Trisomy 18 Causes and Survival Rates
Trisomy 18, also called Edwards syndrome, is a genetic disorder similar to Down syndrome. In each disorder an extra chromosome becomes part of the persons genetic makeup. Normally, we have two of each chromosome ( one from each parent). In Trisomy, the individual has three. In Down syndrome it is chromosome 21, in Edwards syndrome it is chromosome 18. The extra genetic material causes a large number of severe complications within the individual and survivability is drastically reduced. Because the syndrome occurs very early in fetal development and the symptoms are drastic, almost 95% of fetuses with Trisomy 18 either are miscarried or are stillborn. Of the 5% that survive birth, more than half die in the first week and less than 10% survive one year. There are instances where individuals survive into their teens, but they have severe health problems as well as mental impairment and require constant living support.
Trisomy 18 can sometimes be detected during an ultrasound, but amniocentesis is required for diagnosis. There are a few different types of Trisomy 18 which depend on whether all cells in the body are effected and if the entirety of the extra chromosome is present. However, it is not clinically significant which type of Trisomy 18 is present because the complications and symptoms are the same. There is a link between the age of the mother and the chances of the fetus having Trisomy 18. The condition is more likely to occur when the mother is in her thirties.
Trisomy 18 and Heart Defects
Trisomy 18 affects a large number of different systems and organs within the body. Many of the more significant complications result in the death of the fetus very early after conception. The most significant of these are heart defects. There are three different errors that can cause when the heart is formed. The upper chambers often develop with a hole between them, and similarly, the bottom chambers can also develop with a hole in the wall separating them. The aorta also frequently is narrowed at the point where it connects to the heart. Either of these three conditions will result in an early miscarriage.
Trisomy 18 Affects the Digestive System, Kidneys and Lungs
The digestive system is also severely deformed in many Trisomy 18 fetuses. The intestines are frequently protruding through the abdominal muscles, and sometimes completely out of the body. Another deformity occurs where the lower portion of the esophagus is not attached to the stomach. As with the heart defects, these also are severe enough to result in early miscarriages.
Aside from the heart and digestive deformities, there are other organs that can be affected such as the kidneys and lungs which may not cause a miscarriage, but will lead to death shortly after birth. The brain will also develop much smaller than normal and water pockets develop within the brain. If the individual survives, this results in severe mental retardation.
Trisomy 18 Head Development
The most significant external deformities of individuals with Edwards syndrome involve the shape and size of the head and face. Most commonly the individuals will have a very small head size (microcephaly) along with a small jaw. The shape of the head will also be abnormal, with frequently the back of the head protruding beyond normal.
Trisomy 18 Summary
Trisomy 18, also called Edwards syndrome, usually will result in a miscarriage or stillbirth. The cells of the fetus will contain an extra copy of chromosome 18, which causes severe defects of the heart, digestive system, brain and kidneys among other organs. While a small percentage of fetuses with Trisomy 18 survive birth, very few make it through the first year of life.