Autism Spectrum Disorder: Definition, Terminology, Signs, and Diagnosis
Current Terminology
Autism Spectrum Disorder (ASD) is the correct and current diagnostic term, replacing outdated labels such as autism, Asperger's syndrome, and pervasive developmental disorder-not otherwise specified (PDD-NOS) 1, 2. The term "spectrum" reflects the wide range of presentations, from mild to severe, with most individuals having average to above-average intellectual ability 3.
Core Diagnostic Features
ASD is defined by two essential symptom domains 1, 4:
Social communication and interaction deficits: Including impaired nonverbal behaviors (eye contact, gestures), difficulty developing peer relationships, lack of social-emotional reciprocity, and deficits in understanding and using communication for social purposes 1
Restricted and repetitive behaviors (RRBs): Including stereotyped motor movements (hand flapping, finger flicking, rocking, spinning), insistence on sameness, highly restricted interests of abnormal intensity, and hyper- or hypo-reactivity to sensory input 1, 5
Early Warning Signs by Age
First Two Years of Life
- No response to name when called 6, 4
- Lack of pointing for interest at 20-42 months 7
- Absence of conventional gestures at 20-42 months 7
- No or limited use of gestures in communication 4
- Lack of imaginative or pretend play 6, 4
- Deficits in directing attention and attention to voice at 24 months 7
- Atypical body movements (stimming behaviors may emerge early or late during the second year) 5
Behavioral Markers
- Avoidance of eye contact 6
- Lack of interactive play 6
- Excessive fear or unusual fear responses 6
- Repetitive behaviors with objects and body (significantly higher than typically developing children) 5
Diagnosis
Criterion Standard
The gold standard for ASD diagnosis is a comprehensive multidisciplinary evaluation using standardized semistructured measures 4:
- Autism Diagnostic Observation Schedule-Second Edition (ADOS-2): Sensitivity 91%, specificity 76% 4
- Autism Diagnostic Interview (ADI): Sensitivity 80%, specificity 72% 4
These tools combine direct observation of the child's behavior with semistructured caregiver interviews focused on development and behaviors 4.
Additional Screening and Diagnostic Tools
- Modified Checklist for Autism in Toddlers, Revised with Follow-Up (M-CHAT-R/F) 6
- Childhood Autism Rating Scale (CARS) 6, 2
- Social Communication Questionnaire (SCQ) 6
- Parents' Evaluation of Developmental Status (PEDS) 6
- Autism Spectrum Disorder-Observation for Children (ASD-OC) 2
- The Developmental, Dimensional, and Diagnostic Interview (3di) 2
Diagnostic Reliability
The ASD diagnosis demonstrates high interrater reliability and temporal stability, with only a small percentage of cases identified in early childhood no longer meeting criteria later in life 1.
Critical Diagnostic Consideration
No biomarkers specific to ASD diagnosis have been identified 4. Diagnosis remains entirely clinical, based on behavioral observation and developmental history.
Cognitive and Functional Profile
Intellectual Ability Distribution
- 30% have co-occurring intellectual disability (ID) 1, 8
- 50% have severe to profound ID (IQ <35-40) 8
- 35% have mild to moderate ID (IQ 40-70) 8
- 15-20% have IQ in the normal range (IQ ≥70) 8
Cognitive level is the primary driver of behavioral presentation variability, more so than the core social communication deficits themselves 8.
Neurobehavioral Patterns
- Verbal skills typically more impaired than nonverbal skills in classic presentations 8
- Working memory and processing speed deficits are prominent 8
- Receptive and expressive language show decreased performance 8
- Motor dysfunction observed in early developmental course 8
Common Co-occurring Conditions
Approximately 90% of individuals with ASD have at least one additional medical or mental health condition 1, 8:
- Depression: 20% (vs. 7% in general population) 4
- Anxiety: 11% (vs. 5% in general population) 4
- ADHD: Affects more than half 1
- Sleep difficulties: 13% (vs. 5% in general population), affects more than half per other estimates 1, 4
- Epilepsy: 21% with co-occurring ID (vs. 0.8% in general population), affects one-fifth to one-third 1, 4
- Gastrointestinal disorders: Affect around half 1
- Irritability/challenging behavior: Affects around one-fifth 1
Distinguishing ASD from Similar Conditions
Differentiating from OCD
- ASD presents early in development (first 2 years), while OCD typically emerges in later childhood or adolescence 7
- In ASD, repetitive behaviors are ego-syntonic (part of the person's identity), whereas in OCD they are ego-dystonic (intrusive, unwanted, causing marked distress) 7
- Prominent social and communicative impairments characterize ASD but not OCD 7
- Individuals with ASD can have comorbid OCD—failing to diagnose this leads to inadequate treatment 7
Distinguishing Stimming from Tics
Stimming behaviors should be distinguished from tics seen in Tourette syndrome, which can co-occur with ASD 5.
Prevalence and Epidemiology
- Current US prevalence: 2.3% of children aged 8 years, 2.2% of adults 4
- Prevalence has increased from 1.1% in 2008 to 2.3% in 2018, likely due to changes in diagnostic criteria, improved screening tools, and increased public awareness 4
- Male predominance is observed, though the gender bias is an active area of research 1
Clinical Pitfalls to Avoid
- Do not assume all repetitive behaviors in ASD are simply autistic traits—screen for comorbid OCD, which requires specific treatment 7
- Do not delay evaluation based on "wait and see"—early identification enables timely intervention with significantly improved developmental outcomes 6, 4
- Do not overlook co-occurring conditions—comprehensive assessment must include screening for ADHD, anxiety, depression, sleep disorders, GI problems, and epilepsy 1, 4
- Do not rely on a single screening tool—diagnosis requires comprehensive multidisciplinary evaluation with standardized measures 4
Treatment Overview
Behavioral Interventions (First-Line)
Intensive behavioral interventions are first-line treatment for children 5 years or younger, such as the Early Start Denver Model, showing small to medium effect sizes for improvement in language, play, and social communication 4.
Pharmacotherapy Indications
Pharmacotherapy is indicated for co-occurring psychiatric conditions and specific target symptoms 4:
For irritability and aggression: Risperidone and aripiprazole (FDA-approved for irritability associated with autistic disorder in ages 5-17 years) show large effect size (standardized mean difference 1.1) 9, 4
For ADHD symptoms: Psychostimulants show moderate effect size (standardized mean difference 0.6) 4
Common adverse effects: Changes in appetite, weight, and sleep 4
Comprehensive Neurobehavioral Evaluation Components
Assessment should include 8:
- Efficient indicators of global ability
- Measures of sustained attention, working memory, and processing speed
- Assessment of receptive, expressive, and pragmatic language skills
- Evaluation of fine and gross motor skills
- Adaptive function assessment
- Autism-specific measures
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