Autism spectrum disorder



Autism spectrum disorder is a neurodevelopment disorder characterised by deficits in social communication/interaction and restricted, repetitive patterns of behaviour, interests, and activities.

Autism spectrum disorder (ASD) is a persistent neurodevelopment disorder that usually presents from early childhood. ASD is an umbrella term for a diverse group of conditions previously considered separate conditions (e.g. classic autism, Asperger disorder). It is the term used by the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V) and is planned to be used by the updated International Classification of Disease, eleventh edition (ICD-11).

ASD may be associated with other conditions and neurodevelopment disorders such as intellectual disability (i.e. deficits in intellectual and adaptive functioning), anxiety and attention deficit hyperactivity disorder (ADHD). The hallmark of ASD is impairment in social communication and social interaction and restricted, repetitive patterns of behaviour, interests, and activities.

Social communication/interaction behaviours

Patients with ASD show difficulties in the following social communication and interaction behaviours:

  • Sharing interests and emotions
  • Engaging in normal conversation
  • Using non-verbal communication
  • Skills to develop, maintain and understand relationships

Restricted / repetitive behaviours

Patients with ASD show very typical restricted, repetitive patterns of behaviour, interest and activities, which include:

  • Repetitive motor movement, use of objects or speech
  • Idiosyncratic (i.e. peculiar) phrases
  • Adherence to daily routines that are inflexible
  • Often have rituals or routines for certain behaviours (e.g. greeting people)
  • Highly restricted, fixed interests
  • Strong attachment to unusual objects
  • Under or overreaction to sensations within the environment

Neurodevelopmental disorders

This refers to a broad range of conditions that affect the development of the nervous system.

Neurodevelopmental disorders (NDD) refer to conditions that have an onset in the developmental period and lead to impairment of important brain functions such as memory, emotion, learning, self-control, and social interaction. These conditions often co-exist (or associate with) a known medical condition (e.g. epilepsy), genetic condition (e.g. fragile X syndrome), or environmental factor (e.g. perinatal infection or hypoxia).

Examples of neurodevelopment disorders include:

  • Autism spectrum disorder (ASD)
  • Intellectual disability (known as learning disability (LD) in the UK)
  • Attention-Deficit/Hyperactivity Disorder (ADHD)
  • Tic disorders
  • Specific learning disorders


The estimated prevalence of ASD in children is 1%.

ASD is a common neurodevelopmental disorder and it is 3-4 times more common in males, although it may under-recognised in females. There is an increased risk of autism if one sibling has the condition and this risk depends on the sex of the index child and the sex of the sibling.

Importantly, ASD is associated with other neurodevelopmental disorders, most notably ADHD and LD.

  • ADHD: seen in up to 50% of patients with ASD
  • LD: seen in up to 45% of patients with ASD

Aetiology & pathophysiology

ASD is a heterogeneous condition related to both genetic and environmental factors.

Both genetic and environmental factors can influence the risk of a child developing ASD. How these factors influence the development of ASD is incompletely understood.

Genetic factors

It is estimated that up to 25% of cases of ASD are associated with a genetic cause. It is suspected that genetic variants, either by themselves or as part of a genetic syndrome (e.g. Down’s, Fragile X), alter the development of the brain impairing social development and leading to restricted interests and repetitive behaviours. Evidence for a genetic role in ASD is further highlighted by an increased risk of developing ASD in siblings and a high concordance in monozygotic twins (as high as 92%).

Examples of genetic conditions commonly associated with ASD include:

  • Fragile X syndrome (up to 30%)
  • Tuberous sclerosis (up to 40%)
  • Rett syndrome (up to 60% in females)
  • Down syndrome (up to 18%)
  • Neurofibromatosis (up to 16%)

Advancing paternal age has also been associated with the development of ASD.

Environmental factors

Several environmental factors have been linked to the development of ASD, which include:

  • Prenatal infections
  • Perinatal complications (e.g. hypoxia)
  • Exposure to toxins
  • Exposure to teratogens

It is most likely that collective environmental insults (e.g. abnormal birth presentation, low weight, perinatal infection) collectively contribute to the development of ASD and interact with genetic factors that predispose children to neurodevelopment disorders. This is further supported by the ‘epigenetic theory’ whereby early experiences affect epigenetic mechanisms that can alter gene expression without altering the underlying genetic sequence. This altered expression subsequently affects brain development and neural connectivity that influences social development.

Clinical features

ASD is associated with a broad range of clinical features.

The presentation of ASD is highly variable. Some children may be affected severely at a young age whereas others may go unnoticed into adulthood due to only mild impairment. It is also important to remember that children may have numerous clinical features due to an underlying genetic syndrome or associated condition(s).

Remember, the clinical features of ASD may differ depending on the age of presentation (e.g. preschool, primary school or secondary school).

Impaired social communication and interaction

This refers to an impairment in both verbal and non-verbal communication skills that are needed to enable successful communication. Delays or deviations in language development are one of the most common findings by parents and caregivers in the presentation of ASD.

There are various domains in social communication that may be affected including:

  • Social-emotional reciprocity: ability to engage in social interactions between two or more people
  • Nonverbal communication: ability to use and interpret nonverbal forms of communication (e.g. facial gestures)
  • Pragmatic communication: ability to select the rights words for the situation
  • Developing and sustaining friendships

Restricted, repetitive behaviours

This refers to a series of behaviours that form the hallmark of ASD. These behaviours can be very restricting and impact the ability to sustain relationships and engage in social activities. Often, when these behaviours are restricted or blocked it can lead to significant anxiety, aggression and/or worsening behaviour patterns.

There are various different behaviours that may be observed:

  • Stereotyped and repetitive motor movements (e.g. flapping hands, rocking)
  • Stereotyped and repetitive speech (e.g. echolalia - repetition of words spoken by another individual)
  • Insistence on sameness (e.g. always eating particular foods in a particular order) - known as cognitive rigidity
  • Resistance to change (e.g. change leads to anxiety, temper tantrums)
  • Restricted interests (e.g. preoccupation with unusual objects, often with intense, unusual focus)
  • Abnormal response to sensory stimuli (e.g. refusal to eat food with certain texture)

ASD in pre-school children

In pre-school children, ASD may be suggested by:

  • A delay in language or regression in language (e.g. not meeting developmental language milestones or loss of use of speech from 2-3 words sentences to single words)
  • Reduced or infrequent use of language (e.g. use of single words when they can speak in sentences)
  • Unusual characteristics of spoken language (e.g. echolalia, non-speech like vocalisation)
  • Reduced or negative response to others (e.g. rejection of cuddles, reduced response to name being called despite normal hearing and vision)
  • Reduced or absent interaction with others (e.g. sharing of enjoyment, enjoyment of situations most children like, preference to play alone)
  • Reduced eye contact, pointing, and other gestures (e.g. reduced facial expressions to communication, eye contact, body orientation)
  • Reduced or absent imagination and variety of pretend play
  • Unusual or restricted interests and/or rigid and repetitive behaviours (e.g. hand flapping, body rocking, overly focused, insistence on own agenda, repetitive play, insistence on same)

ASD in primary school children

In primary school children, ASD may be suggested by:

  • Unusual characteristics of spoken language (e.g. very limited speech, monotonous tone, frequent use of learnt speech and talking ‘at’ rather than with another individual)
  • Reduced, absent or negative response to others (e.g. lack of facial expressions, lack of feelings, abnormal response to name being called despite normal hearing, misunderstanding sarcasm and metaphors)
  • Reduced or absent interaction with others (e.g. absent awareness of personal space, absent interest in others including children same age, absent ability to share in social play, absent enjoyment of situations children like)
  • Reduced or absent eye contact, pointing, and other gestures when interacting with others
  • Reduced or absent ideas and imagination
  • Unusual or restricted interests and/or rigid and repetitive behaviours (e.g. Repetitive movements, playing repetitively with objects rather than people, overly focused, unusual interests, rigid in play, insistence on own agenda, preference for things being ‘just right’, dislike of change)

ASD in secondary school children

In secondary school children, ASD may be suggested by:

  • Unusual characteristics of spoken language (e.g. very limited speech, monotonous tone, frequent use of learnt speech and talking ‘at’ rather than with another individual, responses that seem rude or inappropriate)
  • Unusual or reduced social interaction with others (e.g. lack of understanding of friendship, problems turn-taking or losing games, do not understand socially expected behaviour, limited ability to adapt to the social situation)
  • Reduced eye contact, pointing, and other gestures when interacting with others
  • Reduced or absent ideas and imagination
  • Unusual or restricted interests and/or rigid and repetitive behaviours (e.g. Repetitive movements, preference for highly specific interests, strong adherence to rules - leads to arguments, highly repetitive rituals, dislike of change)

ASD in adults

The identification of ASD in adults is very challenging due to the co-existence of other mental health disorders with close similarities in symptoms. ASD in adults may be suggested by:

  • Persistent difficulties in social interaction
  • Persistent difficulties in social communication
  • Stereotypic (rigid and repetitive) behaviours, resistance to change, or restricted interests
  • Problems in obtaining or sustaining employment or education
  • Difficulties in initiating or sustaining social relationships
  • Previous or current contact with mental health or learning disability services
  • A history of a neurodevelopmental condition (including learning disabilities and attention deficit hyperactivity disorder) or mental disorder

Asperger’s disorder

Asperger’s disorder (formally known as a syndrome) is now part of ASD rather than a separate entity. People with Asperger’s typically have a normal or above-average intelligence without learning disabilities. Learning disability is common amongst people with autism. Overall, they usually have fewer or no problems with speech and language.

Associated conditions

ASD is associated with many other conditions including other neurodevelopment disorders.

Patients with ASD often have co-existent neurodevelopment disorders (e.g. ADHD) or clinical features commonly seen in other neurodevelopment disorders. It can be difficult to differentiate between these conditions and determine which is predominant. ASD may also be associated with an underlying genetic disorder (e.g. Down syndrome) or medical condition (e.g. epilepsy).

Neurodevelopment disorders associated with ASD include:

  • Anxiety disorder
  • ADHD
  • Tic disorders
  • Learning disability
  • Specific learning disorder
  • Specific language delay or disorder
  • Stereotypic movement disorder

The presence or absence of these conditions is usually stated as part of the diagnosis. For example, ASD with learning disability or ASD with neurofibromatosis.

Diagnosis & investigations

The diagnosis of ASD is based on a clinical assessment to determine whether they meet diagnostic criteria.

The DSM-V outlines the diagnostic criteria for the diagnosis of ASD. These criteria centre on the clinical features relating to the two major themes of ASD, the deficits in social communication/interaction and the restricted, repetitive behaviours. DSM-V also provides a framework for the severity of ASD. The need for further evaluation and comprehensive investigations depends on the suspected contribution of the underlying genetic and medical conditions.

Diagnostic criteria

The diagnosis of ASD should be made if there are:

  • Persistent deficits in social communication and social interaction in multiple settings (evidenced by deficits in all 3 of the following):
    • Social-emotional reciprocity
    • Nonverbal communicative behaviours
    • Developing, maintaining and understanding relationships
  • Restricted, repetitive patterns of behaviour, interests, or activities (evidence by ≥2 of the following):
    • Stereotyped or repetitive movements, use of objects, or speech
    • Insistence on sameness, unwavering adherence to routines, or ritualised patterns of verbal or nonverbal behaviour
    • Highly restricted, fixated interests that are abnormal in strength or focus
    • Increased or decreased response to sensory input or unusual interest in sensory aspects of the environment

For a formal diagnosis, these symptoms must impair daily function as evidenced by their effect on social interaction, academic studies and daily routines. They should be present in the early developmental period and they are not better explained by learning disability or global developmental delay.

Interestingly, although symptoms should be present in the early development period, it is noted that symptoms may be masked until later in life when social demands exceed the capacity of the individual. They may have previously managed through learned strategies.


The severity of ASD can be determined using the DSM-V criteria into varying levels (1-3) for each domain: social communication/interaction and restricted, repetitive behaviours. These essentially grade patients into how much support they require.


When autism is suspected, patients should be referred to professionals specialising in autism and neurodevelopment disorders. This may be a designated autism team, paediatrician, or child and adolescent psychiatrist. When there are major concerns, significant limitations/impairment or regression in development immediate referral is advised.

Those with less severe impairments may have a period of watchful waiting while more information is gathered. If concerns persist or new symptoms develop then a referral to a specialist team should be made.

Further evaluation

A variety of investigations can be requested by specialists depending on whether an underlying genetic condition or associated medical condition (e.g. epilepsy) is suspected. This is often highly dependent on the age of presentation, severity, associated symptoms, and clinical examination (e.g. any dysmorphic features).

Example investigations include:

  • Genetic testing
  • Metabolic testing
  • Neuroimaging (e.g. MRI brain)
  • Electroencephalogram


The management of Autism spectrum disorder should be individualised as the needs and severity differ greatly between patients.

The management of ASD is guided by professionals who specialise in autism. This may be a designated autism team, paediatrician or psychiatrist that depends on the local services available. Individuals with ASD have varying degrees of impairment and social needs so management has to be individualised with the use of a multidisciplinary team.

The goal of managing ASD is to achieve as much functional independence as possible and improve the quality of life for the patient. This can be achieved through improving social functioning, communication, adaptive skills, academic ability and behaviours.The National Institute of Clinical Excellence (NICE) sets out some broad guidelines for managing patients with ASD.

Delivery of care

Care should be coordinated by healthcare professionals with a specialist interest in ASD as part of a multidisciplinary team. They will help to identify particular patient needs and support getting access to education, accommodation, carers, training, employment services, and many other essential aspects.

Patient and family involvement

Children and young people with autism, and their families and carers, should be provided with information about support available on an ongoing basis. This support should be suitable for the child or young person's needs and developmental level. This includes written information, training and advice.

Specific interventions

Children and young adults with ASD may require specific treatment interventions depending on their personal needs. These can include:

  • Behavioural & educational interventions (e.g. high staff-to-student ratio, highly supportive teaching environment, predictability and structure)
  • Psychosocial interventions (e.g. be appropriate for the child or young person’s developmental level and sensitive and responsive to their patterns of communication and interaction, include techniques to expand communication, interactive play and social routines)
  • Interventions for life skills (e.g. coping strategies for leisure activities, public transport and employment)
  • Interventions for speech and language problems (e.g. involvement of speech and language team)
  • Intervention for sleep disorders

Pharmacological interventions

Medications should not be used routinely to treat the core features of ASD. They may be used to treat concurrent medical or psychiatric co-morbidities such as hyperactivity, inattention, and impulsivity, aggressive behaviour, anxiety or depressive symptoms. In addition, they may be needed when behavioural symptoms interfere with learning, health, safety, quality of life, or overall functioning. However, they do not treat the underlying core deficits in ASD and must be used in conjunction with behavioural interventions.

Managing co-morbidities

Children and young adults with autism should who have co-morbidities (medical and other neurodevelopmental disorders) should be offered both psychosocial and pharmacological interventions for these coexisting conditions in association with recognised national health guidelines. In-depth management of these co-morbidities is beyond the scope of these notes.

Last updated: May 2022

NICE. Autism spectrum disorder in under 19s: recognition, referral and diagnosis. CG128. 2017.
Hodges, H. et al. Autism spectrum disorder: definition, epidemiology, causes, and clinical evaluation. Transl Pediatr. 2020
Author The Pulsenotes Team A dedicated team of UK doctors who want to make learning medicine beautifully simple.

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