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Mental retardation is a condition in which people have substantial limitations in their intellectual abilities. People with mental retardation score significantly below average on tests designed to measure intelligence. As a result of their intellectual limits, people with retardation experience difficulties in daily activities such as learning, working, and caring for themselves. They also have difficulty with such social skills as understanding other people's behavior or communicating thoughts and feelings.
To be considered mentally retarded, people must show signs of the condition before they reach 18 years of age. The most common symptom of retardation is a delay in achieving milestones of development. Many children with severe mental retardation fail to sit up or walk at the usual age for these accomplishments. Children with less severe retardation may be slow in learning to talk. Mild retardation may escape detection until a child starts school and has trouble learning.
Physicians and social workers once advised parents of children with mental retardation to place them in residential institutions. Experts now believe that living in a community is a better arrangement for all but the most seriously retarded. People with retardation and their families need varying levels of support services to help them live successfully in a community.
Diagnosis of mental retardation first requires determining that a person has a serious intellectual impairment. The best available--but still imperfect and inexact--means for measuring intelligence is a test called an IQ test. IQ tests assign an approximate numerical value to intelligence, and people of average ability score from about 90 to 110. People with mental retardation score below about 70 to 75.
No IQ test score alone is a sufficient basis for a diagnosis of mental retardation. The score must be confirmed by expert evaluation of an individual's ability to perform daily activities and to function in everyday situations. There are four broad categories of mental retardation based on approximate IQ: (1) mild, (2) moderate, (3) severe, and (4) profound.
People with mild retardation form the largest group. These people have IQ's from about the low 50's to the low 70's. Many people with mild retardation can master reading and other schoolwork up to about year six level. Most attend some special education classes and some regular classes. Many mildly retarded adults with good social skills can live with minimal supervision and work at jobs suited to their abilities.
People with moderate retardation have IQ's from about 40 to the low 50's. Educational goals for moderately retarded children focus on teaching them to care for themselves and other practical skills. Some can learn a little reading and writing. In some cases, adults with moderate retardation can work at suitable jobs at home or in special workshops.
People with severe retardation have IQ's from about the mid-20's to the high 30's. Training for the severely retarded emphasizes learning to care for themselves and developing language and social skills. Severely retarded individuals require close supervision throughout life.
People with profound retardation have IQ's below about 25. They remain at about the mental age of a baby or toddler. Their abilities to talk or care for themselves are usually limited. Most profoundly retarded people need constant care to survive.
Causes. Mental retardation can result from any factor that hinders healthy brain development. In many cases, doctors cannot identify any single cause for the condition. Factors involved in mental retardation fall into the broad categories of: (1) genetic and (2) environmental.
Genetic factors involve faulty chemical instructions in genes, the hereditary material in every cell. In human cells, genes are carried on 46 microscopic threads called chromosomes that are arranged in 23 pairs. Scientists have made great progress in learning how genes function. Some genetic errors cause mental retardation. In some cases, such errors are inherited. Others occur spontaneously for unknown reasons.
One common genetic cause of retardation is a disorder called Down syndrome, in which people have an entire extra chromosome, for a total of 47. Another common genetic factor is called fragile-X syndrome. This condition involves an abnormality in the X chromosome, one of the chromosomes that determines a person's sex.
Many less common genetic factors affect genes that control a particular chemical pathway in the body. One such condition is phenylketonuria, often abbreviated PKU. People with PKU cannot process one of the subunits called amino acids in proteins. This amino acid builds up and leads to mental retardation if their diet is not controlled.
Environmental factors include a wide variety of influences that can affect brain development before, during, or after birth. During pregnancy, a woman's general health and nutrition greatly affect her unborn baby. Consumption of alcohol during pregnancy can lead to a condition in the baby called fetal alcohol syndrome. This syndrome causes many serious health problems, including mental retardation. Use of prescription, nonprescription, or illegal drugs can also harm a developing baby's brain. Exposure before birth to certain infections, including rubella (also called German measles) and HIV, can also cause retardation.
Events at birth can also harm the brain. Premature birth, injury during delivery, and failure of the newborn to breathe properly can all lead to retardation. During childhood, retardation can result from such causes as brain infections, head injuries, prolonged high fevers, or lead poisoning.
Prevention. Good health habits and professional medical care during pregnancy can prevent many cases of mental retardation. Skilled delivery and care of sick or premature infants also help avoid some cases. Damage resulting from PKU and a few other disorders can be controlled after birth by a special diet or medication.
Some couples know that such factors as their age, family history, or ethnic background increase their risk of having children with certain genetic conditions. In some cases, genetic tests are available to determine if the parents or the unborn baby carry damaged genes. Some couples decide to have such tests and take the results into account in planning or continuing a pregnancy.
Treatment of mental retardation focuses on identifying each individual's strengths and limitations and devising approaches that encourage the greatest possible intellectual and social development. One of the key parts of treatment consists of appropriate education or training. In many countries, it is the legal right of every school-age child, no matter how seriously retarded, to have the opportunity to learn to function to the best of his or her ability. In some cases, training can begin in infancy. It may continue until the individual is well established in an adult role.
As children with retardation grow up, their education tends to centre increasingly on the skills they will need as adults. Many mildly retarded adults are able to work and live with some independence. Mildly retarded adults who cannot work independently and some moderately to severely retarded adults may work in a sheltered workshop, a centre that employs people with disabilities. Most such individuals live with their families or in group homes in the community. Only the most severely retarded, who require constant care, are likely to live permanently in institutions.
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