Intellectual Disability (ID), also called intellectual developmental disability (IDD), general learning disability, or mental retardation (MR), is a generalized neurodevelopment disorder characterized by significantly impaired intellectual and adaptive functioning. It is defined by an IQ score under 70 in addition to deficits in two or more adaptive behaviours that affect every day, general living. Once focused almost entirely on cognition, the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environments. As a result of this focus on the person's abilities in practice, a person with an unusually low IQ may not be considered intellectually disabled. Intellectual disability is subdivided into syndrome intellectual disability, in which intellectual deficits associated with other medical and behavioural signs and symptoms are present, and non-syndromic intellectual disability, in which intellectual deficits appear without other abnormalities. Down syndrome and fragile X syndrome are examples of syndromic intellectual disabilities.
Intellectual disability affects about 2–3% of the general population. 75–90% of the affected people have mild intellectual disability. Non-syndromic or idiopathic cases account for 30–50% of cases. About a quarter of cases are caused by a genetic disorder. Cases of unknown cause affect about 95 million people as of 2013.
The signs and symptoms of intellectual disability are all behavioural. Most people with intellectual disability do not look like they are afflicted with such, especially if the disability is caused by environmental factors such as malnutrition or lead poisoning. The so-called typical appearance ascribed to people with intellectual disability is only present in a minority of cases, all of which are syndromic.
Children with intellectual disability may learn to sit up, to crawl, or to walk later than other children, or they may learn to talk later. Both adults and children with intellectual disability may also exhibit some or all of the following characteristics:
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Delays in oral language development
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Deficits in memory skills
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Difficulty learning social rules
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Difficulty with problem solving skills
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Delays in the development of adaptive behaviors such as self-help or self-care skills
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Lack of social inhibitors
Children with intellectual disability learn more slowly than a typical child. Children may take longer to learn language, develop social skills, and take care of their personal needs, such as dressing or eating. Learning will take them longer, require more repetition, and skills may need to be adapted to their learning levels. Nevertheless, virtually every child is able to learn, develop and become a participating member of the community.
Among children, the cause is unknown for one-third to one-half of cases. Down syndrome, velocariofacial syndrome, and fetal alcohol spectrum disorders are the three most common inborn causes. However, doctors have found many other causes. The most common are:
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Genetic conditions. Sometimes disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons. The most prevalent genetic conditions include Down syndrome, Klinefelter's syndrome, Fragile X syndrome (common among boys), neurofibromatosis, congenital hypothyroidism, Williams’s syndrome, phenylketonuria (PKU), and Prader-Willi syndrome. Other genetic conditions include Phelan-McDermid syndrome (22q13del), Mowat-Wilson syndrome, genetic ciliopathy, and Siderius typeX-linked intellectual disability (OMIM 300263) as caused by mutations in the PHF8 gene (OMIM 300560). In the rarest of cases, abnormalities with the X or Y chromosome may also cause disability. 48, XXXX and 49, XXXXX syndrome affects a small number of girls worldwide, while boys may be affected by 47, XYY, 49, XXXXY, or 49, XYYYY.
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Problems during pregnancy. Intellectual disability can result when the fetus does not develop properly. For example, there may be a problem with the way the fetus' cells divide as it grows. A pregnant person who drinks alcohol (see fetal alcohol spectrum disorder) or gets an infection like rubella during pregnancy may also have a baby with intellectual disability.
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Problems at birth. If a baby has problems during labor and birth, such as not getting enough oxygen, he or she may have developmental disability due to brain damage.
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Exposure to certain types of disease or toxins. Diseases like whooping cough, measles, or meningitis can cause intellectual disability if medical care is delayed or inadequate. Exposure to poisons like lead or mercury may also affect mental ability.
