How many people are affected by mental retardation?
The Arc reviewed a number of prevalence studies in the early 1980's
and concluded that 2.5 to 3 percent of the general population have
mental retardation (The Arc, 1982).
Based on the 1990 census, an estimated 6.2 to 7.5 million people
have mental retardation. Mental retardation is 10 times more common
than cerebral palsy and 28 times more prevalent than neural tube
defects such as spina bifida. It affects 25 times as many people
as blindness (Batshaw, 1997).
Mental retardation cuts across the lines of racial, ethnic, educational,
social and economic backgrounds. It can occur in any family. One
out of ten American families is directly affected by mental retardation.
How does mental retardation affect individuals?
The effects of mental retardation vary considerably among people,
just as the range of abilities varies considerably among people
who do not have mental retardation. About 87 percent will be mildly
affected and will be only a little slower than average in learning
new information and skills. As children, their mental retardation
is not readily apparent and may not be identified until they enter
school. As adults, many will be able to lead independent lives in
the community and will no longer be viewed as having mental retardation.
The remaining 13 percent of people with mental retardation, those
with IQ's under 50, will have serious limitations in functioning.
However, with early intervention, a functional education and appropriate
supports as an adult, all can lead satisfying lives in the community.
How is mental retardation diagnosed?
The AAMR process for diagnosing and classifying a person as having
mental retardation contains three steps and describes the system
of supports a person needs to overcome limits in adaptive skills.
The first step in diagnosis is to have a qualified person give
one or more standardized intelligence tests and a standardized adaptive
skills test, on an individual basis.
The second step is to describe the person's strengths and weaknesses
across four dimensions. The four dimensions are:
1. Intellectual and adaptive behavior skills
2. Psychological/emotional considerations
3. Physical/health/etiological considerations
4. Environmental considerations
Strengths and weaknesses may be determined by formal testing, observations,
interviewing key people in the individual's life, interviewing the
individual, interacting with the person in his or her daily life
or a combination of these approaches.
The third step requires an interdisciplinary team to determine
needed supports across the four dimensions. Each support identified
is assigned one of four levels of intensity - intermittent, limited,
extensive, pervasive.
Intermittent support refers to support on an "as needed basis."
An example would be support that is needed in order for a person
to find a new job in the event of a job loss. Intermittent support
may be needed occasionally by an individual over the lifespan, but
not on a continuous daily basis.
Limited support may occur over a limited time span such as during
transition from school to work or in time-limited job training.
This type of support has a limit on the time that is needed to provide
appropriate support for an individual.
Extensive support in a life area is assistance that an individual
needs on a daily basis that is not limited by time. This may involve
support in the home and/or support in work. Intermittent, limited
and extensive supports may be needed in all life areas for an individual.
Pervasive support refers to constant support across environments
and life areas and may include life-sustaining measures. A person
requiring pervasive support will need assistance on a daily basis
across all life areas.
What does the term "mental age" mean when used to
describe the person's functioning?
The term mental age is used in intelligence testing. It means that
the individual received the same number of correct responses on
a standardized IQ test as the average person of that age in the
sample population.
Saying that an older person with mental retardation is like a person
of a younger age or has the "mind" or "understanding"
of a younger person is incorrect usage of the term. The mental age
only refers to the intelligence test score. It does not describe
the level and nature of the person's experience and functioning
in aspects of community life.
What are the causes of mental retardation?
Mental retardation can be caused by any condition which impairs
development of the brain before birth, during birth or in the childhood
years. Several hundred causes have been discovered, but in about
one third of the people affected, the cause remains unknown. The
three major known causes of mental retardation are Down syndrome,
fetal alcohol syndrome and fragile X.
The causes can be categorized as follows:
Genetic conditions - These results from abnormality of genes inherited
from parents, errors when genes combine, or from other disorders
of the genes caused during pregnancy by infections, over exposure
to x-rays and other factors. More than 500 genetic diseases are
associated with mental retardation. Some examples include PKU (phenylketonuria),
a single gene disorder also referred to as an inborn error of metabolism
because it is caused by a defective enzyme. Down syndrome is an
example of a chromosomal disorder. Chromosomal disorders happen
sporadically and are caused by too many or too few chromosomes,
or by a change in a structure of a chromosome. Fragile X syndrome
is a single gene disorder located on the X chromosome and is the
leading inherited cause of mental retardation.
Problems during pregnancy - Use of alcohol or drugs by the pregnant
mother can cause mental retardation. Recent research has implicated
smoking in increasing the risk of mental retardation. Other risks
include malnutrition, certain environmental contaminants, and illnesses
of the mother during pregnancy, such as toxoplasmosis, cytomegalovirus,
rubella, and syphillis. Pregnant women who are infected with HIV
may pass the virus to their child, leading to future neurological
damage.
Problems at birth - Although any birth condition of unusual stress
may injure the infant's brain, prematurity and low birth weight
predict serious problems more often than any other conditions.
Problems after birth - Childhood diseases such as whooping cough,
chicken pox, measles, and Hib disease which may lead to meningitis
and encephalitis can damage the brain, as can accidents such as
a blow to the head or near drowning. Lead, mercury and other environmental
toxins can cause irreparable damage to the brain and nervous system.
Poverty and cultural deprivation - Children in poor families may
become mentally retarded because of malnutrition, disease-producing
conditions, inadequate medical care and environmental health hazards.
Also, children in disadvantaged areas may be deprived of many common
cultural and day-to-day experiences provided to other youngsters.
Research suggests that such under-stimulation can result in irreversible
damage and can serve as a cause of mental retardation.
Can mental retardation be prevented?
During the past 30 years, significant advances in research have
prevented many cases of mental retardation. For example, every year
in the United States, we prevent:
250 cases of mental retardation due to phenylketonuria (PKU) by
newborn screening and dietary treatment;
1,000 cases of mental retardation due to congenital hypothyroidism
thanks to newborn screening and thyroid hormone replacement therapy;
1,000 cases of mental retardation by use of anti-Rh immune globulin
to prevent Rh disease and severe jaundice in newborn infants;
5,000 cases of mental retardation caused by Hib diseases by using
the Hib vaccine;
4,000 cases of mental retardation due to measles encephalitis thanks
to measles vaccine; and
untold numbers of cases of mental retardation caused by rubella
during pregnancy thanks to rubella vaccine (Alexander, 1998).
Other interventions have reduced the chance of mental retardation.
Removing lead from the environment reduces brain damage in children.
Preventive interventions such as child safety seats and bicycle
helmets reduce head trauma. Early intervention programs with high-risk
infants and children have shown remarkable results in reducing the
predicted incidence of subnormal intellectual functioning.
Finally, early comprehensive prenatal care and preventive measures
prior to and during pregnancy increase a woman's chances of preventing
mental retardation. Pediatric AIDS is being reduced by AZT treatment
of the mother during pregnancy, and dietary supplementation with
folic acid reduces the risk of neural tube defects.
Research continues on new ways to prevent mental retardation, including
research on the development and function of the nervous system,
a wide variety of fetal treatments, and gene therapy to correct
the abnormality produced by defective genes.
