Mental retardation is a developmental disability that first appears in
children under the age of 18. It is defined as an intellectual functioning
level (as measured by standard tests for
quotient) that is well below average and significant limitations in daily
living skills (adaptive functioning).
According to statistics made available by the Centers for Disease Control
and Prevention in the 1990s, mental retardation occurs in 2.5 to 3 percent
of the general population. About 6 to 7.5 million mentally retarded
individuals live in the United States alone. Mental retardation begins in
before the age of 18. In most cases, it persists throughout adulthood. A
diagnosis of mental retardation is made if an individual has an
intellectual functioning level well below average and significant
limitations in two or more adaptive skill areas. Intellectual functioning
level is defined by standardized tests that measure the ability to reason
in terms of mental age (intelligence quotient or IQ). Mental retardation
is defined as IQ score below 70 to 75. Adaptive skills are the skills
needed for daily life. Such skills include the ability to produce and
understand language (communication); home-living skills; use of community
, leisure, self-care, and social skills; self-direction; functional
academic skills (reading, writing, and arithmetic); and work skills.
In general, mentally retarded children reach developmental milestones such
as walking and talking much later than the general population. Symptoms of
mental retardation may appear at birth or later in childhood. Time of
onset depends on the suspected cause of the disability. Some cases of mild
mental retardation are not diagnosed before the child enters
. These children typically have difficulties with social, communication,
and functional academic skills. Children who have a neurological disorder
or illness such as
may suddenly show signs of cognitive impairment and adaptive
Mental retardation varies in severity. There are four different degrees of
mental retardation: mild, moderate, severe, and profound. These categories
are based on the functioning level of the individual.
Approximately 85 percent of the mentally retarded population is in the
mildly retarded category. Their IQ score ranges from 50 to 75, and they
can often acquire academic skills up to the sixth grade level. They can
become fairly self-sufficient and in some cases live independently, with
community and social support.
About 10 percent of the mentally retarded population is considered
moderately retarded. Moderately retarded individuals have IQ scores
ranging from 35 to 55. They can carry out work and self-care tasks with
moderate supervision. They typically acquire
in childhood and are able to live and function successfully within the
community in a supervised environment such as a group home.
About 3 to 4 percent of the mentally retarded population is severely
retarded. Severely retarded individuals have IQ scores of 20 to 40. They
may master very basic self-care skills and some communication skills. Many
severely retarded individuals are able to live in a group home.
Only 1 to 2 percent of the mentally retarded population is classified as
profoundly retarded. Profoundly retarded individuals have IQ scores under
20 to 25. They may be able to develop basic self-care and communication
skills with appropriate support and training. Their retardation is often
caused by an accompanying neurological disorder. The profoundly retarded
need a high level of structure and supervision.
The American Association on Mental Retardation (AAMR) has developed
another widely accepted diagnostic classification system for mental
retardation. The AAMR classification system focuses on the capabilities of
the retarded individual rather than on the limitations. The categories
describe the level of support required. They are: intermittent support,
limited support, extensive support, and pervasive support. Intermittent
support, for example, is support needed only occasionally, perhaps during
times of stress or crisis. It is the type of support typically required
for most mildly retarded individuals. At the other end of the spectrum,
pervasive support, or life-long, daily support for most adaptive areas,
would be required for profoundly retarded individuals.
For children, the mental retardation rate is 11.4 per 1,000 and varies
approximately nine fold, ranging from 3.2 in New Jersey to 31.4 in
Alabama. For adults, the rate is 6.6 and varies approximately six fold,
ranging from 2.5 in Alaska to 15.7 in West Virginia. In 42 states, the
rate for children is higher than that for adults; in seven states, the
rate for adults is higher, and in two states, both rates are similar. The
correlation between state-specific rates for children and for adults is
0.66. Overall, 69 percent of the state-specific variation in prevalence
rates for adults is accounted for by median household income, the
percentage of total births to teenaged mothers, and the percentage of the
population with less than a ninth-grade education. Low educational
attainment was the most important correlate of mental retardation rates
Causes and symptoms
Low IQ scores and limitations in adaptive skills are the hallmarks of
mental retardation. Aggression, self-injury, and
are sometimes associated with the disability. The severity of the
symptoms and the age at which they first appear depend on the cause.
Children who are mentally retarded reach developmental milestones
significantly later than expected, if at all. If retardation is caused by
chromosomal or other genetic disorders, it is often apparent from infancy.
If retardation is caused by childhood illnesses or injuries, learning and
adaptive skills that were once easy may suddenly become difficult or
impossible to master. In about 35 percent of cases, the cause of mental
retardation cannot be found. Biological and environmental factors that can
cause mental retardation include genetics, prenatal illnesses and issues,
childhood illnesses and injuries, and environmental factors.
About 5 percent of mental retardation is caused by hereditary factors.
Mental retardation may be caused by an inherited abnormality of the genes,
fragile X syndrome
. Fragile X, a defect in the chromosome that determines sex, is the most
common inherited cause of mental retardation. Single gene defects such as
(PKU) and other inborn errors of metabolism may also cause mental
retardation if they are not found and treated early. An accident or
mutation in genetic
development may also cause retardation. Examples of such accidents are
development of an extra chromosome 18 (trisomy 18) and
. Down syndrome is caused by an abnormality in the development of
chromosome 21. It is the most common genetic cause of mental retardation.
Fetal alcohol syndrome
affects one in 600 children in the United States. It is caused by
excessive alcohol intake in the first twelve weeks (trimester) of
pregnancy. Some studies have shown that even moderate alcohol use during
pregnancy may cause learning disabilities in children. Drug abuse and
during pregnancy have also been linked to mental retardation.
Maternal infections and illnesses such as glandular disorders,
may cause mental retardation. When the mother has high blood pressure (
) or blood poisoning (toxemia), the flow of oxygen to the fetus may be
reduced, causing brain damage and mental retardation.
Birth defects that cause physical deformities of the head, brain, and
central nervous system frequently cause mental retardation. Neural tube
defect, for example, is a birth defect in which the neural tube that forms
the spinal cord does not close completely. This defect may cause children
to develop an accumulation of cerebrospinal fluid on the brain (
). By putting pressure on the brain hydrocephalus can cause learning
Hyperthyroidism, whooping cough, chickenpox, measles
, and Hib disease (a bacterial infection) may cause mental retardation if
they are not treated adequately. An infection of the membrane covering the
brain (meningitis) or an inflammation of the brain itself (encephalitis)
cause swelling that in turn may cause brain damage and mental retardation.
Traumatic brain injury caused by a blow or a violent shake to the head may
also cause brain damage and mental retardation in children.
Ignored or neglected infants who are not provided the mental and physical
stimulation required for normal development may suffer irreversible
learning impairments. Children who live in poverty and suffer from
, unhealthy living conditions, and improper or inadequate medical care are
at a higher risk. Exposure to lead can also cause mental retardation. Many
by eating the flaking lead-based paint often found in older buildings.
If mental retardation is suspected, a comprehensive physical examination
and medical history should be done immediately to discover any organic
cause of symptoms. Conditions such as hyperthyroidism and PKU are
treatable. If these conditions are discovered early, the progression of
retardation can be stopped and, in some cases, partially reversed. If a
neurological cause such as brain injury is suspected, the child may be
referred to a neurologist or neuropsychologist for testing.
The symptoms of mental retardation are usually evident by a child's
first or second year. In the case of Down syndrome, which involves
distinctive physical characteristics, a diagnosis can usually be made
shortly after birth. Mentally retarded children lag behind their peers in
developmental milestones such as smiling, sitting up, walking, and
talking. They often demonstrate lower than normal levels of interest in
their environment and responsiveness to others, and they are slower than
other children in reacting to visual or auditory stimulation. By the time
a child reaches the age of two or three, retardation can be determined
using physical and
. Testing is important at this age if a child shows signs of possible
retardation because alternate causes, such as impaired hearing, may be
found and treated.
A complete medical,
, social, and educational history is compiled from existing medical and
school records (if applicable) and from interviews with parents. Children
are given intelligence tests to measure their learning abilities and
intellectual functioning. Such tests include the Stanford-Binet
Intelligence Scale, the Wechsler Intelligence Scales, the Wechsler
Preschool and Primary Scale of Intelligence, and the Kaufmann Assessment
Battery for Children. For infants, the
Bayley Scales of Infant Development
may be used to assess motor, language, and problem-solving skills.
Interviews with parents or other caregivers are used to assess the
child's daily living, muscle control, communication, and social
skills. The Woodcock-Johnson Scales of Independent Behavior and the
Vineland Adaptive Behavior Scale (VABS) are frequently used to test these
Federal legislation entitles mentally retarded children to free testing
and appropriate, individualized education and skills training within the
school system from ages three to 21. For children under the age of three,
many states have established early intervention programs that assess,
recommend, and begin treatment programs. Many day schools are available to
help train retarded children in basic skills such as bathing and feeding
and social programs are also important in helping retarded children and
Training in independent living and job skills is often begun in early
adulthood. The level of training depends on the degree of retardation.
Mildly retarded individuals can often acquire the skills needed to live
independently and hold an outside job. Moderate to profoundly retarded
individuals usually require supervised community living.
can help relatives of the mentally retarded develop coping skills. It can
also help parents deal with feelings of guilt or anger. A supportive, warm
home environment is essential to help the mentally retarded reach their
full potential. However, as of 2004, there is no cure for mental
A promising but controversial treatment for mental retardation involves
stem cell research. In the early 2000s scientists are exploring the
potential of adult stem cells in treating mental retardation. They have
transplanted bone marrow cells into living embryos in the uteri of animals
to approach congenital diseases, birth defects, and mental retardation.
Stem cells are primitive cells that are capable of forming diverse types
of tissue. Because of this remarkable quality, human stem cells hold huge
promise for the development of therapies to regenerate damaged organs and
heal people who are suffering from terrible diseases. Embryonic stem cells
are derived from human embryos. Their use is controversial because such
stem cells cannot be used in research without destroying the living
embryo. Other sources of stem cells are available, however, and can be
harvested from umbilical cord blood as well as from fat, bone marrow, and
other adult tissue without harm to the donor. An enormous amount of
research involving adult stem cells is going on as of 2004 in laboratories
in the United States.
Individuals with mild to moderate mental retardation are frequently able
to achieve some self-sufficiency and to lead happy and fulfilling lives.
To reach these goals, they need appropriate and consistent educational,
community, social, family, and vocational supports. The outlook is less
promising for those with severe to profound retardation. Studies have
shown that these individuals have a shortened life expectancy. The
diseases that are usually associated with severe retardation may cause the
shorter life span. People with Down syndrome develop in later life the
brain changes that characterize Alzheimer's disease and may develop
the clinical symptoms of this disease as well.
Immunization against diseases such as measles and Hib prevents many of the
illnesses that can cause mental retardation. In addition, all children
should undergo routine developmental screening as part of their pediatric
care. Screening is particularly critical for those children who may be
neglected or undernourished or may live in disease-producing conditions.
Newborn screening and immediate treatment for PKU and hyperthyroidism can
usually catch these disorders early enough to prevent retardation. Good
prenatal care can also help prevent retardation. Pregnant women should be
educated about the risks of drinking and the need to maintain good
during pregnancy. Tests such as
and ultrasonography can determine whether a fetus is developing normally
in the womb.
—A procedure performed at 16–18 weeks of pregnancy in
which a needle is inserted through a woman's abdomen into her
uterus to draw out a small sample of the amniotic fluid from around the
baby for analysis. Either the fluid itself or cells from the fluid can
be used for a variety of tests to obtain information about genetic
disorders and other medical conditions in the fetus.
—The failure of a child to meet certain developmental milestones,
such as sitting, walking, and talking, at the average age. Developmental
delay may indicate a problem in development of the central nervous
—A chromosomal disorder caused by an extra copy or a
rearrangement of chromosome 21. Children with Down syndrome have varying
degrees of mental retardation and may have heart defects.
—An infection caused by
type b (Hib). This disease mainly affects children under the age of
five. In that age group, it is the leading cause of bacterial
meningitis, pneumonia, joint and bone infections, and throat
Inborn error of metabolism
—One of a group of rare conditions characterized by an inherited
defect in an enzyme or other protein. Inborn errors of metabolism can
cause brain damage and mental retardation if left untreated.
Phenylketonuria, Tay-Sachs disease, and galactosemia are inborn errors
—A rare, inherited, metabolic disorder in which the enzyme
necessary to break down and use phenylalanine, an amino acid necessary
for normal growth and development, is lacking. As a result,
phenylalanine builds up in the body causing mental retardation and other
—An abnormal condition where three copies of one chromosome are
present in the cells of an individual's body instead of two, the
—A medical test in which sound waves are directed against
internal structures in the body. As sound waves bounce off the internal
structure, they create an image on a video screen. Ultrasonography is
often used to diagnose fetal abnormalities, gallstones, heart defects,
and tumors. Also called ultrasound imaging.
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444 North Capitol Street NW, Suite 846, Washington, DC 20001–1512.
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