Last edited 07/2022 and last reviewed 07/2022

Autism is characterized by impaired reciprocal social interaction and communication, as well as stereotyped behaviour and interests, with an onset typically before 3 years of age (1)

  • a broader range of phenotypes is also recognized, termed autism spectrum disorders (ASDs), which include 'strict' autism, atypical autism, Asperger syndrome and pervasive developmental disorder- not otherwise specified
  • ASDs are estimated to affect approximately 1% of children (1,2)
    • are significantly skewed towards boys, with a sex ratio of 4:1 (1)
    • among siblings of children with an ASD, the prevalence increases to 2-8% (1)
    • concordance of autism in monozygotic twins is 36-60%, versus 0% in same sex dizygotic twins (1)
    • the heritability of ASDs has been estimated to be about 90%, making ASDs the most heritable of the childhood onset neuropsychiatric disorders(1)
    • core autism behaviours are typically present in early childhood, although some features may not manifest until a change of situation, for example, the start of nursery or school or, less commonly, the transition to secondary school. Regression or stasis of language and social behaviour is reported for at least a third of children with autism. This usually, but not exclusively, occurs between the ages of 1 and 2 years, and the reasons for regression and stasis are unknown (3)
    • around 70% of people with autism also meet diagnostic criteria for at least one other (often unrecognised) psychiatric disorder that further impairs psychosocial functioning, for example, attention deficit hyperactivity disorder (ADHD) or anxiety disorders (3)
    • intellectual disability (IQ below 70) coexists in approximately 50% of children and young people with autism (3)

The term autism describes qualitative differences and impairments in reciprocal social interaction and social communication, combined with restricted interests and rigid and repetitive behaviours (4)

  • autism spectrum disorders are diagnosed in children, young people and adults if these behaviours meet the criteria defined in the International Statistical Classification of Diseases and Related Health Problems (ICD-10) and the Diagnostic and Statistical Manual of Mental Disorders Fifth Edition (DSM-5) and have a significant impact on function
  • the overarching category term used in ICD-10 is pervasive developmental disorder (PDD), a term now used synonymously with autism spectrum disorder (excluding Rett's syndrome); it is a behaviourally defined group of disorders, which is heterogeneous in both cause and manifestation.

DSM-5 refers to autism spectrum disorder as a single condition with different levels of symptom severity in 2 core domains:

    • 1) deficits in social communication and social interaction and
    • 2) restricted repetitive behaviours (RRBs), interests, and activities and sensory anomalies

      • in addition to these features, children and young people with autism frequently experience a range of cognitive, learning, language, medical, emotional and behavioural problems, including: a need for routine; difficulty in understanding other people, including their intentions, feelings and perspectives; sleeping and eating disturbances; and mental health problems such as anxiety, depression, problems with attention, self-injurious behaviour and other challenging, sometimes aggressive behaviour (3)
      • these features may substantially impact on the quality of life of the individual, and their family or carer, and lead to social vulnerability (3)
      • clinical picture of autism is variable because of differences in the severity of autism itself, the presence of coexisting conditions and levels of cognitive ability, from profound intellectual disability in some people to average or above average intelligence quotient (IQ) in others (3)

  • Autism spectrum disorder encompasses the 4 separate disorders from the previous DSM-IV:
    • autistic disorder (autism),
    • Asperger's disorder,
    • childhood disintegrative disorder and
    • pervasive developmental disorder not otherwise specified

Because both components are required for diagnosis of autism spectrum disorder, social (pragmatic) communication disorder is diagnosed if no RRBs are present. Note that childhood disintegrative disorder is excluded from ASD in DSM-5.

The core autism behaviours are typically present in early childhood, but features are not always apparent until the circumstances of the child or young person change, for example when the child goes to nursery or primary school or moves to secondary school. Autism is strongly associated with a number of coexisting conditions

  • studies have shown that approximately 70% of people with autism also meet diagnostic criteria for at least one other (often unrecognised) psychiatric disorder that is further impairing their psychosocial functioning (4)
  • a learning (intellectual) disability (intelligence quotient [IQ] below 70) occurs in approximately 50% of young people with autism (4)

Autism was once thought to be an uncommon developmental disorder, but recent studies have reported increased prevalence and the condition is now thought to occur in at least 1% of children. This rising prevalence has increased demand for diagnostic services for children and young people of all ages in the health service:

  • however, using nationally representative data in the US, the estimated ASD prevalence was 3.14% among children and adolescents in the US in 2019 and 2020 (5)