Intellectual Disability (Intellectual Developmental Disorder)


  • A global deficit in cognitive functioning evidenced by a significant difference in mental and chronologic ages (IQ) and significantly impaired adaptive functioning (1)
  • The term “mental retardation” was deleted from the DSM-5 and replaced with “intellectual disability” (ID) or “intellectual developmental disorder.” The term “mental retardation” is pejorative and insensitive and should be avoided.
  • Although cognitive issues typically have a pervasive impact, patients with ID display highly variable levels of functioning and subsequent service needs.
  • Tailor evaluation and treatment to individual needs.


  • ID is defined as an IQ ≤70 + 5 and a significant impairment in intellectual functioning. This includes verbal and nonverbal reasoning, planning, academic learning, problem solving, and experiential learning. Intellectual function is confirmed by IQ testing and clinical assessment (1).
  • A diagnosis of ID also requires deficits in adaptive functioning, such as communication, socialization, and independent living (1).
  • By definition, ID is a neurodevelopmental disorder that is typically present from birth or recognized during early childhood because developmental milestones are delayed (1).
  • ID is currently subgrouped according to adaptive function and level of needed support: mild (typical development in some domains, mild impairment in others), moderate (skills are markedly behind typically developing, same-age peers), severe (skills quite limited compared to peers), profound (limited awareness of concepts, language; dependent on others for adaptive functioning) (1).
  • The three most common causes of ID are Down syndrome, fragile X syndrome, and fetal alcohol syndrome (FAS).
  • If ID reflects a loss of previously acquired intellectual skills, consider a comorbid neurocognitive disorder.
  • Stereotypes of people with ID (e.g., always happy, poor prognosis, unable to function independently) have been categorically refuted. People with ID show levels of functional variability that parallels the non-ID population.
  • Under the Individuals with Disabilities Education Act (IDEA), caregivers should familiarize themselves with their child’s legal rights to facilitate coordination of services with the school district.
  • Under the Americans with Disabilities Act (1990), people with ID cannot be discriminated against based on their disability, and employers are legally required to ensure that appropriate accommodations are in place.
For some causes of ID, prenatal testing is available.



  • 1 of 6 children (2)
  • Predominant sex: male > female: 1.6:1 for mild ID, 1.2:1 for severe ID (1)

In the United States, 1% of the general population. The prevalence of severe ID is 6/1,000 (1).

Etiology and Pathophysiology

  • Causes:
    • Maternal substance abuse (e.g., alcohol); FAS is a leading environmental cause of ID.
    • Maternal infections: TORCH viruses (toxoplasma, other infections, rubella, cytomegalovirus, and herpes simplex virus)
    • Down syndrome
    • Sex chromosome abnormalities: fragile X, Turner syndrome, Klinefelter syndrome
    • Autosomal dominant conditions: neurocutaneous syndromes (e.g., neurofibromatosis, tuberous sclerosis)
    • Autosomal recessive conditions:
      • Amino acid metabolism (e.g., phenylketonuria, maple syrup urine disease)
      • Carbohydrate metabolism (e.g., galactosemia, fructosuria)
      • Lipid metabolism
      • Tay-Sachs disease
      • Gaucher disease
      • Niemann-Pick disease; mucopolysaccharidosis
      • Purine metabolism (e.g., Lesch-Nyhan disease)
      • Other (e.g., Wilson disease)
  • Maternal medications (e.g., isotretinoin, phenytoin [Dilantin])
  • Perinatal factors:
    • Prematurity
    • Birth injuries
    • Perinatal anoxia
  • Postnatal factors:
    • Childhood diseases (e.g., meningitis, encephalitis, hypothyroidism, seizure disorders)
    • Trauma (e.g., accidents, physical abuse, hypoxia)
    • Severe deprivation
    • Poisoning (e.g., lead, carbon monoxide, household products)

A number of genetic and epigenetic causes are known, and more are under investigation.

Risk Factors

  • Maternal substance abuse during pregnancy
  • Maternal infection during pregnancy
  • For some causes, family history

General Prevention

  • Reduce alcohol and drug use by pregnant women.
  • Prenatal folic acid supplementation

Commonly Associated Conditions

  • Seizures
  • Mood disorders
  • Behavioral disorders
  • Constipation

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