[Autism in adults with intellectual disabilities]

According to the World Health Organization (WHO) the estimated prevalence of intellectual disabilities (ID) is about 1-3% and 1 out of 4 individuals with ID suffer from an additional autistic spectrum disorder (ASD) (arithmetic mean 24.6%, 19 studies, n=9,675) whereby the prevalence increases with the severity of ID (IQ 50-70: 9.9%, IQ<50: 31.7%). Therefore, it is of particular importance for physicians treating individuals with ID who have psychiatric disorders or behavioral problems to take ASD into account as a differential diagnosis so that appropriate treatment can be initiated.Irrespective of the IQ the diagnosis is based on an impairment of social interaction and communication and restricted repetitive interests presenting before the age of 3 (infantile or Kanner autism). ASD can be diagnosed as a separate disorder in adults with ID, however, the social and communicative abilities in respect of the cognitive and developmental level have to be considered.Due to reduced verbal capacity, high prevalence of physical and mental disorders, difficulties in taking the past medical history and presentation of atypical symptoms, the diagnostic assessment for autism in adults with ID is challenging.This article describes the typical symptoms, diagnostic approach, frequent comorbidities, differential diagnoses treatment options and their limitations for adults with ID suspected of having ASD.