Intellectual Disability

The terminology for intellectual disability (ID) has changed in the past decade. In 2010, President Obama signed Rosa’s Law, stating that the term“mental retardation” should be replaced with “intellectual disability” in all federal contexts. In 2013, the Diagnostic and Statistical Manual of Mental Disorders, fifth edition (DSM-5) changed the diagnosis of mental retardation to intellectual disability, and in 2022, intellectual disability was renamed to intellectual developmental disorder (IDD) in the DSM-5-TR. The Centers for Disease Control and Prevention indicates that ID affects approximately 1% of the U.S. population.

The American Association on Intellectual and Developmental Disabilities (AAIDD) defines intellectual disability as “a condition characterized by significant limitations in both intellectual functioning and adaptive behavior that originates before the age of 22.” The AAIDD definition emphasizes severity based on the intensity of supports that are needed instead of the severity of functional limitations.

• Intellectual functioning (intelligence) refers to general mental capacity (e.g., learning, problem-solving, reasoning).

• Adaptive behavior is the collection of conceptual, social, and practical skills that are learned and performed by people in their everyday lives.

The American Psychological Association (APA) uses a similar definition:

• ID is a developmental disability characterized by mild-to-profound limitations in cognitive function (e.g., learning, problem-solving, reasoning, planning) and in adaptive behavior, impairing one’s ability to acquire skills typical for one’s age group as a child or necessary for one’s later independent functioning as an adult.

Evaluation and Diagnosis

The diagnosis of ID is ideally made at age 5 years or older. The diagnosis is generally not made in younger children because cognitive and adaptive skills functioning may improve, consistent with ongoing changes in myelination and brain development in the early years (see Global Developmental Delay in this rotation guide). Children younger than 5 years who are delayed in two or more milestones meet criteria for global developmental delay (GDD). ID refers to individuals whose cognitive and adaptive skills functioning is significantly reduced before the age of 22 years.

From a cognitive perspective, the criterion for ID is an intelligence quotient (IQ) below 70. Both the DSM-5-TR and AAIDD put greater emphasis on the adaptive skills function than IQ score alone, indicating that how a person actually functions takes precedence. Of note, adaptive skills are not static and, with appropriate supports, may continue to improve into adulthood.

Etiology

As with GDD, the etiology of ID is multifactorial and may include infectious, genetic, toxic, traumatic, metabolic, nutritional, and environmental factors.

Risk factors for ID and GDD are the same:

• a known genetic diagnosis (e.g., trisomy 21 or Down syndrome, fragile X syndrome, phenylketonuria)

• prenatal exposure (e.g., alcohol)

• prenatal infections (e.g., cytomegalovirus, Zika virus)

• perinatal or postnatal insult (e.g., hypoxic-ischemic event)

• postnatal bacterial meningitis infection

• postnatal severe malnutrition

• postnatal exposure to toxins (e.g., lead)

Recent advancements in genetic testing have improved the identification of genetic diagnoses associated with ID. (See the Pediatric Genetic and Metabolic Disorders rotation guide for more on genetic syndromes, screening, and testing.)

Screening and Testing

ID is typically identified when children with GDD remain delayed beyond 5 years of age. As such there are no screening tests to identify ID in the general population.

When there is concern for ID, testing is performed to make a diagnosis and identify appropriate interventions. Diagnosis of ID should be made by using standardized cognitive and adaptive measures.

• Examples of cognitive measures:

  • Weschler Intelligence Scale for Children (age-based cognitive assessment)

  • Stanford-Binet Intelligence Scales

• Examples of adaptive measures:

  • Vineland Adaptive Behavior Scales

  • Adaptive Behavior Assessment System

  • Scales of Independent Behavior - Revised

Medical Workup

A medical workup should be systematically performed to identify a potential cause of GDD and ID, especially when the etiology is unclear (see risk factors above). If an individual presentation is consistent with a specific diagnosis, then testing and evaluation should initially focus on that disorder. Testing can include, but is not limited to, the following common disorders in children with ID:

• fragile X syndrome

• fetal alcohol syndrome

• trisomy 21

• inborn errors of metabolism

If a potential diagnosis is uncertain, genetic testing should be performed (e.g., chromosomal microarray, fragile X DNA analysis). Other genetic tests (e.g., autism spectrum disorder/intellectual disability [ASD/ID] panel and whole-exome sequencing) are increasingly being used but are not currently included in the first tier of genetic tests. Evaluation of inborn errors of metabolism may need to be considered, especially in a child with a history of regression in skills, since some may present after infancy. An algorithm depicting the recommended medical genetic-diagnostic-evaluation flow process for a new patient with GDD/ID can be found here.

If a specific cause is identified, families may find it helpful to learn about the natural history of the condition and recurrence risk and be informed about community resources (e.g., diagnosis-specific support groups). If an etiology is not identified, periodic reevaluation is advised in case new advances and discoveries provide further insight and information.

Community Inclusion and Integration

It is important for individuals with ID to be included and integrated into the community to the best of their abilities. Resources often can be utilized to support an individual to engage in meaningful work in the community (see the Individuals with Disabilities Education Act [IDEA] in the Screening, Referrals, Evaluation, and Services section in this rotation guide).