What are the diagnostic criteria and management options for intellectual disability?

Intellectual Disability Diagnosis

Diagnose intellectual disability when both intellectual functioning (IQ ≤70, approximately 2 standard deviations below the mean) AND adaptive functioning deficits across multiple domains are present, with onset during the developmental period before age 18. 1, 2

Diagnostic Criteria

The diagnosis requires three core components that must all be met:

1. Intellectual Functioning Deficits

• IQ score ≤70 (approximately 2 SD below mean) on individually administered, standardized, culturally appropriate intelligence tests 1, 3
• Assess profile of IQ subtests rather than relying solely on composite scores, as subtest patterns reveal specific cognitive strengths and weaknesses 1, 3
• Clinical judgment is essential because test performance can be affected by motivation, cooperation, physical/mental health, test setting, communication barriers, sensory impairments, and motor factors 1, 3
• Results are less reliable in individuals with severe ID or significant language impairment 1, 3

2. Adaptive Functioning Deficits

• Must demonstrate significant limitations in adaptive behavior across conceptual, social, AND practical domains 1, 2
• Conceptual domain: language, reading, writing, math, reasoning, memory, practical knowledge 1
• Social domain: interpersonal communication, social judgment, empathy, understanding others' thoughts/feelings, social problem-solving 1
• Practical domain: self-care, job responsibilities, money management, recreation, organizing school/work tasks 1
• Severity classification is based on adaptive functioning, not IQ scores 1, 2

3. Developmental Period Onset

• Deficits must be present during the developmental period (before age 18) 1
• Severe ID: typically identified in first 2 years with delayed motor, language, and social milestones 1
• Mild ID: often not identified until school age when academic learning difficulties become apparent 1

Age-Specific Diagnostic Considerations

Children Under 5 Years

• Use "Global Developmental Delay" instead of intellectual disability, as IQ measures are unreliable in this age group 1
• Requires significant limitations in ≥2 developmental domains 1
• Must demonstrate developmental delay ≥1.5 SD below the mean 1

Children 5 Years and Older

• Use "Unspecified Intellectual Disability" when assessment is impossible due to sensory/physical impairments, severe communication difficulties, locomotor disability, or severe behavioral/psychiatric problems 1
• Reassessment required every 3 years by federal law for school-aged children 1

Differential Diagnosis

Rule out these conditions before confirming ID diagnosis:

• Specific learning disorders: isolated academic deficits without global intellectual impairment 1
• Communication disorders: language deficits without broader cognitive impairment 1
• Autism spectrum disorder: social communication deficits may occur without intellectual impairment 1
• Major/mild neurocognitive disorders: acquired rather than developmental 1
• Affective disorders or psychosis: can temporarily impair test performance 1

Diagnostic Assessment Tools

Screening for Developmental Delay

• Ages and Stages Questionnaire (ASQ-III) for ages 1-66 months: 30-item parent report with sensitivity 0.83-0.89 and specificity 0.80-0.92 1

Psychiatric Comorbidity Assessment

• Developmental Behaviour Checklist (DBC): 96 items, 5 subscales, strongest psychometric properties 1
• Nisonger Child Behavior Rating Form (NCBRF): 76 items total (10 social competence, 66 problem behavior) 1
• Reiss Screen for Children's Dual Diagnosis (RSCDD): 60 items, 10 subscales 1

Diagnostic Manuals

• DM-ID-2 (Diagnostic Manual-Intellectual Disability, 2nd edition): complements DSM-5 with ID-specific diagnostic criteria modifications 1

Management Approach

Initial Evaluation

• Identify underlying etiology through genetic testing, metabolic screening, and neuroimaging, particularly in moderate-to-severe ID where specific causes are more likely identified 4
• Screen for common comorbidities: epilepsy (30-50% in severe ID), autism spectrum disorder (7.5-15%), ADHD, anxiety disorders, and oppositional defiant disorder occur at 3 times higher rates than in typically developing children 1, 5

Ongoing Management

• Treat co-occurring medical conditions (epilepsy, hearing/vision impairments) as these can improve cognitive and adaptive functioning 1
• Provide individualized supports based on specific adaptive functioning deficits across conceptual, social, and practical domains 1
• Monitor for psychiatric disorders using validated behavioral assessment tools, as psychiatric comorbidity is 3-fold higher 1
• Reassess diagnosis periodically: adaptive skills may improve with interventions, potentially changing severity level or even eliminating the diagnosis if improvements represent stable skill acquisition rather than support-dependent gains 1

Critical Pitfalls to Avoid

• Do not rely solely on IQ scores for diagnosis or severity classification; adaptive functioning determines severity 1, 2
• Do not assume static disability: cognitive and adaptive skills can improve with early intervention, treatment of comorbidities, and environmental supports 1
• Do not overlook cultural/linguistic factors: standardized tests may underestimate abilities in minority populations 1, 3
• Do not miss psychiatric comorbidities: use structured behavioral assessment tools as self-report is unreliable in this population 1
• Do not assume genetic cause: while genetic abnormalities are common in severe ID, most mild ID cases lack identified specific etiology 4