How do you differentiate between Autism Spectrum Disorder (ASD) and Attention Deficit Hyperactivity Disorder (ADHD)?

Differentiating Autism Spectrum Disorder from ADHD

Distinguish ASD from ADHD by focusing on core social-communication deficits and the quality of repetitive behaviors: children with ASD demonstrate fundamental impairments in joint attention, nonverbal communication, and social reciprocity that are absent in pure ADHD, while their repetitive behaviors serve self-regulatory functions rather than being driven by impulsivity. 1

Core Distinguishing Features

Social-Communication Domain (Primary Differentiator)

ASD-specific markers that are absent in ADHD:

• Failure to respond to name at 12 months is highly specific for ASD (86% specificity), distinguishing it from other developmental delays including ADHD 1
• Deficits in joint attention initiation: Children with ASD show significantly fewer nonverbal behaviors to initiate shared experiences and marked differences in frequency of requesting behaviors 1
• Lack of pointing for interest and absence of conventional gestures at 20-42 months strongly suggests ASD 2
• Qualitatively impaired eye contact: Less frequent and poorly modulated, not simply reduced due to distractibility as in ADHD 1
• Deficits in directing attention and attention to voice at 24 months indicate ASD 2

Behavioral and Emotional Regulation

Children with ASD at 24 months demonstrate:

• Lower positive affect and higher negative affect compared to non-ASD siblings 1
• Difficulty controlling behavior with lower sensitivity to social reward cues 1
• Poor effortful emotion regulation that is qualitatively different from ADHD impulsivity 1

Repetitive Behaviors (Quality Matters More Than Presence)

ASD repetitive behaviors (stimming):

• Serve self-regulatory functions including hand flapping, finger flicking, rocking, spinning, and atypical movements 3
• Are ego-syntonic (not distressing to the child) and often increase with stress or excitement 3
• Correlate with overall ASD symptom severity 3

ADHD repetitive behaviors:

• Driven by impulsivity and hyperactivity rather than self-regulation 1
• Typically involve fidgeting, difficulty remaining seated, and excessive talking 1
• Respond to external redirection more readily than ASD behaviors 1

Diagnostic Evaluation Algorithm

Step 1: Assess Core Social-Communication Skills

Use standardized tools:

• ADOS (Autism Diagnostic Observation Schedule) for direct observation of social-communication behaviors 1
• ADI-R (Autism Diagnostic Interview-Revised) for developmental history 1
• M-CHAT for screening at 24 months 1

Step 2: Evaluate Attention and Executive Function

For ADHD diagnosis, confirm:

• DSM-5 criteria met with documentation of impairment in more than one major setting (home, school, social) 1
• Information obtained from parents, teachers, and other observers using standardized rating scales 1
• Symptoms of inattention, hyperactivity, or impulsivity present before age 12 1

Step 3: Assess Adaptive Functioning

Critical differentiator:

• Children with ASD show significantly worse adaptive functioning across all VABS-II domains (Communication, Socialization, Daily Living Skills, Motor skills) compared to ADHD-only 4
• ADHD children typically have better preserved adaptive skills despite attention difficulties 4
• Combined ASD+ADHD shows the worst adaptive profile overall 4, 5

Step 4: Cognitive Assessment Pattern Recognition

Neuropsychological testing reveals:

• ASD pattern: Lower scores on Picture Concept (perceptual reasoning subscale of WISC), reflecting difficulties with abstract social reasoning 6
• ADHD pattern: Lower scores on spatial working memory tests (CANTAB), reflecting executive function deficits 6
• ASD shows lower mean IQ when comorbid with ADHD compared to either condition alone 5

Critical Comorbidity Consideration

Both conditions can coexist (now permitted in DSM-5, unlike DSM-IV): 7, 4, 8

• Screen for ADHD symptoms in every child with ASD, as inattention, impulsivity, and hyperactivity are among the most frequent associated symptoms 7
• Screen for ASD features in children presenting with ADHD who have prominent social difficulties 1
• The ASD+ADHD phenotype shows higher autistic symptom severity, lower IQ, and worse adaptive functioning than either condition alone 4, 5

Common Diagnostic Pitfalls

Avoid these errors:

• Mistaking ADHD social difficulties for ASD: Children with ADHD may have peer relationship problems due to impulsivity and poor self-regulation, but they understand social cues and desire social connection 1, 8
• Overlooking comorbid ADHD in ASD: Assuming all attention problems in ASD are part of the autism itself, missing treatable ADHD that requires different intervention 7, 5
• Confusing hyperactivity with stimming: ADHD hyperactivity is purposeless motor activity, while ASD stimming serves self-regulatory functions 3
• Age-related presentation changes: Adolescents with ADHD show less overt hyperactivity, making diagnosis more challenging and requiring information from multiple sources 1

Assessment for Coexisting Conditions

Both ASD and ADHD require screening for:

• Anxiety and depression (increased risk in both, especially adolescents) 1
• Learning disabilities and language disorders 1
• Sleep disorders 1
• Oppositional defiant disorder and conduct disorders (more common in ADHD) 1
• Tic disorders (can coexist with both) 1, 3