DESCRIPTION

• The Diagnostic and Statistical Manual of Mental Disorders, 5th edition (DSM-5) defines intellectual disability as impairment in both cognitive and adaptive functioning, with onset during the developmental period.
• Cognitive function is often measured as an IQ, and the distribution of IQ scores in the community generally follows a normal distribution, with an average of 100 and a standard deviation of 15. Cognitive functions include reasoning, memory, problem solving, and learning. Intellectual disability is recognized as an IQ that falls 2 standard deviations below the population mean (IQ <70).
• Adaptive functions include communication, activities of daily living, and social interactions. Failure to establish age-expected levels of independence in activities of learning, self-care, or socialization across living environments should raise suspicion for intellectual disability.
• Severity levels correspond to an IQ range, but it is the degree of impairment in adaptive functioning that determines the level of support needed.
  • Mild (associated with an IQ range of 55 to 70)
    • May need academic support in school, handling money, and managing time; socially immature; poor risk assessment; may need supervision with more complex daily living skills and personal transportation
    • Mild cases make up about 85% of those with intellectual disability.
  • Moderate (associated with an IQ range of 40 to 55)
    • Expect slow attainment of academic and social skills throughout school age, with ceiling achievement at an elementary school level; needs greater support and supervision to achieve independence in daily living skills; language is less complex than is expected for age.
    • Moderate cases make up about 5–10% of those with intellectual disability.
  • Severe (associated with an IQ range of 25–40)
    • Limited achievement in academics and learning of conceptual skills, limited spoken language, needs ongoing supervision, may display behavioral challenges
    • Severe cases make up about 3–4% of those with intellectual disability.
  • Profound (associated with an IQ range of <25
    • Very limited achievement of conceptual skills and use and understanding of language, needs ongoing supervision, completely dependent on others for self-care and recreation, may display behavioral challenges
    • Profound cases make up about 1–2% of those with intellectual disability.
  • Unspecified
    • This classification is generally used for those aged >5 years when the degree of intellectual impairment cannot be tested using traditional standard measures due to sensory or motor impairments or for behavioral reasons; those who are given a diagnosis of “intellectual disability: unspecified” should be reassessed periodically.
    • In 2010, Rosa’s law was passed to revise the wording in federal law, replacing the term mental retardation with intellectual disability to destigmatize the diagnosis.

EPIDEMIOLOGY

Current prevalence of intellectual disability in the United States is estimated at 2–3%.

ETIOLOGY

• For many mild cases, the etiology is unknown. In cases when an underlying etiology is diagnosed, it can impact prognosis, management of associated medical conditions, and family counseling.
• Those with more significant levels of intellectual disability are more likely to have an organic etiology, such as birth trauma, early infection, genetic syndrome, metabolic errors, and to have comorbid associated conditions such as:
  • Hearing or vision impairment
  • Cerebral palsy
  • Micro/macrocephaly
  • Seizures
• There are genetic, metabolic, neurologic, infectious, traumatic, and environmental causes of intellectual disability. Some common examples include the following:
  • Genetic mutations, deletions, additions, or chromosome abnormalities; examples include fragile X syndrome, Rett syndrome, and Down syndrome.
  • Metabolic: Examples include phenylketonuria, galactosemia, Hunter syndrome, Lesch-Nyhan syndrome, and untreated congenital hypothyroidism.
  • Neurologic: Examples include hypoxic-ischemic encephalopathy and intraventricular hemorrhage.
  • Exposure to infection in utero or perinatally: Examples include TORCH (toxoplasmosis, “other” infections [e.g., syphilis, HIV], rubella, cytomegalovirus, herpes simplex virus) infections and bacterial meningitis.
  • Trauma: Examples include traumatic brain injury and shaken baby syndrome.
  • Exposure to environmental toxins or lack of nutrients in utero or in infancy or early childhood: Examples include fetal alcohol spectrum disorder, severe malnutrition, and chronic iron deficiency.