Autism and genetic syndromes
Some rare syndromes are characterised by the large number of individuals with the syndrome who also meet the criteria for an autism spectrum disorder. Autism is characterised by difficulties with communication and social interaction, as well as repetitive and stereotypical behaviour (15). There is an increased prevalence rate of autism in some genetic syndromes; fragile X syndrome and tuberous sclerosis being the most well-known (15, 16). Individuals with these syndromes can have different degrees of intellectual disability, and it is estimated that approximately 30 % of men with fragile X syndrome and 36 % of people with tuberous sclerosis also meet the criteria for an autism spectrum disorder (8, 16). The risk is highest amongst those with the most severe intellectual disabilities, but the prevalence of autism in tuberous sclerosis is also higher among those with normal cognitive levels (16). It was long believed that everyone with Down syndrome was sociable and outgoing and thus almost protected against autism, but it is now known that autism in Down syndrome is not uncommon (16).
The cause of autism is largely genetic, but people with autism are a highly heterogeneous group, and different people have different genetic makeup (15, 17). Diagnosing autism in addition to a genetic syndrome is useful because people with autism have special educational needs (16). Moreover, communication difficulties in autism increase the risk of diagnostic overshadowing, causing physical or mental health problems to be overlooked.
Structure and predictability, language and communication support, and help with social interaction are core educational principles for individuals with autism. Advances in medical technology are leading to the continual discovery of more genetic variants associated with autism (18); however, there is no evidence of 'autism-specific' genes (17). Genes associated with autism are also associated with intellectual disability or other neurodevelopmental disorders.
Brain development and formation of neural connections start in the womb, while the symptoms that define autism become apparent during the first two years of life. Genetics, brain development, neural connections, and the functional impairments that define autism are descriptions at different levels. Among those with average or above average intelligence, the genetic risk for autism is typically due to the sum of effects of several common genetic variants, whereas in others the genetic cause of autism is explained by the effect of a single rare genetic variant.