|
|
|
Down Syndrome
|
|
This syndrome affects approximately 1:660 newborns, and is therefore considered
to be the number one cause of mental retardation. The main features are mental
retardation of varying degrees and characteristic facial features. The main clinical
signs are a large tongue, a tendency for the mouth to be open, upwardly slanted
eyes, and a single lateral line on the palms of the hands (simian crease). During
pregnancy, findings such as a thick neck,
echogenic intestines and other markers may be detected in the fetal system
ultrasound scan or in the fetal
nuchal translucency test. Such findings are an indication that the fetus
may have Down syndrome. In some cases there are additional defects such as heart defects, a blockage in the digestive tract,
etc.
The syndrome is usually caused by the presence of an extra chromosome 21. Instead of the normal situation where
there are two number 21 chromosomes, in this case there are three, resulting in
a total of 47 chromosomes instead of 46. In rare cases there is a normal total number
of chromosomes - 46 - and here the syndrome is caused when a number 21 chromosome
becomes attached to one of the other chromosomes (a situation called translocation). The individual has two normal number
21 chromosomes as well as the one that is attached to another chromosome, and therefore
he possesses 3 copies of chromosome 21. This can be diagnosed by performing chromosome
analysis on blood cells in the adult or on amniotic cells or the chorionic villi of the placenta in the fetus.
(See: "Down
syndrome in the family", "Maternal blood biochemical screening
test").
|
Down syndrome
|
|
|