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Trisomy 18
Thursday, 16 October 2003
Wednesday, 11 February 2004
Edwards Syndrome

What

Trisomy 18 syndrome is a genetic disorder that occurs when some or all of the cells of the body have three copies of chromosome 18, instead of the usual two copies. Most fetuses affected with trisomy 18 end in miscarriages. Infants who are born with trisomy 18 can have multiple physical problems affecting their ability to eat, breath, move, and grow. They are generally quite developmentally delayed and have difficulties learning in school. Other children born with trisomy 18 mosaicism can lead normal lives.

Who

Trisomy 18 is a rare chromosomal disorder affecting approximately 1 out every 7500 births. The disorder is known to affect both genders, however girls are affected about four times more often than boys. The disorder is seen in people of all ethnicities and racial backgrounds. There are two forms of trisomy 18: complete or mosaic. When all of the cells in the body have three copies of chromosome 18, it is called “complete trisomy 18”. Complete trisomy 18 is generally not compatible with life and 95% of pregnancies that are affected result in miscarriages. Of the remaining 5% that are live births, only 5-10% will survive beyond the first year of life. When some of the cells in the child’s body have three copies of chromosome 18, and the other cells of the body are normal (have two copies of chromosome 18), it is called “trisomy 18 mosaicism”.

Signs and Symptoms

Those infants born with trisomy 18 who survive the first few years are almost always those with mosaicism. Their features are variable depending on the degree of mosaicism. That is, the more cells that are normal, the milder the symptoms. The more cells that have trisomy 18, the more severe the symptoms. When the mother is pregnant with an affected child, her uterus may be larger than it should be. This is because of excess fluid around the baby (“polyhydramnios”). When the infant is born, he/she may have a low birth weight and a small head. The back of the head may look very prominent. The appearance of the infant’s face may include: small eyes, droopy eyelids, eyes that are widely spaced apart; colobomas (abnormal shapes of the pupils) in the eyes; an upturned nose; abnormally shaped ears that may be set abnormally lower on the head; abnormal palate (roof of the mouth). An unusually small mouth and lower jaw can contribute to trouble with feeding and breathing. The hands may be clenched into a fist. The finger and toenails are not developed well. The breastbone, or “sternum”, is abnormally shaped. There may be hernias through the umbilicus (belly-button) or groin. The muscles on the front of the stomach may be separated (“diastasis recti”). In boys, the testes may not be located in the proper place within the scrotal sac. There may be heart and/or kidney problems. Over time, the affected child can exhibit developmental delay and mental retardation.

Possible Causes

Each pair of chromosomes is made up of one chromosome from the mother’s egg and one chromosome from the father’s sperm. Complete trisomy 18 occurs when the mother’s egg and the father’s sperm combine, and instead of resulting in two copies of chromosome 18, the baby inherits an extra copy for a total of three. Cells destined to become eggs or sperm have to go through a unique process called meiosis. Normal cells in the body have 46 chromosomes (23 pairs), and must divide so that when the egg and sperm combine the baby has the right amount of genetic material. If this division does not go as it should, then an extra copy of chromosome 18 is left and the baby ends up with one extra chromosome 18. The older the mother’s age beyond 35 years, the higher the risk that the extra copy of chromosome 18 came from the mother’s egg: the sperm contributes one chromosome 18 and the egg contributes two. Since all of the body’s cells are derived from the egg and sperm, then all of the cells will have trisomy 18 (complete trisomy 18). Most cases of trisomy 18 result from this type of genetic accident. Up to 20% of cases result from a translocation, where a part of one chromosome moves onto another chromosome. The few remaining cases result from mosaicism. The cause of trisomy 18 mosaicism is different. Instead of the extra chromosome 18 coming from the egg or the sperm, the extra chromosome 18 occurs because of a mistake in the growing cells of a developing fetus’ body. Trisomy 18 mosaicism means that the genetic abnormality occurs after fertilization and therefore does not affect all cells as is the case of complete trisomy 18. The signs and symptoms associated with trisomy 18 mosaicism are proportional to the amount of cells that are abnormal.

Diagnosis

Prenatal testing such as amniocentesis and ultrasound can detect trisomy 18. Amniocentesis can look for the number of chromosomes that the baby has. Ultrasound can show some of the physical features that are described above in the signs and symptoms section. After the infant is born, a physical examination may show the above symptoms suggesting the diagnosis. This can be confirmed with chromosome testing to show trisomy 18, translocation, or mosaicism.

Treatment

Heroic measures to try to prolong life for infants born with complete trisomy 18 are generally not recommended. An infant with trisomy 18 mosaicism with more severe symptoms may need to be treated surgically for abnormalities of his or her body such as heart, kidney, or genital malformations. Infections should also be treated accordingly since there is a higher risk for middle ear infection, pulmonary infection, and urinary tract infection. Some children may benefit from psychological counseling as well as academic support to avoid depression, social isolation and difficulties at school. On the milder end of the mosaic spectrum, a minimally affected child will be treated the same way one would treat a normal child. It is important to raise a special child in an encouraging and supportive environment.

Prognosis

Complete trisomy 18 is usually incompatible with life, hence the mortality (death) rate is high in the form of miscarriages and the live birth of such an infant is a rare event. If live born, the abnormalities of trisomy 18 are generally so severe that fifty percent of these infants do not survive more than the first week and less survive for a few months. More than 10 children have been known to survive to teenage years, but have usually marked handicaps

Connect with other parents

In the spirit of community and support, Madisons Foundation offers the unique service of connecting parents of children with rare diseases. If you would like to be connected to other parents of children with this disease, please fill out this brief form.

Weblinks

Support Organization for Trisomy 18,13 and Related Disorders
A great website with information about Trisomy 13 as well as available support for families affected by the disorder.

Medline Plus
A well written article on Trisomy 13 with nice definitions of medical terms.

Google Search for Trisomy 18

References and Sources

search.nlm.nih.gov/medlineplus/query?DISAMBIGUATION=true&FUNCTION=search&SERVER2=server2&SERVER1=server1&PARAMETER=trisomy+18&x=28&y=10 www.emedicine.com www.icondata.com/health/pedbase/files/trisomy2.htm www.geneticalliance.org trisomy.org