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

Differentiating ASD from ADHD in Adults

In adults, differentiate ASD from ADHD by establishing whether core social-communication deficits and restricted interests have been present since early childhood (ASD) versus symptoms of inattention and hyperactivity that began before age 12 and persist across multiple settings (ADHD), recognizing that both conditions frequently co-occur and require comprehensive assessment including collateral information from parents or significant others. 1, 2

Critical Developmental History

Obtain detailed childhood history to establish symptom onset and trajectory:

• ASD presents with lifelong social-communication deficits from early childhood with no period of normal development, whereas ADHD symptoms must have onset before age 12 but may not involve pervasive social impairment 2, 3
• Adults with ADHD often have poor insight and underestimate symptom severity, making collateral information from parents, spouses, or significant others essential 1
• Early developmental markers distinguishing ASD include absent or impaired pointing for interest and lack of conventional gestures at 20-42 months 3, 4

Core Symptom Patterns

Social and Communication Impairments:

• Prominent, persistent social and communicative impairments are the hallmark of ASD and include deficits in nonverbal behaviors, lack of developed social insight, and impaired emotion regulation that are chronic rather than episodic 2, 3, 4
• ADHD may involve social difficulties secondary to impulsivity and inattention, but these lack the qualitative social-communication deficits seen in ASD 2

Attention and Executive Function:

• ADHD presents with inattention, hyperactivity, and impulsivity that must be present across multiple settings (work, home, social) and cause functional impairment 1, 2
• Spatial working memory deficits on neuropsychological testing (such as CANTAB) may help distinguish ADHD from ASD 5
• Both conditions show attention problems, but the mechanisms differ—ASD involves attention to social cues and shifting focus, while ADHD involves sustained attention and distractibility 5, 6

Repetitive Behaviors and Restricted Interests:

• In ASD, repetitive behaviors serve self-regulatory functions, are ego-syntonic, and include hand flapping, rocking, spinning, and restricted interests that are persistent and pervasive 4
• These behaviors in ASD are part of the core disorder and present from early development, unlike the disorganization and restlessness in ADHD 4

Structured Assessment Protocol

Use validated rating scales and obtain multiple perspectives:

• Administer the Autism-Spectrum Quotient (AQ), Conners Adult ADHD Rating Scale (CAARS), Wender Parent's Rating Scale, or Brown Attention-Deficit Disorder Scale for Adults 1, 7, 6
• The Autism Diagnostic Observation Schedule, Second Edition (ADOS-2) Module 4 can identify ASD symptoms in adults with ADHD, though symptom overlap is substantial 7
• Obtain information from parents regarding childhood symptoms, as parent reports at age 25 remain clinically useful and show stronger associations with cognitive problems and polygenic risk scores than self-reports 8
• Screen for IQ, emotional and behavioral problems, and adaptive functioning using comprehensive neuropsychological battery 5, 9

Addressing Symptom Overlap

Recognize that substantial overlap exists but mechanisms differ:

• Adults with ADHD show elevated AQ scores (higher than controls but lower than ASD), and adults with ASD show elevated CAARS scores (higher than controls but lower than ADHD) 6
• ASD symptoms in adult ADHD comprise two distinct mechanisms: one qualitatively similar to ASD and another arising from ADHD characteristics 7
• The absence of correlation between AQ and CAARS scores suggests these represent distinct underlying processes despite apparent clinical similarity 6
• Lower perceptual reasoning scores (Picture Concept subtest on WISC) may suggest ASD, while spatial working memory deficits suggest ADHD 5

Comorbidity Considerations

Screen for co-occurring conditions that complicate diagnosis:

• Both ASD and ADHD can coexist—individuals meeting criteria for both show lower IQ, higher autistic symptom severity, shared inattention/hyperactivity with ADHD, and shared adaptive behavior impairment with ASD 9
• Screen for anxiety, depression, substance use disorders, learning disabilities, and other psychiatric comorbidities that are common in both conditions 1, 2
• Obtain detailed substance use history and consider urine drug screen given high rates of comorbid substance abuse in adult ADHD 1

Differential Diagnosis Algorithm

Rule out conditions that mimic ASD or ADHD:

• Exclude bipolar disorder, depression, personality disorders, learning disabilities, narcolepsy, and borderline intellectual functioning through comprehensive psychiatric evaluation 1
• Medical history, physical examination, and screening laboratory tests help rule out medical conditions masquerading as ADHD 1
• Trauma history is essential, as PTSD can present with attention problems and emotional dysregulation that mimic both conditions 2

Common Diagnostic Pitfalls

• Do not assume all social difficulties in ADHD are secondary to impulsivity—adults with ADHD can have comorbid ASD requiring separate treatment 7, 9
• Avoid diagnostic overshadowing where clinicians fail to diagnose one condition when the other is already present 3
• Do not rely solely on self-report in adults, as insight is often poor, particularly in ADHD 1, 8
• Sex differences are less pronounced in adulthood than childhood, but males still show higher trait levels for both conditions 8

Treatment Implications

Treatment approach differs based on diagnosis:

• For ADHD, FDA-approved stimulant medications combined with behavioral interventions are first-line treatment 1, 2
• For ASD, focus on social skills development and management of comorbid conditions, though evidence for social skills training as primary ADHD intervention is lacking 2
• When both conditions coexist, address the most impairing symptoms first while managing both conditions following chronic care model principles 2
• Untreated ADHD increases risk for early death, suicide, psychiatric comorbidity, lower educational achievement, and incarceration 2