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 Down syndrome (DS) is a chromosomal disorder occurring in about 1 out
of every 800 births. People with DS may have mild to severe learning
disabilities and physical symptoms that include a small skull, extra
folds of skin under the eyes, a flattened nose bridge, and a large,
protruding tongue. Muscle tone throughout the body is usually low. The
condition was formerly known as "mongolism" because the features of
people with Down syndrome were thought to resemble those of Mongolian
Asians. This term is now considered offensive and inappropriate and is
no longer used.
DS results when a person inherits all or part of an extra copy of
chromosome 21. This can occur in a variety of ways, the causes of
which are unknown. The most common chromosomal abnormality that
produces DS (accounting for about 95 percent of all cases) is Trisomy
21, a defect in which an extra, third copy of chromosome 21 is present
in every cell in the body.
Two other chromosomal abnormalities cause Down syndrome and occur in
about 2 to 3 percent of all cases. The first, translocation, takes
place when a child inherits a small, extra piece of the 21st
chromosome that is attached to another chromosome. If, in addition to
the translocation, two normal 21st chromosomes are also present, the
person will have some of the features of DS. If there is only one
normal 21st chromosome, the person will not display symptoms but the
children may inherit Down syndrome. Mosaic DS results from a second
type of chromosomal abnormality in which only some cells in the body
have an extra chromosome.
People with DS are subject to a variety of medical conditions. Heart
abnormalities that may require surgery are present in about half of
all DS cases. Thyroid problems (underproduction or overproduction of
thyroid hormones) affect 10 to 20 percent of people with DS, but these
problems respond well to treatment. The risk of acute leukemia is
somewhat increased, although treatment is successful in the majority
of cases. -excerpted from "Down Syndrome," Microsoft(r) Encarta(r)
Online Encyclopedia 2000
http://encarta.msn.com (c) 1997-2000 Microsoft Corporation. All rights
reserved.
Abnormal levels of maternal serum alpha-fetoprotein (MsAFP) alone may
identify 10% to 20% of Down syndrome babies. MsAFP may also be used in
a triple screen combination with estriol and beta-human chorionic
gonadotropin values to detect Down syndrome more accurately, but still
missing about 30% to 40% of cases.
According to one source, in 1,000 maternal serum alpha-fetoprotein
(MsAFP) tests, approximately 950 women will have normal results. Of
the remaining 50, 48 to 49 will have false positive tests and normal
babies. This leaves 1 out of every 1,000 with a defect. (Blatt, 1988)
Another source predicts that out of an 8% positive test rate, 6% will
have lower results and only 2% of those babies will have Down
syndrome. (Meagher, 1993) The principal conditions associated with
false positive values are multiple pregnancy (60% of twins and nearly
all triplets have elevated results if judged as for a singelton
pregnancy), threatened miscarriage, underestimation of gestation age
(MsAFP normally increases as gestation advances), small-for-dates
fetal growth, or a sample contaminated with fetal blood (which might
occur after a fetal death). However, women who are carrying perfectly
normal singleton pregnancies may show a false positive result as well.
Maternal liver disease or alcoholism may raise levels. Newborns with Down syndrome have an increased risk of having a
condition called transient leukemia (also called transient abnormal
myelopoiesis, transient blastemia or myeloproliferative syndrome).
This condition resembles leukemia, but disappears on its own without
treatment in just a few weeks or months. One researcher found that up
to 10% of newborns with DS had evidence of transient leukemia on blood
tests. On the blood test, the number of white blood cells is greatly
above normal and there are immature white blood cells ("blasts")
present in the blood as well. While this condition resolves without
treatment, the concern is that there is an increased risk of these
infants to develop leukemia later in childhood. In one study of 85
infants with transient leukemia, 30% went on to develop a type of
leukemia called "nonlymphoid" (also called "myelocytic," or AML)
within the next 3 years. The reason for the development of transient
leukemia isn't understood, but is definitely linked to the extra 21st
chromosome.
Leukemia is more common in children with DS, being seen anywhere from
10 to 30 times more often than in the general population of children.
The vast majority of cases occur in the first 5 years of life. In the
first 3 years of life, nonlymphoid leukemia is the most common form of
leukemia in children with DS; after age 3, approximately 80% have
acute lymphocytic leukemia and 20% have nonlymphocytic leukemia.
There are also functional defects of white blood cells in Down
syndrome. White blood cells in people with DS have a decreased
response to infection, and a decreased killing ability of
microorganisms. This may be one reason for the decreased immunity to
infection seen in children with diseases of the blood and Down
syndrome.-Dr. Len Leshin, MD, http://www.ds-health.com/hemat.htm
Reprinted from Midwifery Today E-News 2:47 Nov. 22, 2000 |
