Autism spectrum disorders in children with physical or mental disability or both. I: Clinical and epidemiological aspects.

The prevalence of autism spectrum disorders was studied in all children with mental retardation and/or motor disability in a defined geographical region over a two-year follow-up period. In the general population, the prevalence of autistic disorder was 0.09% at the end of the follow-up period -a minimum estimate, as children with average intelligence were not screened. Autism spectrum disorders were found in 19.8% of children with mental retardation, including strictly defined autistic disorder (DSM-III-R criteria) in 8.9%; the two-year follow-up yielded a higher prevalence of 11.7% with autistic disorder. Among children with cerebral palsy, 10.5% had an autism spectrum disorder. Clear co-variation was found between mental retardation, epilepsy and autism spectrum disorders in this population of children with neurodevelopmental disorders.