Trisomy 18 or Edward's Syndrome, is the second most common trisomy after Down's Syndrome. Edward's Syndrome occurs when three sets (trisomy) of chromosome 18 occur.
Trisomy 18 is therefore caused by a genetic abnormality occurring before conception, when egg and sperm cells are made. A healthy egg or sperm cell contains 23 individual chromosomes - one to contribute to each of the 23 pairs of chromosomes needed to form a healthy, 46 chromosome cell. However, sometimes egg and sperm cells are left with 24 (or more) chromosomes. It is the joining of these egg or sperm cells that eventually cause a trisomy fetus to be formed.
It is this extra genetic information that causes all the trouble for individuals with Edward's Syndrome. As each and every cell in their body contains extra information, its ability to grow and develop appropriately is delayed or confused. This results in characteristic physical abnormalities such as low birth weight; a small, abnormally shaped head; small jaw; small mouth; low-set ears; and clenched fists with overlapping fingers. Those with Edward's syndrome also have heart defects, and other organ malformations such that most systems of the body are affected.
Edward's Syndrome also results in significant developmental delays. For this reason a full-term Edward's syndrome baby may well exhibit the breathing and feeding difficulties of a premature baby. Given time and the assistance regularly offered premature babies, however, some of these infants are able to overcome these initial difficulties.
The survival rate for Edward's Syndrome is still incredibly low. Only 5 - 10% of liveborn infants will survive their first year of life. Major causes of death include apnea and heart abnormalities. It is impossible to predict the prognosis of an Edward's Syndrome child during pregnancy or the neonatal period. Some children with Edward's Syndrome develop only a small degree, while others gain many of the skills of their Down's Syndrome peers - walking, talking, attending school, holding down a job, etc. As major medical interventions are routinely withheld from these children, it is also difficult to determine what the survival rate or prognosis would be for the condition if they were treated with the same aggressiveness as their genetically 'normal' peers.
The rate of occurance for Edward's Syndrome is ~ 1:3000 conceptions and 1:6000 livebirths, as 50% of those diagnosed prenatally with the condition will not survive the prenatal period. Although there is an increased risk of conceiving a child with Edward's Syndrome as a woman's age increases, women in their 20's and 30's still conceive Edward's Syndrome babies.
A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 18, resulting in a mixed population of cells with a differing number of chromosomes. Such cases are sometimes called mosaic Edwards syndrome. Very rarely, a piece of chromosome 18 becomes attached to another chromosome (translocated) before or after conception. Affected people have two copies of chromosome 18, plus extra material from chromosome 18 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 18 and often the physical signs of the syndrome differ from the typical Edward's syndrome.
External link
Last updated: 05-07-2005 07:06:57
