Autism Spectrum Disorder (ASD)

Definition

Autism spectrum disorder (ASD) describes a wide range of impairments in social communication and restricted and repetitive behaviors. It is a phenotypically heterogeneous group of neurodevelopmental syndromes with polygenic heritability. DSM-5 collapses the core diagnostic impairments into two domains:

Deficits in social communication and social interaction
Restricted and repetitive patterns of behavior, interests, or activities

Prior to DSM-5, five overlapping disorders captured the spectrum: autistic disorder, Asperger disorder, childhood disintegrative disorder, Rett syndrome, and pervasive developmental disorder not otherwise specified (PDD-NOS). DSM-5 unified these into a single continuum model.

— Kaplan and Sadock's Synopsis of Psychiatry

Epidemiology

Prevalence: Approximately 1 in 54 children in the United States carry an ASD diagnosis; the rate has risen significantly over the past two decades.
Sex ratio: Diagnosed 4× more often in males than females. Girls with ASD more often exhibit intellectual disability; girls without intellectual disability may be underidentified.
Age of detection: Typically evident during the second year of life; in severe cases, impaired social interest may appear even in the first year. Average diagnosis age: ~3.1 years for classic autistic disorder, ~7.2 years for former Asperger disorder.
About one-third of children with ASD also meet criteria for intellectual disability.

DSM-5 Diagnostic Criteria

Domain A — Persistent deficits in social communication and social interaction (across multiple contexts):

Deficits in social-emotional reciprocity
Deficits in nonverbal communicative behaviors
Deficits in developing, maintaining, and understanding relationships

Domain B — Restricted, repetitive patterns of behavior, interests, or activities (at least 2 of):

Stereotyped or repetitive motor movements, use of objects, or speech
Insistence on sameness, inflexible adherence to routines
Highly restricted, fixated interests, abnormal in intensity or focus
Hyper- or hyporeactivity to sensory input

Symptoms must be present in the early developmental period, cause clinically significant impairment, and not be better explained by intellectual disability or global developmental delay.

Specifiers: Severity levels (Levels 1–3 requiring support), with/without intellectual impairment, with/without language impairment.

Clinical Features

Social communication deficits:

Poor eye contact, failure to share interest or emotions
Limited response to name, reduced social smile
Impaired theory of mind ("mind blindness") — difficulty inferring others' mental states, beliefs, and intentions
Language deviance as much as language delay is characteristic; in up to 25% of cases, some language develops and is subsequently lost

Restricted and repetitive behaviors:

Intense, idiosyncratic interests in narrow topics
Resistance to change; distress over minor environmental alterations
Motor stereotypies: hand-flapping, toe-walking, spinning, rocking
Compulsive behaviors (e.g., lining objects in rows)
Sensory sensitivities

Associated features:

Attention deficits and hyperactivity
Anxiety, mood disorders
Sleep dysfunction
Self-injurious behavior
Seizures (epilepsy occurs in ~25–33% of individuals with ASD)

Etiology and Pathogenesis

Genetic Factors

Highly heritable: monozygotic twin concordance ~36–96% vs. dizygotic ~0–27% across studies
Up to 15% of cases associated with a known genetic mutation; most cases involve multiple genes
Sibling recurrence risk: up to 50% in families with two or more affected children
Chromosomal regions implicated: chromosomes 2, 7, 16, 17

Genetic syndromes associated with ASD:

Syndrome	Mechanism
Fragile X syndrome	X-linked recessive; trinucleotide repeat in FMR1; 2–3% of ASD cases
Tuberous sclerosis	Autosomal dominant; benign tumor growth; ~2% of ASD
Rett syndrome	MECP2 mutation; affects females; normal development then regression
Angelman/Prader-Willi	Chromosome 15 imprinting defects
Williams syndrome	Chromosome 7q11.23 deletion

Neurobiological Factors

Elevated platelet serotonin (5-HT): a consistent biomarker finding
mTOR pathway dysfunction: disrupts synaptic plasticity
Neuroimaging: increased total brain volume in children <4 years; frontal lobe abnormalities; decreased connectivity between cortical regions
Neuroanatomy: postmortem studies show abnormal cortical lamination, reduced Purkinje cells in cerebellum, and altered neuronal organization
Some evidence of failed cerebral lateralization

Environmental Factors

Higher-than-expected incidence of prenatal and perinatal complications in children later diagnosed with ASD
Parental emotional or rearing factors have been disproven as contributors

Differential Diagnosis

Condition	Key Distinguishing Features
Social (pragmatic) communication disorder	Social communication deficits only — no restricted/repetitive behaviors
Childhood-onset schizophrenia	Hallucinations/delusions; rare before age 5; fewer seizures
Intellectual disability	Global verbal and nonverbal impairment; social relatedness proportional to mental age
Congenital deafness	Audiological testing distinguishes
ADHD	May co-occur with ASD (allowed under DSM-5); ADHD lacks social communication core deficits
Psychosocial deprivation	Social features may improve with enriched environment

Treatment

ASD has no cure; management is multimodal and targets specific symptom domains.

Behavioral and Educational Interventions

Applied behavior analysis (ABA): structured behavioral therapy; best evidence base; early intensive intervention improves outcomes
Social skills training groups: improve peer interaction strategies
Special education: combination of special and mainstream classes
Speech and language therapy: core for communication deficits
Occupational therapy: sensory integration, fine motor skills

Pharmacotherapy

ASD-specific pharmacotherapy targets associated symptoms, not core deficits:

Drug	Target Symptom	Notes
Risperidone (FDA-approved)	Irritability, aggression, self-injury, tantrums	Atypical antipsychotic
Aripiprazole (FDA-approved)	Irritability	Atypical antipsychotic
α2-agonists (clonidine, guanfacine)	Hyperactivity, inattention, impulsivity, sleep dysfunction	Second-line agents
SSRIs	Repetitive behaviors, anxiety, OCD features	Evidence mixed
Stimulants (methylphenidate)	ADHD symptoms comorbid with ASD	Reduced efficacy vs. ADHD alone

Case example from Kaplan & Sadock: An 8-year-old boy (Brett) with ASD and verbal ability was prescribed risperidone, which reduced tantrums in frequency and severity. He attended a social skills group and progressed through a combination of special and regular education into high school with plans for community college.

Prognosis

ASD is a lifelong condition; core symptoms persist into adulthood
Best predictors of functional outcome: language development by age 5 and IQ level
A minority achieve independent living and employment, particularly those with higher cognitive and language functioning (former Asperger profile)
Early intensive intervention significantly improves adaptive outcomes

Sources: Kaplan and Sadock's Synopsis of Psychiatry | Eric Kandel's Principles of Neural Science, 6th ed.

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