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It is caused by the presence of three — instead of two — chromosomes 18 in a fetus or baby's cells.
The additional chromosome usually occurs before conception. 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 normal cell with 46 chromosomes. Numerical errors arise at either of the two meiotic divisions and cause the failure of segregation of a chromosome into the daughter cells (non-disjunction). This results in an extra chromosome making the haploid number 24 rather than 23. Fertilization of these eggs or sperm that contain an extra chromosome results in trisomy, or three copies of a chromosome rather than two.
It is this extra genetic information that causes all the abnormalities characteristic of individuals with Edwards Syndrome. As each and every cell in their body contains extra information, the ability to grow and develop appropriately is delayed or impaired. 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. Babies with Edwards syndrome also have heart defects, and other organ malformations such that most systems of the body are affected.
Edwards Syndrome also results in significant developmental delays. For this reason a full-term Edwards syndrome baby may well exhibit the breathing and feeding difficulties of a premature baby. Given the assistance offered to premature babies, some of these infants are able to overcome these initial difficulties, but most eventually succumb.
The survival rate for Edwards Syndrome is very low. About half die in utero. Of liveborn infants, only 50% live to 2 months, and only 5 - 10% will survive their first year of life. Major causes of death include apnea and heart abnormalities. It is impossible to predict the exact prognosis of an Edwards Syndrome child during pregnancy or the neonatal period. 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. They are typically severely to profoundly developmentally delayed.
The rate of occurrence for Edwards Syndrome is ~ 1:3000 conceptions and 1:6000 live births, as 50% of those diagnosed prenatally with the condition will not survive the prenatal period. Although women in their 20's and 30's may conceive Edwards Syndrome babies, there is an increased risk of conceiving a child with Edwards Syndrome as a woman's age increases.
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 the abnormalities are often less than for the typical Edwards syndrome.
Features and characteristicsEdit
Symptoms and findings may be extremely variable from case to case. However, in many affected infants, the following may be found:
- Growth deficiency
- Feeding difficulties
- Breathing difficulties
- Developmental delays
- Mental retardation
- Undescended testicles in males
- Prominent back portion of the head
- Small head (microcephaly)
- Low-set, malformed ears
- Abnormally small jaw (micrognathia)
- Small mouth
- Cleft lip/palate
- Upturned nose
- Narrow eyelid folds (palpebral fissures)
- Widely-spaced eyes (ocular hypertelorism)
- Dropping of the upper eyelids (ptosis)
- Overlapped, flexed fingers
- Underdeveloped or absent thumbs
- Underdeveloped nails
- Absent radius
- Webbing of the second and third toes
- Clubfeet or Rocker bottom feet
- Small pelvis with limited movements of the hips
- Short breastbone
- Kidney malformations
- Structural heart defects at birth (i.e., ventricular septal defect, atrial septal defect, patent ductus arteriosus)
- Stenson, Carol M. (1999). Trisomy 18: A Guidebook for Families. University of Nebraska Medical Center. ISBN 1-889843-29-6.
- Barnes, Ann M. (2000). Care of the infant and child with trisomy 18 or 13: medical problems, reported treatments and milestones. University of Nebraska Medical Center. ISBN 1-889843-58-X.
- Trisomy 18 Support Foundation
- Support Organisation For Trisomy 18, 13, and Related Disorders (SOFT)
- The Chromosome 18 Registry & Research Society
- Who Named It synd/3438