Mental Retardation- Causes, Symptoms, Diagnosis, Treatment and Ongoing care


  • Mental retardation (MR) is a global deficit in cognitive functioning, as evidenced by a significant difference between one’s mental and chronological ages (AKA “intelligence quotient” or “IQ”) and significantly impaired adaptive functioning (1).
  • Although these cognitive issues typically have a pervasive impact, patients with MR 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 current DSM-IV-TR diagnosis is “Mental Retardation.” However, since that term is deemed to be somewhat pejorative, the new DSM-V diagnosis will be “Intellectual Disability.”


  • MR is defined as an IQ ≤70 and a significant impairment in an area of adaptive functioning such as communication, self-care, activities of daily living, socialization, use of community and resources, or health/safety (1).
  • These issues are present prior to the age of 18.
  • Currently, MR is subgrouped according to IQ level [mild (IQ ∼55–69), moderate (∼40–54), severe (∼25–39), and profound (∼0–24)]. A diagnosis of “Mental Retardation, severity unspecified” may be used for individuals who are unable to undergo formal assessment.
  • The 3 most common causes of MR are Down syndrome, Fragile X syndrome, and fetal alcohol syndrome (FAS).
  • Synonym(s): Intellectual disability; Cognitive disability


For some causes of MR, prenatal testing is available.



  • 1/63–1/83 (in two surveillance studies in Atlanta, in 1996 and 2000) (2)
  • By definition, MR begins in childhood and must be diagnosed prior to the age of 18.
  • Predominant sex: Male > Female; 2:1 for mild MR, 1.5:1 for severe MR.


In the US, 1–2.5% of the population

Risk Factors

  • Maternal substance abuse during pregnancy
  • Maternal infection during pregnancy
  • For some causes, family history
  • Mild MR is more common in children of women who did not complete high school; likely related to genetic and socioeconomic factors (e.g., nutritional deficiencies, poverty).


A number of genetic and epigenetic causes are known, and more are under investigation (3).

General Prevention

  • Public health efforts to reduce alcohol and drug use by pregnant women
  • Prenatal folic acid supplementation


  • The cause of mild MR is identified in <50% of cases. The cause of severe MR is identified in >75% of cases.
  • Causes:
    • Maternal substance abuse (e.g., alcohol); FAS is a leading environmental cause of MR.
    • 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)
    • Trauma (e.g., accidents, physical abuse)
    • Severe deprivation
    • Poisoning (e.g., lead, carbon monoxide, household products)

Commonly Associated Conditions

  • Seizures
  • Mood disorders
  • Behavioral disorders

Intelligence quotient, Intellectual disability, fragile x syndrome, maternal substance abuse, cognitive disability, prenatal testing,


A diagnosis of MR/intellectual disability should only be made through a psychodiagnostic assessment conducted by a mental health provider trained and licensed to conduct formal psychological testing.


  • Three-generation family history
  • Children with profound or severe MR typically are diagnosed at birth or during the newborn period and are more likely to have dysmorphic features.
  • Children with MR often are identified because they fail to meet motor or language milestones.

Physical Exam

Careful examination by a physician trained in the assessment of morphologic features suggestive of a specific etiology for MR (e.g., microcephaly) (4)

Diagnostic Tests & Interpretation

  • Visual and hearing tests to rule out these etiologies as a cause of impairment and provide an assessment of visual and auditory functioning, which often are impaired in children and adults with MR
  • 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 or limited expressive or 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, Weschler Intelligence Scales) are determined by age/developmental level of the child.
    • Common tests of adaptive functioning include the Vineland Adaptive Behavior Scales Second Edition and Adaptive Behavior Assessment System Second Edition. These tests assess areas of functioning, such as age-appropriate communication, social skills, activities of daily living, and motor skills.


  • Metabolic screening not routine unless evidence in history and physical, or no newborn screening records (5)[B]
  • Lead as per current targeted guidelines (5)[B]
  • Thyroid stimulating hormone (TSH) if systemic features present or no newborn screening (5)[B]
  • Routine cytogenetic testing (karyotype) (5)[B]
    • Fragile X screening (FMR1 gene), particularly if family history of intellectual disability (5)[B]
    • Rett syndrome (MECP2 gene) in females with unexplained moderate-to-severe intellectual disability (5)[B]
  • Molecular screening such as array comparative genomic hybridization (aCGH) is used increasingly and may yield diagnosis in 10% 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 MR (4).

Follow-Up & Special Considerations

EEG not routine unless presence of epilepsy or a specific epileptiform syndrome (5)[C]

Differential Diagnosis

  • Brain tumors
  • Auditory, visual, and/or speech/language impairment
  • Autistic 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 MR.
  • Cerebral palsy
  • Emotional or behavioral disturbance
  • Lack of environmental opportunities for appropriate development


  • Early intervention services tailored to the individual’s specific needs
  • Caregiver support, including:
    • Training parent(s) to address behavioral issues 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 parent(s) with an opportunity to address their reactions to the diagnosis and their child’s special needs
  • Individualized education programs and, depending on the level of impairment, social skills and behavioral plans/training
  • Referral to job training programs and independent living opportunities if appropriate
  • Taking notice of all changes in behavior, which may be indicative of pain or illness, particularly in individuals with limited communication skills


Support's development and hosting

Medication may be appropriate for comorbid conditions (e.g., anxiety, ADHD, depression).

Ongoing Care

The physician should match his/her communication of exam procedures, test results, and treatment recommendations to the patient’s level of cognitive functioning and communication.

  • The vast majority of patients with MR will fall within the mild range and are fully capable of understanding information if it is provided at the appropriate level.
  • Provide explanations directly to the patient, instead of solely to his/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/her intellectual disability.

Follow-Up Recommendations

  • Many adults and children with MR exhibit poor physical fitness. Preliminary studies suggest that structured exercise programs are effective to engage this population in healthy activities (6)[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 (age 30 in Down syndrome) and every 2 years thereafter (7)[B]
  • Hearing evaluations every 5 years after age 45 (every 3 years throughout life in Down syndrome) (7)[B]
  • Screen for sexual activity and offer contraception and testing for sexually transmitted infections (7)[B].
  • Abuse and neglect of people with MR are common. Screen at least annually and assess for abuse if behavior change noted. Report abuse or neglect to appropriate protective agencies (7)[B].
  • Dysphagia and aspiration common; consider speech pathology evaluation and swallowing study (8)[B].
  • Monitor for and treat constipation (8)[B].
  • Osteoporosis is common; low threshold to order imaging studies after traumatic injury (8)[B].


No restrictions, except in case of metabolic and storage disorders (e.g., phenylketonuria)

Patient Education

  • Families should be referred to the local Association for Retarded Citizens: http://www.thearc.or
  • American Association of Intellectual and Developmental Disabilities:
  • Refer to local family support group (e.g., Parent To Parent, local Down Syndrome Association, local Autism Association)


Although MR is a lifelong diagnosis, individuals with MR 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.


1. American Psychiatric Association (2000). Diagnostic and statistical manual of mental disorders: Fourth edition, text revision. Arlington, VA: American Psychiatric Association.


3. Grant ME. The epigenetic origins of mental retardation. Clin Genet. 2008;73:528–30.

4. van Karnebeek CDM, Jansweijer MCE, Offringa M. Diagnostic investigations in individuals with mental retardation: A systematic literature review of their usefulness. Eur J Human Genet. 2005;13:6–25.

5. Shevell M, Ashwal S, Donley D, Flint J, Gingold M, Hirtz D, Majnemer A, Noetzel M, Sheth RD, Quality Standards Subcommittee of the American Academy of Neurology, Practice Committee of the Child Neurology Society 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;60:367–80.

6. 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;109:175–85.

7. Sullivan WF, Heng J, Cameron D, et al. Consensus guidelines for primary health care of adults with developmental disabilities. Can Fam Physician. 2006;52:1410–8.

8. Prater CD, Zylstra RG. Medical care of adults with mental retardation. Am Fam Physician. 2006;73:2175–83.

Additional Reading

Moeschler JB, Shevell M, American Academy of Pediatrics Committee on Genetics. Clinical genetic evaluation of the child with mental retardation or developmental delays. Pediatrics. 2006;117:2304–16.

See Also (Topic, Algorithm, Electronic Media Element)

ADD/ADHD; Cerebral Palsy; Down Syndrome; Fragile X Syndrome; Lead Poisoning;

Algorithm: Mental Retardation



  • 317 Mild mental retardation
  • 318.0 Moderate mental retardation
  • 318.1 Severe mental retardation
  • 318.2 Profound mental retardation
  • 319 Unspecified mental retardation


  • 91138005 mental retardation (disorder)
  • 86765009 mild mental retardation (I.Q. 50–70) (disorder)
  • 61152003 moderate mental retardation (I.Q. 35–49) (disorder)
  • 40700009 severe mental retardation (I.Q. 20–34) (disorder)
  • 31216003 profound mental retardation (I.Q. below 20) (disorder)

Clinical Pearls

  • The term “mental retardation” may be interpreted as highly limited, culturally insensitive, and disrespectful to patients and their caregivers. “Intellectual disability” is the preferred term.
  • Overall functioning with MR 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 MR (e.g., always happy, poor prognosis, unable to function independently) have been refuted. People with MR are showing a level of functioning variability that parallels what is found in the non-MR population.
  • Be aware of the unique parenting needs that caregivers may face. Link families to community resources that can provide practical and emotional support when appropriate.
  • Since 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.

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