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Intellectual Disability (Intellectual Developmental Disorder)
Jennifer L. Ayres, PhD
image BASICS
  • Intellectual disability (ID) is a global deficit in cognitive functioning evidenced by a significant difference between one's mental and chronologic ages (also known as intelligence quotient [IQ]) and significantly impaired adaptive functioning (1).
  • Although these cognitive issues typically have a pervasive impact, patients with ID will display highly variable levels of functioning and subsequent service needs.
  • Patients must be evaluated individually. Treatment plans must be tailored to specific needs.
  • The term mental retardation was deleted from the DSM-5 and replaced with “intellectual disability” or “intellectual developmental disorder.” The rationale was that the term mental retardation is considered pejorative and culturally insensitive.
  • ID is defined as an IQ ≤70 + 5 and a significant impairment in intellectual functioning, including verbal and nonverbal reasoning, planning, academic learning, problem solving, and experiential learning. The intellectual functioning is confirmed by both 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 determined during early childhood (1).
  • Currently, ID is subgrouped according to the patient's level of adaptive functioning and level of support needed. Severity levels: mild (typical development in some domains, mild impairment in others), moderate (skills are markedly behind sameage peers), severe (skills are quite limited, compared to peers), profound (very limited awareness of concepts, language, dependent on others for all aspects of adaptive functioning) (1).
  • The three most common causes of ID are Down syndrome, fragile X syndrome, and fetal alcohol syndrome (FAS).
  • If the ID reflects a loss of previously acquired intellectual skills, comorbid diagnoses of ID and neurocognitive disorder may be appropriate.
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 for severe ID is 6/1,000 (1).
  • 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)
    • 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 (e.g., mucopolysaccharidosis)
      • Purine metabolism (e.g., Lesch-Nyhan disease)
      • Other (e.g., Wilson disease)
  • Maternal use of prescription medications (e.g., Accutane, 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 (3).
  • Maternal substance abuse during pregnancy
  • Maternal infection during pregnancy
  • For some causes, family history
  • Public health efforts to reduce alcohol and drug use by pregnant women
  • Prenatal folic acid supplementation
  • Seizures
  • Mood disorders
  • Behavioral disorders
  • Constipation
A diagnosis of ID should be made only through a psychodiagnostic assessment conducted by a mental health provider who is trained and licensed to conduct formal psychological testing.
Careful examination by a physician trained in the assessment of morphologic features suggestive of a specific etiology for ID (e.g., microcephaly) (4)
  • Brain tumors
  • Auditory, visual, and/or speech/language impairment
  • Autism spectrum disorder (language and social skills are more affected than other cognitive abilities); however, 75% of individuals with an autistic disorder may meet criteria for a comorbid diagnosis of ID.
  • Expressive/receptive language disorders
  • Cerebral palsy
  • Emotional/behavioral disturbance
  • Learning disorders (reading, math, written expression)
  • Auditory/sensory processing difficulties
  • Lack of environmental opportunities for appropriate development
  • Visual and hearing tests to rule out these etiologies as a cause of impairment and provide an assessment of visual and auditory functioning, which are often impaired in children and adults with ID.
  • Formal testing of intellectual and adaptive functioning:
    • A child's communication skills must be considered in test selection. For example, a patient with auditory processing issues/limited expressive/receptive language skills may need to be assessed using a nonverbal IQ test, such as the Leiter-R, Test of Nonverbal Intelligence, or other nonverbal measures.
    • Commonly used intelligence tests (e.g., Bayley Scales of Infant Development, Stanford-Binet Intelligence Scale, Wechsler Intelligence Scales) are determined by age/developmental level of the child.
    • Common tests of adaptive functioning include the Vineland Adaptive Behavior Scales, 2nd ed., and Adaptive Behavior Assessment System, 2nd ed. These tests assess areas of functioning such as age-appropriate communication, social skills, activities of daily living, and motor skills.
  • Metabolic screening is not routine unless history and physical suggest or no newborn screening done (5).
Initial Tests (lab, imaging)
  • Lead (5)[B]
  • Thyroid-stimulating hormone if systemic features present/no newborn screening (5)[B]
  • Routine cytogenetic testing (karyotype) (5)[B]:
    • Fragile X screening (FMR1 gene), particularly with a family history of ID
    • Rett syndrome (MECP2 gene) in women with unexplained moderate to severe ID (5)
  • Molecular screening, such as array comparative genomic hybridization, is used increasingly and may yield a diagnosis in 10% of undiagnosed cases (4)[B].
  • Neuroimaging (MRI more sensitive than CT) is routinely recommended. The presence of physical findings (microcephaly, focal motor deficit) will increase the yield of a specific diagnosis (5)[B].
  • MRI may show mild cerebral abnormalities but is unlikely to establish etiology of ID (4).
Follow-Up Tests & Special Considerations
  • Electroencephalogram is not routine unless epilepsy or a specific epileptiform syndrome is present (5)[C].
  • Assessment of quality of life provides salient information about a patient's general sense of well-being and life satisfaction; however, quality of life may be difficult to assess when significant behavioral issues confound an individual's self-report and socialization.

  • Early intervention services tailored to the individual's specific needs
  • Caregiver support, including:
    • Training caregiver(s) to address behavioral issues, discipline, and support socialization development
    • Encouraging caregivers to create a structured home environment that is based on the child's developmental level and specific needs rather than age-appropriate expectations.
    • Providing caregiver(s) with an opportunity to address their reactions to the diagnosis and their child's special needs
    • Informing caregivers about advocacy groups and available community, state, and national resources (6,7,8)
    • Encouraging caregiver(s) to seek social support to increase overall sense of well-being
    • Encouraging caregivers to seek respite care as needed to ensure that they have an opportunity to engage in health self-care.
  • Individualized education plans and, depending on the level of impairment, social skills and behavioral plans/training
  • Refer to job training programs and independent living opportunities, if appropriate.
  • Notice all changes in behavior, which may be indicative of pain/illness, particularly in individuals with limited communication skills.
  • Assess for abuse and neglect.
Medication may be appropriate for comorbid conditions (e.g., anxiety, ADHD, depression).
The physician should match his or her communication of exam procedures, test results, and treatment recommendations to the patient's level of cognitive functioning and receptive language skills.
  • Most patients with ID will fall within the mild range and are fully capable of understanding information if it is provided at the appropriate level.
  • Provide oral and written explanations directly to the patient instead of solely to his or her caregivers. The dignity of the patient must be respected at all times. This includes providing honest information, responding to patient's questions with respect, and not infantilizing the patient due to his or her ID.
  • Many adults and children with ID exhibit poor physical fitness. Preliminary studies suggest structured exercise programs are effective to engage this population in healthy activities (9)[A].
  • Linkage to community-based resources for job training, independent living, caregiver support, school-based services
Patient Monitoring
  • Primary care with attention to associated medical conditions
  • Vision testing at least once before age 40 years (age 30 years in Down syndrome) and every 2 years thereafter (10)[B]
  • Hearing evaluations every 5 years after age 45 years (every 3 years throughout life in Down syndrome) (10)[B]
  • Screen for sexual activity and offer contraception and testing for STIs (10)[B].
  • Abuse and neglect of people with ID are common. Screen at least annually and if behavior change is noted. Report abuse/neglect to appropriate protective agencies (10)[B].
  • Dysphagia and aspiration are common; consider speech pathology evaluation and swallowing study (11)[B].
  • Monitor for and treat constipation (11)[B].
  • Osteoporosis: common; low threshold to order imaging studies after traumatic injury (11)[B]
No restrictions, except in cases of metabolic and storage disorders (e.g., phenylketonuria)
  • The Arc of the United States (The Arc): www.thearc.org
  • American Association of Intellectual and Developmental Disabilities: www.aaidd.org
  • Family support groups (Parent to Parent, local Down Syndrome, or Autism Association)
  • Special Olympics: www.specialolympics.org
Although ID is a lifelong diagnosis, individuals with ID are capable of living a fulfilling, purposeful life that includes having a career, living independently, marrying/participating in a committed relationship, and becoming a parent. Also, the level of severity and support needed may vary over the course of the individual's life.
1. American Psychiatric Association. Diagnostic and Statistical Manual of Mental Disorders. 5th ed. Arlington, VA: American Psychiatric Association; 2013.
2. Centers for Disease Control and Prevention. Developmental disabilities. http://www.cdc.gov/ncbddd/developmentaldisabilities/index.html. Accessed 2014.
3. Grant ME. The epigenetic origins of mental retardation. Clin Genet. 2008;73(6):528-530.
4. van Karnebeek CD, Jansweijer MC, Leenders AG, et al. Diagnostic investigations in individuals with mental retardation: a systematic literature review of their usefulness. Eur J Hum Genet. 2005;13(1):6-25.
5. Shevell M, Ashwal S, Donley D, et al. Practice parameter: evaluation of the child with global developmental delay: report of the Quality Standards Subcommittee of the American Academy of Neurology and the Practice Committee of the Child Neurology Society. Neurology. 2003;60(3):367-380.
6. Shogren KA, Bradley VJ, Gomez SC, et al. Public policy and the enhancement of desired outcomes for persons with intellectual disability. Intellect Dev Disabil. 2009;47(4):307-319.
7. Rizzolo MC, Hemp R, Braddock D, et al. Family support services for persons with intellectual and developmental disabilities: recent national trends. Intellect Dev Disabil. 2009;47(2):152-155.
8. Samuel PS, Hobden KL, LeRoy BW, et al. Analysing family service needs of typically underserved families in the USA. J Intellect Disabil Res. 2012;56(1):111-128.
9. Heller T, Hsieh K, Rimmer JH. Attitudinal and psychosocial outcomes of a fitness and health education program on adults with down syndrome. Am J Ment Retard. 2004;109(2):175-185.
10. Sullivan WF, Heng J, Cameron D, et al. Consensus guidelines for primary health care of adults with developmental disabilities. Can Fam Physician. 2006;52(11):1410-1418.
11. Prater CD, Zylstra RG. Medical care of adults with mental retardation. Am Fam Physician. 2006;73(12):2175-2183.
  • F79 Unspecified intellectual disabilities
  • F70 Mild intellectual disabilities
  • F71 Moderate intellectual disabilities
Clinical Pearls
  • The term mental retardation may be interpreted as culturally insensitive and disrespectful to patients and their caregivers. ID or intellectual developmental disorder is the correct diagnosis.
  • Overall functioning with ID is highly variable and influenced by multiple factors, including appropriateness of school placement/special education services, exposure to early intervention, behavioral therapy, parent training, self-esteem, and social skills.
  • Previous stereotypes of people with ID (e.g., always happy, poor prognosis, unable to function independently) have been refuted. People with ID are showing a level of functioning variability that parallels what is found in the non-ID population.
  • Be aware of the unique parenting needs that caregivers may face. Link families to community and national resources that can provide practical and emotional support when appropriate.
  • Because children with developmental disabilities are at higher risk of being abused than their peers without developmental disabilities, discuss with caregivers how to educate children about safety precautions in a developmentally appropriate manner.