In general, people with intellectual disabilities have an IQ below 70, but clinical discretion may be necessary for individuals who have a somewhat higher IQ but severe impairment in adaptive functioning.It is formally diagnosed by an assessment of IQ and adaptive behavior. A third condition requiring onset during the developmental period is used to distinguish intellectual disability from other conditions, such as traumatic brain injuries and dementias (including Alzheimers disease). Intelligence quotient
The first English-language IQ test, the Stanford–Binet Intelligence Scales, was adapted from a test battery designed for school placement by Alfred Binet in France. Lewis Terman adapted Binets test and promoted it as a test measuring "general intelligence". Termans test was the first widely used mental test to report scores in "intelligence quotient" form ("mental age" divided by chronological age, multiplied by 100). Current tests are scored in "deviation IQ" form, with a performance level by a test-taker two standard deviations below the median score for the test-takers age group defined as IQ 70. Until the most recent revision of diagnostic standards, an IQ of 70 or below was a primary factor for intellectual disability diagnosis, and IQ scores were used to categorize degrees of intellectual disability. Since the current diagnosis of intellectual disability is not based on IQ scores alone, but must also take into consideration a persons adaptive functioning, the diagnosis is not made rigidly.
The discovery that trinucleotide repeats could expand during intergenerational transmission and could cause disease was the first evidence that not all disease-causing mutations are stably transmitted from parent to offspring.There are several known categories of trinucleotide repeat disorder. Category I includes Huntingtons disease (HD) and the spinocerebellar ataxias. These are caused by a CAG repeat expansion in protein-coding portions, or exons, of specific genes. Category II expansions are also found in exons, and tend to be more phenotypically diverse with heterogeneous expansions that are generally small in magnitude. Category III includes fragile X syndrome, myotonic dystrophy, two of the spinocerebellar ataxias, juvenile myoclonic epilepsy, and Friedreichs ataxia. These diseases are characterized by typically much larger repeat expansions than the first two groups, and the repeats are located in introns rather than exons. Types
Some of the problems in trinucleotide repeat syndromes result from causing alterations in the coding region of the gene, while others are caused by altered gene regulation. In over half of these disorders, the repeated trinucleotide, or codon, is CAG. In a coding region, CAG codes for glutamine (Q), so CAG repeats result in a polyglutamine tract. These diseases are commonly referred to as polyglutamine (or polyQ) diseases. The repeated codons in the remaining disorders do not code for glutamine, and these are classified as non-polyglutamine diseases. Polyglutamine (PolyQ) diseases
Non-polyglutamine diseases
Symptoms and signs
A common symptom of polyQ diseases is the progressive degeneration of nerve cells, usually affecting people later in life.