Intellectual disability is a broad concept encompassing various intellectual deficits, including mental retardation (MR), deficits too mild to properly qualify as MR, various specific conditions (such as specific learning disability), and problems acquired later in life through acquired brain injuries or neurodegenerative diseases like dementia. Intellectual disabilities may appear at any age.
Intellectual disability is also increasingly being used as a synonym for the term mental retardation as used in standard medical references.[note 1]
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Mental retardation is a generalized disorder characterized by significantly impaired cognitive functioning and deficits in two or more adaptive behaviors that appears before adulthood. It has historically been defined as an Intelligence Quotient (IQ) score under 70, but the definition now includes both a component relating to mental functioning and one relating to individuals' functional skills in their environment, so IQ is not the only factor.
Although the clinical term[note 1] mental retardation is a subtype of intellectual disability the latter is now preferred by most advocates in most English-speaking countries as a euphemism for mental retardation and is replacing it in United States official documents following the passing of Rosa's Law on October 6, 2010.[1] Some bodies are using developmental disability[2] which also has an established wider meaning.
By contrast, people with cognitive impairment have, or had, normal IQ, but show confusion, forgetfulness and difficulty concentrating; cognitive impairment is typical of brain injuries, side effects from medications, and dementia.
Specific learning disability is a classification including several disorders in which a person has difficulty learning in a typical manner, usually caused by an unknown factor or factors. This is not indicative of intelligence level. Rather, people with a learning disability have trouble performing specific types of skills or completing tasks if left to figure things out by themselves or if taught in conventional ways. Dyslexia is a well known example of such disorders. A learning disability cannot be cured or fixed. Individuals with learning disabilities can face unique challenges that are often pervasive throughout the lifespan. Social support can be a crucial component for students with learning disabilities in the school system. With the right support and intervention, people with learning disabilities can succeed in school and be successful later in life.
An acquired brain injury (ABI) is brain damage caused by events after birth, rather than as part of a genetic or congenital disorder. It usually affects cognitive, physical, emotional, social or independent functioning and can result from either traumatic brain injury or nontraumatic injury such as stroke, infection or substance abuse. Most definitions of ABI exclude neurodegenerative disorders.
People with a brain injury may have difficulty controlling, coordinating and communicating their thoughts and actions, but they retain their intellectual abilities. However, the intellectual abilities of a person with a brain injury are likely to be interfered with by the resulting thought coordination and communication difficulties, which can make it difficult for them to express themselves in a manner intelligible to others. This may give the impression of a damaged intelligence, even though such is not the case.
Neurodegeneration is the umbrella term for the progressive loss of structure or function of neurons, including death of neurons. Many neurodegenerative diseases including Parkinson’s, Alzheimer’s, and Huntington’s occur as a result of neurodegenerative processes. As research progresses, many similarities appear which relate these diseases to one another on a sub-cellular level. Discovering these similarities offers hope for therapeutic advances that could ameliorate many diseases simultaneously.
Dementia is a serious loss of cognitive ability in a previously unimpaired person, beyond what might be expected from normal aging. Both dementia and mental retardation are defined by neurologists as having an IQ that is two standard deviations below median (below about 70, when 100 is the median); the difference between these two classifications for intellectual disability is whether the low IQ represents a lifelong condition (mental retardation), or a condition that is acquired later (dementia).[3]
Dementia may be static, the result of a unique global brain injury, or progressive, resulting in long-term decline due to damage or disease in the body. In the early stages of Alzheimer's disease, whose symptoms of dementia are called mild cognitive impairment, the person typically loses 8 to 10 IQ points per year, with the result that a person of previously normal intelligence usually becomes intellectually disabled in less than five years.[4]
Although dementia is far more common in the geriatric population, it may occur in any stage of adulthood. The overwhelming factor emerging from genetic studies of the dementias and other central nervous system neurodegenerative conditions is abnormalities of protein handling.
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