Mental retardation

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Classification according to ICD-10
F70 Mild intellectual disability (IQ 50-69)
F71 Moderate intellectual disability (IQ 35-49)
F72 Severe intellectual disability (IQ 20–34)
F73 Severe intellectual disability (IQ <20)
F78 Other mental retardation
F79 Unspecified intellectual disability
ICD-10 online (WHO version 2019)

Intelligence is defined as the mental abilities associated with a person's cognitive capabilities in their potential and dynamic meaning. The average IQ is 100. In the 70–84 range one speaks of a learning disability. Reduced intelligence (comparatively less developmental progress) or talentedness (comparatively low level of development) is understood to mean a development of the mental abilities that is manifested, stalled or incomplete, with particular impairment of skills that contribute to the level of intelligence, such as cognition , language and motor skills as well as social skills.

Measurement of talentedness

The intellectual disability is measured with standardized intelligence tests and indicated in an intelligence quotient , the mean value of which is traditionally calibrated to 100 points with a standard deviation of 15 points. The first intelligence tests did not give a point value, but a development lag (or lead) in months. Over time, this method was abandoned and the one used today, as it only works with positive values. In addition to these cognitive tests, development tests are used in children , since intelligence tests are only of limited significance in this early phase of life.

Levels of intellectual disability

The WHO shares in ICD-10 (Chapter V F7) mental retardation as follows:

slight mental retardation
or mild intellectual disability , ICD 10 F70, IQ 50 to 69. Individuals with mild intellectual disabilities can achieve the level of primary school at the age of 18 to 19 years. In adults, this corresponds roughly to the intelligence age of a nine to twelve year old child. Prevalence : 2.5%. Low-level intellectual disabilities lead to difficulties in acquiring knowledge as well as in acting and thinking (caused by impaired concentration or poor memory ), limited interest and delayed intellectual maturity. Those affected are capable of schooling, but mostly only in special schools for people with learning disabilities. In addition to intellectual disability, there can be social and emotional immaturity, so that those affected cannot meet the demands of marriage or bringing up children on their own. The term used earlier was debility.
moderate intellectual disability
or moderate intellectual disability, ICD 10 F71, IQ 35 to 49, the adult intelligence age is six to nine years. Significant developmental delay in childhood. Special schools for the mentally handicapped offer adequate funding opportunities. As adults, with good support, they can work, read and write in a protected setting. Prevalence: 0.4%. Moderate and severe intellectual disabilities used to be known as imbecility .
severe mental retardation
or severe intellectual disability, ICD 10 F72, IQ 20 to 34, the adult intelligence age is three to less than six years. Since affected people cannot learn to read or write, they are not capable of schooling, but are usually eligible for support within the framework of a special school for the mentally handicapped (can be trained in a practical way). Continuous support is necessary. Prevalence: 0.3%. This type of intellectual disability was once called severe imbecility .
severe intellectual disability
or the most severe intellectual disability, ICD 10 F73, IQ below 20, the maximum achievable intelligence age for adults is below three years. Mobility , continence and language skills are severely restricted. Prevalence: 0.04%. This type of intellectual disability was also known as idiocy in the past .
mental handicap without further details
In the ICD-10 classification, the intellectual disability is also listed without further details F79. It should only be coded if z. B. because of physical disability or behavioral disorder no intelligence test can be performed.


A variety of causes can be considered for intellectual disability. A distinction is made between:

  • prenatal genetically chromosomal (the best known example is trisomy 21 ),
  • prenatal metabolic-related (e.g. phenylketonuria or galactosemia ),
  • Prenatal environmental. This can be viral (e.g. rubella ) or bacterial (e.g. listeriosis ) etiology, including intoxication (e.g. through medication or drugs such as alcohol; see also fetal alcohol syndrome ) and hypoxia (e.g. through maternal smoking during pregnancy) play a causal role.
  • Prenatal multifactorial causes are caused by organic brain developmental defects with subsequent epilepsies or hydrocephalus .
  • Perinatal causes are premature birth or birth trauma.
  • Postnatal causes can be etiologically caused by infections (e.g. meningitis or encephalitis ), endocrinological (e.g. hypothyroidism ), traumatic, neoplastic, dystrophy (e.g. as a result of a kwashiorkor ) or vaccine damage.
  • Low levels of vitamin D in the blood may be detrimental to brain performance . This is suggested by data from a US study with 858 participants over 65 years of age. In participants with low vitamin D 25 OH values ​​at the start of the study (below 25 nmol / l), the rate of cognitive impairment was 60 percent higher after six years than in participants with high values ​​(above 75 nmol / l) and around 31 percent higher than with sufficient initial values.

In the context of some syndromes , intellectual disability occurs, such as in Coffin-Siris syndrome .

In many cases, however, the causes are unknown and their parts cannot be separated.


Nowadays people with severe cognitive disabilities are often no longer placed in homes because this has led to symptoms such as hospitalism and societal segregation and exclusion. Assisted living or integrative therapy programs, for example, offer more opportunities and better social contacts .

Legal consequences

A cognitive disability can lead to impaired legal capacity or even legal incapacity. The incapacitation was abolished in Germany in 1992, the legal support introduced in its place has since then only related to individual matters. Cognitive deficiency does not lead to a loss of legal capacity.

See also

Individual evidence

  1. ICD 10
  2. U. Knölker, F. Mattejat, M. Schulte-Markwort : child and adolescent psychiatry and psychotherapy, systematically . Pp. 211-215.
  3. U. Knölker, F. Mattejat, M. Schulte-Markwort: child and adolescent psychiatry and psychotherapy, systematically . Pp. 217-224.
  4. Archives of Infernal Medicine (2010; 170: p. 1135), quoted from Ärzte Zeitung , July 14, 2010, p. 4.