability | 30–50% | Most common; impacts prognosis and adaptive functioning | | ADHD | 30–50% | High overlap; shared neurobiological pathways | | Specific learning disorder | 20–30% | Often co-occurs with language/communication deficits | | Anxiety disorders | 40–50% | Common in higher-functioning ASD | | Childhood disintegrative disorder | <1% | Rare; now subsumed under ASD in DSM-5 | **High-Yield:** The DSM-5 specifies that intellectual disability frequently accompanies ASD. The severity of intellectual disability ranges from mild to profound and significantly influences the level of support required and functional outcomes. ### Why Intellectual Disability Is Most Common 1. **Shared neurobiological substrate** — Both ASD and ID involve atypical neurodevelopment and brain connectivity patterns. 2. **Diagnostic overlap** — Social communication deficits in ASD often co-occur with cognitive delays that meet criteria for ID. 3. **Longitudinal studies** — Follow-up data from birth cohorts show that approximately 1 in 3 children with ASD have concurrent ID. **Clinical Pearl:** Children with ASD and comorbid ID typically have: - More severe social and communication impairments - Greater need for structured support and supervision - Poorer adaptive functioning in daily living skills - More challenging behavioral management **Warning:** Do not confuse the high prevalence of ADHD symptoms in ASD (which can be 30–50%) with it being the *most common* comorbid *condition*. While ADHD features overlap significantly with ASD, intellectual disability remains the most frequently diagnosed distinct comorbid neurodevelopmental disorder.