Edward’s syndrome is the name given to trisomy 18, a genetic condition in which a third chromosome 18 is present in the person’s cells. The condition was independently described in 1960 by two research groups: Edwards et al. and Smith et al. It is the second most common trisomy, following Down’s syndrome (trisomy 21). The syndromes share similar features of varying degrees, such as mental impairment and heart conditions, but trisomy 18 is usually fatal. Most cases (95%) of trisomy 18 do not make it to birth, but the estimated incidence of Edward’s syndrome is 1:8000 to 1:3000 live births, 80 percent of which are usually girls. Of those who are born with the syndrome, less than 10 percent make it to one year of age. Only roughly 1 percent live to ten years of age, but cases of survival into the mid-20s have been reported. Trisomy 18 is one of only three trisomies (despite the 23 chromosome pairs that can be duplicated) that result in live births due to the disruption chromosomal aberrations cause in embryonic development: the other two are 21 (Down’s) and 13 (Patau).
Trisomy 18 usually occurs as full trisomy, that is, the mutation occurs in every cell of the body. Thus, it is not inherited, but occurs as a defect in meiosis rather than translocations or defects in the parental cells. Physically, babies with the chromosomal aberration typically have heart defects, including ventricular and atrial septal defects (holes in the inner heart wall preventing proper blood flow for oxygenation and distribution) and coarctation of the aorta, a narrowing of the artery leading from the heart, preventing proper blood flow to the body. They also have kidney and digestive tract abnormalities, clenched hands, choroid plexus (fluid in the brain cavity), rocker bottom feet, small jaw, low set ears, strawberry shaped head, and delayed growth. In the womb they exhibit excess amniotic fluid (polyhydramnios). Mentally, the children with this disorder show impairment and developmental delays of varying severity.
Prenatal screening and diagnostic tests can determine if a baby is at risk or has trisomy 18. Ultrasound and maternal serum screening (also known as quad screen, triple screen, or AFP) can determine the risk of trisomy based on the fetal characteristics and development. In order to diagnose the chromosomal aberration, karyotyping (FISH analysis) after amniocentesis or chorionic villus sampling have to be performed. All prenatal testing carries risk, both mentally and physically, and should be discussed fully with the doctor. Genetic counselors can help provide guidance and answer questions before and after test results are received.
For more information: Trisomy 18 Foundation, Genetics Home Reference from the NIH, and eMedicine.