Mental retardation (MR) is a symptom with multiple etiologies including chromosomal abnormalities, genetic defects, intrauterine, perinatal, neonatal, and postnatal causes. Mental retardation refers to substantial limitations in present functioning. It is characterized by significantly subaverage intellectual functioning, existing concurrently with related limitations in two or more of the following applicable adaptive skills areas: communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure and work. Cognitive and adaptive behavior deficits are manifested before age 18.

• Subgroups of mentally retarded:
  • Mild: IQ 55-69 (85%)
  • Moderate: IQ 40-54 (10%)
  • Severe: IQ 25-39 (5%)
  • Profound: IQ 0-24 (<1%)
• System(s) affected: Nervous
• Genetics:
  • Autosomes: Trisomies and rearrangements - approximately 1500 variations are all associated with MR
  • Sex chromosomes: 80 of 336 disorders cause MR
  • Autosomal dominant: 180 of 3,000 disorders cause MR
  • Autosomal recessive: 400 of 1,550 disorders cause MR
• Incidence/Prevalence in USA: Incidence and prevalence are closely related to social, economic, and health conditions of the society. The general incidence of mental retardation in the United States has been estimated at 125,000 births per year by the American Association on Mental Retardation. This would correspond to approximately 3% of the population. The research on both incidence and prevalence of mental retardation in the US is exceedingly scant. A comprehensive Canadian study of the maritime Provinces found prevalences of 3.65 per 1000. Most professionals associated with the American Association on Mental Retardation accept a prevalence of 2.5% and they recognize that the prevalence varies with chronological age. Specifically, mildly retarded preschoolers are able to meet societies demands but are identified by the school system due to demands for cognitive processing. Conversely, once they leave the requirements of the educational system they may adapt to societies demands and the diagnosis need not apply.
• Predominant age: By definition mental retardation occurs during the developmental period. Older adults who lose mental faculties are more accurately diagnosed as demented. Patients may present the MR after traumatic brain injury.
• Predominant sex: Male > Female (1.5:1)

• Profoundly and severely retarded children are frequently diagnosed at the time of birth or during the newborn period. Children with profound or severe retardation are more likely to have dysmorphic features.
• Moderately retarded children may go undiagnosed until they fail to meet normal developmental milestones.
• Mildly retarded children may go undiagnosed until well into the school years.

• Chromosomal abnormalities
  • Autosomal abnormalities
  • Trisomy, e.g., Down syndrome
  • Translocations
  • Inversions
  • Duplications
  • Deletions, e.g., Prader-Willi
• Sex chromosome abnormalities
  • Fragile X mutation
  • Turner syndrome
  • Klinefelter syndrome
  • Rett syndrome
  • Various multiple X and/or Y conditions
• 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
    • Gaucher
    • Niemann-Pick, e.g., mucopolysaccharidosis
  • Purine metabolism, e.g., Lesch-Nyhan
  • Other, e.g., Wilson
• Multifactorial and sporadic conditions
  • Cornelia de Lange syndrome
  • Spinal cord disorders (spina bifida, Arnold-Chiari malformation)
  • Disorders of brain and skull
  • Prenatal factors
  • Rh incompatibility
  • Maternal infections - all the TORCH viruses: rubella, toxoplasmosis, cytomegalic virus, and herpes simplex
  • Maternal diseases, e.g., diabetes mellitus, toxemia
  • Maternal substance abuse, e.g., alcohol use/abuse. Fetal alcohol syndrome is a leading environmental cause of mental retardation.
  • Prescription medications, such as Accutane or Dilantin
• Perinatal factors
  • Prematurity
  • Postmaturity
  • Birth injuries
  • High risk mothers
• Postnatal factors
  • Childhood diseases, e.g., meningitis, encephalitis, general inflammatory disease with high fever, hypothyroidism
  • Trauma, e.g., accidents, physical abuse, marked deprivation
  • Poisoning, e.g., lead, carbon monoxide, household products

Risk factors for future offspring of the parent couple must be calculated based upon the specific etiology of related retarded individuals