Prevalence of autism among adolescents with intellectual disabilities

To estimate the prevalence of autism in an epidemiologically-derived population of adolescents with intellectual disabilities (ID).

The prevalence of autism was examined using the Autism Diagnostic Interview-Revised, with appropriate care taken in assessing lower functioning individuals and those with additional physical and sensory impairments. Individual assessment during psychological evaluation, and consensus classification of complex cases, involving clinicians experienced in the assessment of autism, contributed to the identification of autism.

Overall, 28% of individuals, or 2.0 of the 7.1/1000 with ID in the target population (as we have previously identified in another study), were identified with autism. Autism rates did not differ significantly across severe ID (32.0%) and mild ID (24.1%); males predominated (2.3 males to 1 female), but less so for severe ID (2 males to 1 female, compared with 2.8 males to 1 female for mild ID). Socioeconomic status did not distinguish the groups with and without autism. Less than one-half of the adolescents who met diagnostic criteria for autism were previously diagnosed as such.

Our overall prevalence estimate for autism is in the higher range of estimates reported in previous studies of ID (more so for mild ID). This likely reflects the changes in diagnostic criteria for autism that have subsequently occurred. Discussion focuses on the identification of autism in the population with ID, and on the implications for service delivery and clinical training.