Handbook of Genetic Counseling/Developmental Delay and Mental Retardation

Developmental Delay and Mental Retardation

Etiology

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

It is a, medical and mental, developmental 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

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

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)

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.

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