Objective: Autism spectrum disorder (ASD) in very young children with significant cognitive impairment is difficult to diagnose, depriving them of the earliest opportunities for autism-specific intervention. This study delineated specific symptoms in this group, compared to symptoms in children with Global Developmental Delay (GDD) and in ASD with milder developmental delays.Method: Autism Diagnostic Observation Schedule, 2nd Edition, Toddler Module revealed symptoms in three groups of toddlers, with mean ages of 17-20 months: (1) ASD and cognitive/language functioning below the 12-month level (ASD-MA < 12 mos; n = 28), (2) GDD (n = 27), and (3) ASD and cognitive/language functioning at or above the 12-month level (ASD-MA ≥ 12 mos; n = 29). Logistic regression models were fit to control for developmental level. Results: Items in all domains (social interaction, communication, repetitive movements) discriminated ASD-MA < 12 mos from GDD. The two ASD groups, matched for age but differing on developmental level, showed strikingly similar ASD symptomatology. Conclusion: ADOS-2 symptoms differentiated ASD-MA < 12 mos from GDD, after controlling for cognitive impairment. Symptoms in the two ASD groups were minimally related to developmental level. The ADOS-2 Toddler Module successfully captured ASD symptomatology even in children whose developmental level was below the recommended ADOS-2 cutoff of 12 months, which may increase their access to early ASD-specific intervention.
Keywords: Autism spectrum disorder; developmental delay; diagnosis; low mental age.