Definition of Down syndrome
Down syndrome: A common chromosome disorder due to
an extra chromosome number 21 (trisomy 21). Down syndrome causes
mental retardation, a characteristic face, and multiple
malformations. Down syndrome is a relatively common birth defect. The
chromosome abnormality affects both the physical and intellectual
development of the individual.
Down syndrome causes mental retardation, a characteristic facial
appearance, and multiple malformations. It is associated with a major
risk for heart malformations, a lesser risk of duodenal atresia (part
of the small intestines is not developed), and a minor but still
significant risk of acute leukemia.
The chromosome abnormality that causes Down syndrome is trisomy
21, a extra copy of chromosome number 21. This means that instead of
having the normal 2 copies of chromosome number 21, the person with
Down syndrome has 3 copies of chromosome number 21. Confirmation of
such a condition requires a chromosome study (analysis under
the microscope of the chromosomes). A chromosome study is also
valuable to rule in or out a translocation (a type of rearrangement)
of chromosome 21 that can be heritable in which case it can give rise
to more cases of Down syndrome in the family. The evaluation of the
Down syndrome baby and the family by a medical geneticist is often
useful.
In Down syndrome there are certain characteristic features in the
appearance which may individually be quite subtle but together permit
a clinical diagnosis of Down syndrome to be made at
birth. These signs of Down syndrome include slight flattening of the
face, minimal squaring off of the top of the ear, a low bridge of the
nose (lower than the usually flat nasal bridge of the normal
newborn), an epicanthal fold (a fold of skin over top of the inner
corner of the eye, which can also be seen less frequently in normal
babies), a ring of tiny harmless white spots around the iris, and a
little narrowing of the palate. There are many, many more minor
malformations in Down syndrome.
Down syndrome is also associated with a number of major
malformations. For example, approximately a half of Down syndrome
children are born with a heart defect, most often a hole between the
two sides of the heart. For another example, Hirschprung's disease
(congenital aganglionic megacolon) which can cause intestinal
obstruction occurs more frequently in children with Down syndrome
than in other children.
The intellectual handicaps in Down syndrome are often the most
important problem. These handicaps may not be evident in early
infancy. However, they tend to become increasingly noticeable later
in infancy and during childhood as developmental delay. In adults
with Down syndrome, the intellectual handicap is manifest as mental
retardation.
Very few adults with Down
syndrome can lead independent lives because of their mental
retardation. It was once thought that nearly all adults with Down
syndrome
developed Alzheimer's disease (dementia) so that on top of their
mental handicap most people with Down syndrome were slated for
premature senility. However, it now appears that a much lower
proportion, perhaps 20 to 25%, of Down's adults develop dementia. The
majority of adults with Down syndrome may thus be spared this fate.
The name Down syndrome refers to a 19th century English physician
Langdon Down. Ironically, he was not the first person to describe the
condition, he added little to our knowledge of it and, in great
error, he attributed the condition to a "reversion" to the mongoloid
race. The disorder was also once called mongolism, a term now
considered obsolete.
Last Editorial Review: 10/24/2004Common Misspellings: down syndrone
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