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Mental Retardation
Mental retardation is defined by the American Association on Intellectual and Developmental Disabilities as “a disability characterized by significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills” that originates before age 18 years.

From: Adolescent Medicine, 2008

Related terms:
FMR1Down SyndromeNested GenePrevalencePhenotypeMutationAutosomal Recessive Inheritance
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Learn more about Mental Retardation
Mental Retardation
Susan L. Hyman MD, in Pediatric Clinical Advisor (Second Edition), 2007

Basic Information
Definition
Mental retardation is cognitive limitation as characterized by scores greater than 2 standard deviations below the mean on a valid intelligence quotient (IQ) measure, with limitation of adaptive function in communication, self‐care, daily living skills at home or in the community, or social skills.

Synonyms

Cognitive limitation

Developmental delay (global)

Learning disability (in Europe, not in United States)

Slow learner

ICD‐9‐CM Codes

317 Mild mental retardation

318.0 Moderate mental retardation

318.1 Severe mental retardation

318 Profound mental retardation

319 Mental retardation, unspecified

783.4 Global delays

Epidemiology & Demographics

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The incidence is 1% of the population, with a male preponderance.

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Most affected individuals have mild mental retardation (IQ of 50 to 70).

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Moderate mental retardation is defined by a tested IQ of between 35 to 40 and 50 to 55.

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Severe mental retardation is defined by a tested IQ of between 20 to 25 and 35 to 40.

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Profound mental retardation is defined by a tested IQ of less than 20 to 25.

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Approximately 70% of cases of severe to profound mental retardation have a known cause.

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About 24% of cases with an IQ in the 50 to 70 range have a specific medical cause identified.

Clinical Presentation

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Relative preservation of motor skills may delay diagnosis.

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Initial manifestation in early childhood may be language delay.

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Medical and family histories help guide the workup (see “Etiology”).

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Careful physical examination with attention to the following:

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Skin examination to rule out neurocutaneous syndromes

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Hearing and vision assessment

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Motor examination

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Head circumference: evaluation for large or small heads

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Syndrome stigmata: examination should include dysmorphic features to allow diagnosis of specific syndromes associated with mental retardation

Etiology

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A cause has been identified for 70% of individuals with severe or profound mental retardation and for 24% with mild mental retardation.

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Embryologic causes

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Microcephaly

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Early decreased cell proliferation (e.g., genetic, embryologic origin)

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Prenatal events with disruption of architecture (e.g., viral infections, vascular insults, migrational errors)

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Early perinatal events (e.g., hypoxic encephalopathy, intracranial bleeding)

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Macrocephaly

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Hydrocephalus

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Sotos' syndrome

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Fragile X syndrome

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Autism

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Chronic subdural bleeding

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Genetic causes

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Fragile X syndrome, also called X‐linked mental retardation, causes developmental delays in male maternal relatives and is the most common inherited form of mental retardation.

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Trisomy 21, also known as Down syndrome, is the most common genetic cause of mental retardation!