How common is
intellectual disability?
Intellectual
disability is the most common developmental disorder. To learn just how
common it is, CDC is tracking the number of children with intellectual
disability in a five-county area in metropolitan Atlanta (Georgia). This
activity is part of the Metropolitan Atlanta Developmental Disabilities
Surveillance Program (MADDSP). For the purposes of tracking, MADDSP
defines intellectual disabilities in 8-year-old children by the presence
of a score of 70 or below on the most recent test of intellectual
functioning. In 1996, an estimated 16 per 1,000 8-year-old children in
metro Atlanta, or about 1 in 63, had an intellectual disability. In
2000, the prevalence was an estimated 12 per 1,000, or 1 in 83,
8-year-olds. In 1996 and 2000, respectively, 29% and 35% of children
with intellectual disabilities also had one or more other developmental
disabilities tracked by MADDSP. [Read more
about MADDSP]
CDC also studied how many children in
metropolitan Atlanta had intellectual disability in the mid-1980s. This
project was done as part of the Metropolitan Atlanta Developmental
Disabilities Study (MADDS), which studied how common certain
disabilities were in 10-year-old children. We found that 12 of
every 1,000 10-year-old children had intellectual disability. Mild
intellectual disability was 3 times more common than severe intellectual
disability. As in MADDSP, intellectual disability was more common in
boys than in girls, and more common in black children than in white
children. [Read
a summary of the article about intellectual disability in MADDS]
[Read more about MADDS]
In another study, CDC used data from the
U.S. Department of Education and the Social Security Administration to
find the number of people with intellectual disability in the United
States in 1993. The study showed that about 1.5 million children and
adults (ages 6-64 years) had intellectual disability. The highest rate
of intellectual disability was found in West Virginia and the lowest
rate was found in Alaska. [Read
the complete article on
intellectual disability
in the United States]
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The Autism and Developmental
Disabilities Monitoring (ADDM) Program tracked the number of
children with autism spectrum disorders and intellectual
disabilities in five sites in 2002 and four sites in 2006.
-
Fetal alcohol syndrome
surveillance programs track the number of children with FAS in
five states.
-
CDC works with vaccine programs
to prevent infectious diseases that may cause intellectual
disabilities.
-
Community-based studies are
exploring effective ways that parents can help improve
developmental outcomes for their children.
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What causes intellectual
disability? Can it be prevented?
Intellectual disability can start anytime
before a child reaches the age of 18 years. It can be caused by
injury, disease, or a brain abnormality. These causes can happen
before a child is born or during childhood. For many children, the
cause of their intellectual disability is not known. Some of the
most common known causes of intellectual disability are Down syndrome,
fetal alcohol syndrome, and fragile X syndrome, all of which occur
before birth. Other causes that take place before a child is born
include genetic conditions (such as Cri-du-chat syndrome or Prader-Willi
syndrome), infections (such as congenital cytomegalovirus), or birth
defects that affect the brain (such as hydrocephalus or cortical
atrophy). Other causes of intellectual disability (such as asphyxia)
happen while a baby is being born or soon after birth. Still other
causes of intellectual disability do not happen until a child is older.
These may include serious head injury, stroke, or certain infections
such as meningitis.
If you would like to learn more about a
specific genetic condition that you think could cause intellectual
disability, you can go to the National Library of Medicine's Genetics
Home Reference Web site. Information on each genetic condition includes
symptoms, how common it is, related genes, treatments, and links to
resources where you can learn more about the condition. The Genetics
Home Reference also can help you learn more about genetics, including
genetic testing, genetic counseling, and gene therapy. [Go
to the Genetics Home Reference Web site]
Right now, we do not know how to prevent
most conditions that cause intellectual disability. However, there
are some causes that can be prevented. Fetal alcohol syndrome (FAS) is
one such cause. A woman can prevent FAS by not drinking when she
is pregnant. CDC funds several projects to study how common FAS
is, how to encourage women not to drink during pregnancy, and how to
help people with FAS and their families. [Read
more about CDC's FAS program]
Some metabolic conditions, such as
phenylketonuria (PKU), galactosemia, and congenital hypothyroidism, can
cause intellectual disability and other problems if babies with these
conditions do not begin treatment soon after birth. Parents and
doctors can find out if a child has one of these conditions through a
simple blood test or heel prick. Newborns in the United States are
tested soon after birth, but different states test for different
conditions. Parents can request that their baby be tested for all
conditions that have tests. Children that do have these conditions
are usually treated with medicine or put on a special diet. If the
correct treatment is started soon enough after the child is born and
continues as long as needed, the child will not have intellectual
disability. [Learn
more about newborn screening in your state] [Learn
more about the conditions screened for in newborn screening programs]
It's also important for women with PKU to
follow a special diet when they are pregnant. If they do not
follow their diets, their babies are very likely to be affected by
intellectual disability and other birth defects. [Read
the CDC press release about this topic] [Read
the complete article]
Another cause of intellectual disability
that can be prevented is kernicterus, a kind of brain damage that
happens when a newborn baby has too much jaundice. In some newborn
babies, the liver makes too much yellow pigment called bilirubin.
If too much bilirubin builds up in a new baby's body, the skin and
whites of the eyes turn yellow. This yellow coloring is called
jaundice. A little jaundice is not a problem. It is actually
very common in newborn babies and usually goes away by itself.
Some babies, however, have too much jaundice. If not treated,
these high levels of bilirubin can damage a baby's brain.
Kernicterus most often causes cerebral palsy and hearing loss, but in
some children it can also cause intellectual disability.
Kernicterus can be prevented by using special lights (phototherapy) or
other therapies to treat babies. [Read
more about kernicterus]
References
Batshaw ML. Children with disabilities
(4th edition). Baltimore MD: Paul H. Brookes Publishing Co.;1997.
Murphy CC, Boyle C, Schendel D, Decouflé
P, Yeargin-Allsopp M. Epidemiology of mental retardation in children.
Mental Retardation and Developmental Disabilities Research Reviews
1998;4:6-13.
Smith R (Editor). Children with mental
retardation: a parents' guide. Rockville, MD: Woodbine House; 1993.
Yeargin-Allsopp M, Murphy CC, Cordero JF,
Decouflé P, Hollowell JG. Reported biomedical causes and associated
medical conditions for mental retardation among 10-year-old children,
metropolitan Atlanta, 1985 to 1987. Developmental Medicine & Child
Neurology 1997;39:142-149. [Read
abstract of this paper]
[Return to top]
Date: October 29, 2005
Content source: National Center on Birth Defects and Developmental
Disabilities