Edwards syndrome

Trisomy 18 or Edwards Syndrome (named after John H. Edwards, who first described the syndrome in 1960) is a genetic disorder. It is the most common autosomal trisomy after Down Syndrome.

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Cause

It is caused by the presence of three — instead of two — copies of chromosome 18 in a fetus or infant's cells.

The additional chromosome usually occurs before conception. A healthy egg or sperm cell contains 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.

Prognosis

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.

Incidence/prevalence

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 20s and 30s may conceive Edwards Syndrome babies, there is an increased risk of conceiving a child with Edwards Syndrome as a woman's age increases.

Variations

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 characteristics

Symptoms and findings may be extremely variable from case to case. However, in many affected infants, the following may be found:

• Arthrogryposis a muscle disorder that causes multiple joint contractures at birth
• 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/Cleft palate
• Upturned nose
• Narrow eyelid folds (palpebral fissures)
• Widely-spaced eyes (ocular hypertelorism)
• Drooping 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)
• Intestines protruding outside the body (omphalocele)

References

• Support Organisation For Trisomy 18, 13, and Related Disorders (SOFT)
• The Chromosome 18 Registry & Research Society
• „Perinatal Hospice Care - Preparing for birth and death”

• synd/3438 at Who Named It