Autism

What it is

Autism, also called autism spectrum disorder (ASD), is a difference in brain development that affects social communication and interaction, and is associated with focused interests, repetitive behaviours and differences in sensory experience^2,3. It is described as a spectrum because autistic people have a wide range of characteristics, strengths, needs and challenges¹².

Autism is present from early development and is lifelong, but it is often not recognised or diagnosed until later childhood or adulthood^1,2. Some autistic people live independently, while others have high support needs throughout life¹. In prevalence studies, about a third of autistic people also have an intellectual disability, meaning significant difficulties with learning and reasoning⁵.

The language used about autism is debated. Many autistic adults prefer identity-first terms such as ‘autistic person’, whereas many professionals use person-first terms such as ‘person with autism’; no single term is universally preferred¹¹.

The neurodiversity perspective, advanced largely by the autistic community, regards autism as one form of natural variation among human minds and challenges an exclusive focus on deficits¹⁰. Clinical services, meanwhile, aim to identify needs and offer support, and the perspectives of autistic people are increasingly considered in both research and practice³.

Signs and symptoms

Autistic characteristics differ greatly between individuals and can change over the lifespan; not every autistic person shows all of them^1,12. Many autistic people also have particular strengths, such as learning things in great detail and remembering information for long periods¹².

• Differences in social communication, such as little or inconsistent eye contact, or difficulty with the back-and-forth of conversation¹²
• Finding it hard to understand another person’s point of view or to adjust behaviour to different social situations¹²
• Repetitive movements or speech, including repeating words or phrases (echolalia)¹²
• Intense, long-lasting interests in particular topics, such as numbers, details or facts¹²
• Distress at small changes in routine and difficulty moving from one activity to another^1,12
• Being more or less sensitive than other people to light, sound, touch, smell, taste or temperature^2,12
• Co-occurring conditions are common, including epilepsy, anxiety, depression, attention deficit hyperactivity disorder (ADHD) and sleep problems^1,2,12

Causes and risk factors

There is no single known cause of autism. Current evidence points to a strong genetic contribution that interacts with environmental factors, mainly acting during early brain development, although how these factors combine is not well understood^1,12.

Large studies have found no link between autism and vaccines, including the measles, mumps and rubella (MMR) vaccine and vaccine components such as thiomersal^1,9. The study that originally suggested a link was shown to be fraudulent and was withdrawn by the journal that published it¹.

Autism is diagnosed about four times more often in males than in females⁵. The following factors are associated with a higher likelihood of autism, but most people exposed to them are not autistic¹².

• Having a brother or sister who is autistic¹²
• Older parental age^1,12
• Certain genetic conditions, such as fragile X syndrome and Down syndrome¹²
• Premature birth, very low birth weight or severe birth complications^1,12
• Diabetes in the mother during pregnancy, and prenatal exposure to some medicines, such as valproate, or to air pollutants¹

Genetics

Autism often runs in families. A study of more than two million children in five countries estimated its heritability, the share of differences between people in a population that is explained by genetic differences, at about 80%⁶. A high heritability does not mean that autism is caused by a single gene, or that the environment plays no part.

Much of the genetic influence comes from many common variants, each with a very small effect. A genome-wide association study (GWAS), which scans the genomes of thousands of people for shared variants, identified the first five regions robustly linked to autism⁷.

Rare variants also contribute. A large exome sequencing study, which reads the protein-coding parts of genes, implicated 102 genes, most of which are active early in brain development⁸. Many autism-linked variants are not specific to autism and are also found in people with other developmental conditions^3,8.

What happens in the brain

Autism is not associated with obvious damage to the brain. Instead, post-mortem, brain imaging and electrophysiological studies, which record the brain’s electrical activity, have found subtle differences in brain structure and function at a group level³.

Genes linked to autism are often active while the cerebral cortex, the brain’s outer layer, is forming, and many influence how nerve cells develop or communicate with one another^7,8. In developing human cortex, these genes are especially active in both excitatory nerve cells, which make other cells more likely to fire, and inhibitory nerve cells, which dampen activity⁸.

This has supported the idea that autism may involve a shift in the balance between excitation and inhibition in brain circuits^8,16. This remains a hypothesis: some studies point to changes in excitation and others to changes in inhibition, the brain may partly compensate for such changes, and the underlying biology probably differs between individuals^16,17.

Genetics and neuroscience have revealed patterns of risk, but these findings have not yet led to much practical benefit for autistic people⁴.

How it is diagnosed

There is no blood test or brain scan for autism. Diagnosis is based on a careful assessment of development and behaviour by experienced professionals, often working as a team^12,15.

Autism can often be reliably diagnosed by around the age of two, although many people are identified later in childhood or as adults^1,12.

• Developmental monitoring at routine child health checks: in India, the Rashtriya Bal Swasthya Karyakram screens children from birth to 18 years for developmental delays and disabilities, including autism¹⁸, and in the United States autism-specific screening is recommended at 18 and 24 months¹²
• Referral to a specialist autism team when parents, carers or professionals raise concerns¹⁵
• A detailed developmental history from parents or carers, and observation of the person’s communication, play and social interaction^12,15
• Assessment of thinking skills, speech and language, and everyday living skills¹²
• Medical and neurological examination, and assessment for co-occurring conditions such as ADHD, anxiety, depression or epilepsy^12,14
• For adults, NICE in the United Kingdom advises against routinely using biological tests, genetic tests or brain imaging to diagnose autism¹⁴

How scientists study it

Current treatments

Research frontiers

Living with the condition and seeking help

When autism is suspected in a child or an adult, an assessment can be discussed with a family doctor or other health professional, or with staff at the school or nursery¹². The earlier autism is identified, the sooner support and services can begin¹².

After diagnosis, autistic people and their carers should be offered information, services and practical support suited to their individual and changing needs¹. Support groups and advocacy organisations, including those led by autistic people, can offer information and community¹².

Autistic people are more likely than others to experience anxiety or depression². If someone is in immediate danger, has seriously harmed themselves or is thinking about suicide, local emergency services or a crisis line should be contacted straight away, or the person should go to the nearest emergency department. In India, emergency help is available on 112²⁰, and Tele-MANAS, the national tele-mental health service, offers free support at all hours on 14416²¹.