In this condition there is an extra chromosome
13, so that there are three number 13 chromosomes instead of two. The frequency
is estimated to be 1 in every 5,000 births. This syndrome ends in death - 86% die
in the uterus or in the first year of life, and only a few survive to the age of
5 years. In most cases, midline structures are affected, and the usual manifestations
are cleft lip and palate, polydactyly
(extra fingers and toes), holoprosencephaly or cyclopia, a small head circumference, abnormalities
in the cranial skin, defects of the heart and kidneys, etc. A small percentage of
cases occur when the trisomy is present in only some of the cells, while others
cells are normal - this condition is known as
mosaicism.
The risk of having a child with this syndrome increases with maternal age.
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