Autism Spectrum Disorder

• Genetic Architecture and De Novo Mutations: ASD involves both common genetic variants (polygenic inheritance across hundreds of loci identified by genome-wide association studies) and rare high-penetrance variants including copy number variations (CNVs), deletion/duplication events, and de novo point mutations in genes critical for synaptic function and neurodevelopment. Key genes implicated include PTEN (associated with macrocephaly and autism), SHANK3 (postsynaptic density scaffolding protein), NLGN3/NLGN4 (neuroligin-neurotensin interactions crucial for synaptogenesis), CHD8 (chromatin remodeling), and NRXN1 (neurexin, presynaptic adhesion molecule). These genetic alterations disrupt synaptic adhesion, scaffold assembly, and the balance between excitatory and inhibitory neurotransmission, fundamentally altering neural circuit development and function.

• Excitation-Inhibition (E-I) Imbalance Hypothesis: This leading mechanistic model proposes that ASD results from relative excess of excitatory glutamatergic neurotransmission compared to inhibitory GABAergic neurotransmission. This imbalance disrupts the precise tuning of neural circuits required for appropriate social processing and behavioral flexibility. In ASD brains, there is evidence of: (1) reduced GABAergic inhibitory tone due to altered GABA synthesis, receptor expression, or function; (2) elevated glutamate signaling through NMDA and AMPA receptors; (3) altered expression of GABA-A receptor subunits; and (4) reduced parvalbumin-positive interneurons in cortical circuits. These alterations result in hyperexcitability in neural networks, particularly affecting distributed circuits for social-emotional processing (amygdala, superior temporal sulcus, fusiform face area) and executive function (prefrontal cortex). The E-I imbalance explains many clinical features: repetitive behaviors reflect reduced top-down inhibition of motor programs; social difficulties arise from impaired processing of rapidly changing facial expressions and prosody; sensory hypersensitivity occurs from weakened lateral inhibition in sensory cortices.

• Synaptic Development and Pruning Abnormalities: Normal brain development involves exuberant synapse formation followed by activity-dependent pruning to refine neural circuits. In ASD, there is evidence of: (1) increased synaptic density, particularly in cortical layers; (2) altered timing and extent of developmental pruning, with overabundant immature synapses persisting into later childhood; (3) impaired myelination in white matter tracts connecting distributed brain regions; (4) altered expression of synaptic proteins including neurexins, neuroligins, and postsynaptic density proteins. Molecular drivers include dysregulation of cell adhesion molecules that stabilize synapses, altered activity-dependent signaling (BDNF-TrkB pathway), and impaired glial pruning mechanisms mediated by complement and microglial activation. This results in cortical hyperconnectivity at local scales (excessive connections between nearby neurons) combined with reduced long-range connectivity between distributed brain regions, creating inefficient information transfer in networks subserving social cognition and integration of multimodal sensory information.

• Altered Brain Maturation and Structural Organization: Neuroimaging reveals: increased head circumference and early brain overgrowth (by 6-14 months); altered gray matter developmental trajectories; abnormal white matter organization including reduced fractional anisotropy; and atypical functional connectivity patterns. These structural differences emerge perinatally and persist throughout development, reflecting fundamental alterations in neural organization. Differential growth rates, with excessive cortical surface area expansion, create organizational differences in columnar architecture and reduce the efficiency of large-scale network integration necessary for coordinating social and communicative functions.

• Altered Sensory Processing and Integration: ASD involves dysregulation of sensory gating—the brain's ability to filter irrelevant sensory information. Sensory information processing is amplified due to reduced lateral inhibition and impaired thalamic gating, resulting in hypersensitivity to stimuli (auditory, visual, tactile, olfactory). Simultaneously, there is evidence of hyposensitivity in some domains, reflecting heterogeneous alterations in sensory-specific processing. These alterations are driven by E-I imbalance affecting sensory cortices and subcortical sensory relay stations, and explain why individuals with ASD may exhibit simultaneous sensory seeking and avoidance.

• Oxytocin and Social Brain Dysfunction: The neuropeptide oxytocin, critical for social bonding and recognition, is dysregulated in ASD. Reduced oxytocin levels correlate with greater social dysfunction. Oxytocin normally facilitates amygdala processing of social cues and enhances activity in social brain networks including the superior temporal sulcus and posterior cingulate. In ASD, altered oxytocin receptor distribution and reduced oxytocin signaling contribute to impaired social motivation and reduced preferential attention to social information, particularly faces and eyes.

• Neuroinflammation and Immune Dysregulation: Post-mortem and in vivo neuroimaging studies reveal activated microglia, elevated cytokines (particularly IL-6, TNF-α, IL-1β), and astrogliosis in ASD brains. While causality remains unclear, neuroinflammation likely contributes to altered synaptic pruning and E-I imbalance. Peripheral immune activation (elevated levels of pro-inflammatory cytokines in blood and cerebrospinal fluid) may reflect a broader immune dysregulation that affects brain development, particularly during critical prenatal and early postnatal windows.

• Genetic Factors (50-90% heritability): Multiple inheritance patterns are observed. Monogenic causes (single genes with high penetrance) account for <10% of ASD cases and include: tuberous sclerosis complex (TSC1/TSC2 mutations cause mTOR pathway dysregulation), fragile X syndrome (FMR1 gene loss), PTEN mutations (with macrocephaly), CHD8 mutations, SHANK3 deletions (associated with Phelan-McDermid syndrome), and mutations in genes affecting synaptic function (NLGN3, NLGN4, NRXN1). Polygenic inheritance involving hundreds of common variants of small individual effect sizes accounts for most ASD cases and is captured by polygenic risk scores. Copy number variations (CNVs) including 16p11.2 deletions/duplications, 22q11.2 deletions, and 1q21.1 duplications carry increased ASD risk. De novo mutations (mutations not inherited from parents but arising during gametogenesis or early embryogenesis) account for approximately 10% of cases and include point mutations in genes encoding synaptic proteins, transcription factors regulating brain development (GRIN2B, KATNAL2, POGZ), and chromatin modifiers.

• Prenatal Environmental Risk Factors: Maternal infection during pregnancy, particularly influenza, rubella, and other viral infections during the first trimester, increases ASD risk, likely through altered prenatal immune activation and neurodevelopmental disruption. Maternal autoimmune conditions (particularly when associated with brain-reactive antibodies) and maternal antibodies to fetal brain proteins (detected by maternal serum antibody profiles) are associated with increased ASD risk. Prenatal exposure to medications including valproic acid (a teratogen with strong ASD association when exposure occurs during the first trimester, with affected offspring having an approximately 20-fold increased risk), thalidomide, corticosteroids (some studies), and selective serotonin reuptake inhibitors (SSRIs, controversial but likely modest effect) increase risk. Prenatal exposures to air pollution, pesticides, and heavy metals (particularly lead and mercury, though the mercury-vaccine connection is definitively disproven) show associations in epidemiologic studies. Maternal diabetes, preeclampsia, and other complications affecting placental function have been associated with increased ASD risk through mechanisms involving altered nutrient delivery and immune activation.

• Perinatal and Early Postnatal Factors: Prematurity, particularly extreme prematurity (<28 weeks), increases ASD risk through multiple mechanisms including altered brain development and greater infection/inflammation exposure. Low birth weight correlates with ASD risk. Neonatal complications including intraventricular hemorrhage, periventricular leukomalacia, seizures, and hypoxia-ischemia increase risk through direct brain injury and developmental disruption. Infections during infancy (cytomegalovirus, herpes simplex virus) and early life inflammation are associated with increased risk.

• Genetic Syndromes with High ASD Prevalence: Multiple genetic conditions exhibit high rates of ASD (30-90%) and should prompt ASD screening. These include Down syndrome (trisomy 21), Williams syndrome, Prader-Willi syndrome, Angelman syndrome, Timothy syndrome (CACNA1C mutations, featuring hypercalcemia and cardiac arrhythmias), PTEN-related overgrowth syndromes, and various mitochondrial disorders.

• Factors Definitively NOT Causative: The measles-mumps-rubella (MMR) vaccine does NOT cause autism—this has been definitively disproven by large-scale epidemiologic studies, cellular mechanism studies, and the retracted Wakefield study revealed as fraudulent. Thimerosal, the mercury-containing preservative previously used in some vaccines, does not cause autism. Post-natal dietary factors, parenting style, and psychological trauma are not causative (though they may influence symptom severity and coping).

• Sociodemographic Patterns: ASD is diagnosed more frequently in children with higher socioeconomic status and greater educational access, reflecting diagnostic bias and variability in screening rather than true prevalence differences. ASD occurs across all ethnic and racial groups with similar frequency, though disparities in diagnosis exist with some minority populations underdiagnosed. The male predominance is real but partially reflects underrecognition in females, who may "mask" or "camouflage" symptoms in social settings.

• Persistent Deficits in Social Communication: This represents the first major domain of ASD symptomatology and includes multiple related impairments: (1) Reduced social reciprocity—difficulty engaging in back-and-forth social interaction, reduced initiation of social contact, and difficulty understanding others' mental states (theory of mind deficits). Individuals may not seek shared experiences or show reduced response to their name. (2) Impaired non-verbal communication—reduced use of eye contact, reduced facial expressions, abnormal body language, and difficulty understanding others' gestures and expressions. Some individuals may have marked difficulty reading facial expressions or recognizing emotions from faces (prosopagnosia spectrum). (3) Speech and language abnormalities ranging from no functional speech to fluent but unusual speech. Common patterns include echolalia (repetition of heard words or phrases, either immediate or delayed), pragmatic language disorder (difficulty using language for social purposes—difficulty with topic maintenance, turn-taking in conversation, understanding implied meanings), unusual prosody (atypical rhythm, intonation, or rate of speech), and literal interpretation of language without understanding metaphor or humor. Language delay is variable, present in approximately 30% of individuals, and when absent should raise consideration of Asperger's syndrome variant. Speech may be pedantic or tangential, with difficulty adjusting communication style to different audiences.

• Restricted, Repetitive Patterns of Behavior, Interests, or Activities: This represents the second core domain and includes: (1) Stereotyped or repetitive motor movements including hand flapping, spinning, walking on tiptoes, arm posturing, or complex whole-body movements. These repetitive behaviors typically begin in early childhood and may decrease with age but persist in more severely affected individuals. (2) Insistence on sameness, routines, and rigid adherence to rules with marked distress when routines are disrupted. Individuals may require specific sequences for activities, consistent room arrangement, or ritualistic daily schedules. (3) Highly restricted and intense interests—preoccupation with specific topics (trains, dinosaurs, mathematics, statistics, specific factual domains) often with remarkable factual knowledge but often without social context or reciprocal interest. In adults with ASD, interests may focus on technology, science, history, or systems. The intensity is notable: individuals may prefer these interests to social engagement and may perseverate on these topics during conversation. (4) Unusual sensory interests—fascination with specific sensory aspects (spinning, watching water, light patterns, specific tactile sensations). (5) Hyper- or hypo-reactivity to sensory stimuli—heightened sensitivity to sounds (covering ears when alarmed), textures (difficulty tolerating clothing tags or specific fabrics), tastes, smells, or pain; or conversely, reduced sensitivity to pain or temperature. Some individuals show simultaneous hyper- and hypo-reactivity in different sensory domains.

• Social Deficits and Impaired Theory of Mind: Individuals with ASD typically show reduced theory of mind—difficulty understanding that others have mental states (beliefs, desires, knowledge) different from their own. This impairment underlies difficulty with false belief tasks, understanding irony or sarcasm, and predicting others' actions based on their mental states. Related are difficulties with shared attention (joint attention), which normally emerges around 12-18 months and involves the coordination of attention between the child and caregiver (pointing to show vs. pointing to request). Reduced joint attention skills predict poorer language development and are an early marker of ASD risk.

• Motor Development Abnormalities: While gross motor milestones are typically achieved on time, fine motor coordination may be impaired, with clumsiness and poor coordination (dyspraxia). Gait abnormalities are common, including toe-walking, unusual arm positioning while walking, or asymmetric gait patterns.

• Cognitive Profile and Intelligence: Intellectual ability is heterogeneous. Approximately 30% of individuals have intellectual disability (IQ <70), 25% have borderline intellectual functioning (IQ 70-85), and 45% have average to above-average intelligence. Some individuals with ASD demonstrate splinter skills—areas of relative cognitive strength in specific domains (memory, pattern recognition, calculation) despite overall lower cognitive function. Many individuals with Asperger's syndrome or higher-functioning ASD have average to above-average intelligence, though executive function deficits may impair real-world functioning despite adequate intellectual capacity.

• Associated Features and Comorbidities: Sleep disturbance is extremely common (50-80% of children), including difficulty falling asleep, frequent night wakings, and irregular sleep architecture, related to both primary circadian rhythm dysfunction and secondary effects of anxiety. Anxiety disorders are highly prevalent (40-50%), often manifesting as generalized anxiety, specific phobias (particularly social anxiety), or obsessive-compulsive behaviors. Attention-deficit/hyperactivity disorder (ADHD) co-occurs in approximately 30-50% of individuals and may be difficult to distinguish from the inattention seen with restricted interests. Mood disorders including depression (particularly in adolescents and adults) are common. Eating disorders including selective eating (restricted to certain textures or tastes, often related to sensory sensitivities) are frequent. Gastrointestinal symptoms including constipation, diarrhea, and non-specific abdominal pain are more common in ASD than in general population, though specific pathophysiology remains unclear.

• Seizure Disorder: Approximately 20-30% of individuals with ASD develop seizures, typically with onset either in early childhood or adolescence/early adulthood. Seizures are more common in those with intellectual disability and early language delay. The relationship between seizure susceptibility and ASD is incompletely understood but likely reflects common neurodevelopmental mechanisms affecting cortical excitability.

• Physical Examination Findings: General physical exam is often unremarkable,

Screening (initial step)

• Developmental surveillance: the AAP (Bright Futures periodicity schedule and the 2020 AAP clinical report on identification and management of ASD) recommends surveillance at every well-child visit, general developmental screening at 9, 18, and 30 months, and ASD-specific screening at 18 and 24 months.
• M-CHAT-R/F: the standard parent-report instrument for toddlers. Scoring is low risk (0–2, no further action), medium risk (3–7, administer the structured follow-up interview), and high risk (8–20, refer immediately for diagnostic evaluation and early intervention without waiting).

Confirmatory evaluation

• DSM-5-TR criteria are the reference standard: (A) persistent deficits in all three social-communication domains — social-emotional reciprocity, nonverbal communicative behaviors, and developing/maintaining relationships; (B) at least two of four restricted/repetitive behaviors — stereotyped movements or speech, insistence on sameness, fixated interests, sensory hyper- or hyporeactivity; (C) onset in the early developmental period; (D) clinically significant impairment; (E) not better explained by intellectual disability or global developmental delay. Severity is coded Level 1–3 by support needed, separately for each domain.
• ADOS-2 (semi-structured direct observation) plus the ADI-R (caregiver interview) are the gold-standard instruments used by developmental-behavioral pediatrics, child psychiatry, or psychology.

Mandatory and etiologic testing

• Formal audiologic evaluation in every child with language delay or suspected ASD — hearing loss is the must-not-miss mimic and is the single best next step when speech regression is described.
• Chromosomal microarray and fragile X (FMR1) testing are first-tier per ACMG for ASD with developmental delay; add MECP2 in girls with regression and PTEN with macrocephaly (occipitofrontal circumference well above the mean).
• EEG only if seizures, staring spells, or language regression suggesting Landau-Kleffner syndrome. Routine MRI, heavy-metal panels, and "autism biomarker" testing are not indicated.

Immediate concerns

• Acute agitation, aggression, or self-injury: ensure safety, remove sensory triggers, use low-stimulation environment and communication supports; reserve short-term pharmacologic sedation for danger to self/others. Always evaluate for an unrecognized medical pain source (constipation, dental caries, otitis, fracture) in a nonverbal child with new behavioral escalation.

First-line (non-pharmacologic, for all patients)

• Early intensive behavioral intervention: applied behavior analysis and naturalistic developmental behavioral interventions (e.g., Early Start Denver Model), plus speech-language and occupational therapy. Refer to IDEA Part C early intervention (<3 years) or the school district's Part B program (≥3 years) at the time of concern — do not wait for the confirmed diagnosis.
• Parent-mediated training and structured school supports (IEP) form the backbone of long-term care.

Pharmacotherapy (targets associated symptoms only)

• Second-generation antipsychotics: risperidone and aripiprazole are the only FDA-approved agents, indicated for irritability associated with autistic disorder (aggression, self-injury, severe tantrums). Monitor weight, BMI, glucose/lipids, and prolactin (risperidone causes hyperprolactinemia and gynecomastia) and screen for extrapyramidal symptoms.
• ADHD symptoms: stimulants (methylphenidate) work but with smaller effect and more irritability than in idiopathic ADHD; alpha-2 agonists (guanfacine) and atomoxetine are reasonable alternatives.
• Anxiety and repetitive behaviors: SSRIs (e.g., sertraline) have limited pediatric evidence in ASD and risk behavioral activation; start low, go slow.
• Insomnia: sleep hygiene first, then melatonin, which has the best evidence in ASD.

Disease-related

• Epilepsy (emergency when prolonged): shared cortical excitation–inhibition imbalance produces a bimodal onset in early childhood and adolescence; highest risk with intellectual disability. New staring spells, nocturnal events, or skill loss warrant EEG; convulsive status epilepticus requires benzodiazepine rescue and standard status protocols.
• Elopement/wandering: impaired danger appreciation plus sensory-driven flight; drowning is the feared outcome and a leading cause of injury death in ASD (emergency). Counsel on door alarms, ID bracelets, and swim training.
• Self-injurious behavior: head-banging, biting; signals unmet communication needs or occult pain. Sudden escalation in a nonverbal child is a red flag for an untreated medical problem.
• Catatonia (emergency): adolescents may develop mutism, posturing, negativism, and food refusal; malignant catatonia brings autonomic instability. Treated with benzodiazepines (lorazepam challenge) and ECT — easily mistaken for "worsening autism."
• Nutritional deficiency from food selectivity: extreme texture-based restriction has produced scurvy (gum bleeding, refusal to bear weight, subperiosteal hemorrhage) and vitamin A deficiency with night blindness. Chronic constipation with impaction is common.
• Psychiatric burden: anxiety, depression, and elevated suicide risk in adolescents and adults, often underrecognized because of atypical symptom expression.

Treatment-related

• Antipsychotic metabolic effects: risperidone/aripiprazole cause weight gain, dyslipidemia, and insulin resistance via H1/5-HT2C antagonism — the most common reason for discontinuation.
• Hyperprolactinemia (risperidone): D2 blockade in the tuberoinfundibular pathway → gynecomastia, galactorrhea, delayed puberty.
• Extrapyramidal symptoms and tardive dyskinesia; neuroleptic malignant syndrome (emergency) — hyperthermia, rigidity, autonomic instability, elevated CK.
• Stimulants: appetite suppression, growth deceleration, and paradoxical irritability.
• SSRIs: behavioral activation and the pediatric suicidality warning.
• Chelation (emergency): iatrogenic hypocalcemia causing arrhythmia and death — the classic harm from non-evidence-based "biomedical" therapy.

• Screen at 18 and 24 months with the M-CHAT-R/F — this is the AAP-mandated ASD-specific screening interval, and it is the most commonly tested "which visit" fact.
• Speech delay or language regression? The single best next step is a formal audiologic evaluation. Examiners reward ruling out hearing loss before invoking a developmental diagnosis.
• Refer to early intervention immediately, before the diagnosis is confirmed. "Wait for the ADOS" and "repeat screening in one year" are the classic wrong answers.
• **Regression + acquired microcephaly + midline hand-wringing in a girl = Rett syndrome (MECP2)**, not ASD — the highest-yield distractor. Loss of purposeful hand use is the discriminator.
• Syndromic associations examiners love: long face, large ears, macroorchidism → fragile X (FMR1, the most common inherited cause of ASD); ash-leaf macules and infantile spasms → tuberous sclerosis (TSC1/TSC2); macrocephaly with ASD → PTEN. First-tier genetics per ACMG is chromosomal microarray plus fragile X testing.
• Only risperidone and aripiprazole are FDA-approved in ASD, and only for irritability/aggression — not for core social-communication deficits. Follow weight, glucose, lipids, and prolactin. There is no drug for the core disorder.
• The MMR vaccine does not cause autism; the originating study was fraudulent and retracted. The correct response to a hesitant parent is to affirm vaccination on schedule.
• Differential trap: restricted/repetitive behaviors are required for ASD — deficits in social pragmatics without them is social (pragmatic) communication disorder. Speech present at home but absent at school is selective mutism; global delay affecting all domains equally is global developmental delay/intellectual disability.
• Sudden mutism, posturing, and food refusal in an adolescent with ASD is catatonia, not "regression" — it is treatable with benzodiazepines and ECT.