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Developmental Delay and Mental Retardation


  • Cause unknown in about 50% of cases
    • Often multifactorial with genetic and environmental components
    • Low birth weight, prematurity, and perinatal complications may be associated - not known if they cause mental retardation or if factor causing these problems also caused MR
  • Approximately 2000 known genetic causes
    • Chromosomal abnormalities
    • Metabolic or endocrine disorders
    • Hereditary degenerative disorders
    • Hormonal deficiency
    • Hereditary syndromes or malformations
  • Acquired causes
    • Prenatal: infection, irradiation, or exposure to toxins
    • Perinatal: prematurity, anoxia, cerebral damage, or infection
    • Postnatal: brain injuries, anoxia, poisons, hormonal deficiencies, metabolic dysfunction, postimmunization encephalopathy, sociocultural, kernicterus, epilepsy
  • In United States, 1-3% of people meet cognitive and functional criteria

Clinical FeaturesEdit

  • Not a medical or mental disorder
  • Affects developmental and cognitive abilities
    • Substantial limitations in functioning
    • IQ scores below 70
      • Mild MR: IQ range 50-55 to 70
      • Moderate MR: IQ range 35-40 to 50-55
      • Severe MR: 20-25 to 35-40
      • Profound MR: Below 20-25
    • Majority of individuals with mental retardation have IQ scores of 55-69
      • Able to live independently or with support in group homes
      • Less than 10% of all people with mental retardation have severe to profound impairments
    • May have limited ability to develop some adaptive skills
      • Communication
      • Home living
      • Work
      • Self-care
      • Social/interpersonal skills
      • Self-direction
      • Functional academic skills
      • Leisure
      • Health and safety
      • Use of community resources
  • Can affect abilities in key developmental areas
    • Language development
    • Visual and auditory perception and discrimination
    • Abstract problem solving
  • Onset must occur before age 18

Management optionsEdit

  • No treatment or cure
  • Early intervention services
    • Provided by the county in Ohio to children between birth and age 3
    • Studies show the earliest experiences in learning sets the pattern for later information processing
    • Begins with comprehensive developmental assessment
      • May be performed here by CCDD or by private service
      • Assessment used to develop intervention strategy
  • After age 3, school system provides special services
    • Develop an Individualized Education Plan (IEP)
    • Early education focuses on cognitive development and special services such as speech therapy
    • Later education may focus on developing life skills
    • Can attend school until age 21
  • Adult services
    • Handled in Ohio by the Board of MRDD
    • Focuses on job training, vocational education
    • Community or group homes are available for semi-independent living

Recurrence RisksEdit

  • Can calculate a much more accurate risk if etiology is known
  • Other factors to consider:
    • Possibility of consanguinity
    • Whether one or both parents are affected
    • Developmental disabilities may be exacerbated by environmental factors
      • Unsafe or unstimulating home environment
      • Substandard health care
      • Unadequate schooling or lack of services
  • Empiric risk figures when parents affected with mental retardation of unknown etiology
    • 39.4% if both parents affected
    • 7.8% if only father affected
    • 19.6% if only mother affected (higher due to prevalence of X-linked inheritance for conditions such as Fragile X)

Psychosocial IssuesEdit

  • Provision of adequate services
  • Burden of taking care of a child/adolescent/adult with mental retardation
  • Impact on siblings and other family members
  • Denial, grief, disappointment, or feeling of loss
  • Interruption of career goals, family routines, or plans for the future
  • Financial and insurance issues


  • Milunsky, Aubry. Prevention of Genetic Disease and Mental Retardation. Philadelphia: W.B. Saunders Company (1975).


The information in this outline was last updated in 2001.

This material has been imported fom the wikibook "Genetic counseling"[] under the GNU Free Documentation License.

Heckert GNU white Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License."

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