Trisomy 18

Trisomy 18, also known as Edwards syndrome, is a chromosomal disorder characterised by the presence of an extra copy of all or part of chromosome 18. This condition affects multiple parts of the body and results in severe developmental and physical anomalies. The syndrome is named after John Hilton Edwards, who first described it in 1960.

Signs and Symptoms

Trisomy 18 manifests through a variety of physical and developmental abnormalities. Children born with this condition may present with a small head (microcephaly), a small jaw (micrognathia), and clenched fists with overlapping fingers. Intellectual disability and developmental delays are profound and common. Heart defects, such as ventricular septal defects and atrial septal defects, are frequently observed.

Additional physical malformations can include kidney malformations, esophageal atresia, omphalocele (intestines protruding outside the body), and a prominent back portion of the head (occiput). Other notable features are low-set, malformed ears, a cleft lip/cleft palate, upturned nose, and widely spaced eyes (ocular hypertelorism).

Genetics

Trisomy 18 is caused by a meiotic nondisjunction event, where a pair of chromosomes fails to separate during cell division, resulting in a gamete with an extra copy of chromosome 18. When this gamete combines with a normal one, the resulting embryo has 47 chromosomes instead of the usual 46, leading to trisomy 18.

In some cases, only a portion of the body's cells may have the extra chromosome, a condition known as mosaic trisomy 18, which can result in milder symptoms. Rarely, a piece of chromosome 18 may become attached to another chromosome, leading to partial trisomy 18.

Diagnosis

Suspicion of trisomy 18 can be raised through ultrasound findings during pregnancy. Definitive diagnosis is achieved via chorionic villus sampling (CVS) or amniocentesis. During pregnancy, levels of PAPP-A, AFP, and uE3 are generally decreased, while free beta HCG is elevated, which can be indicative of the condition.

Prognosis

The prognosis for trisomy 18 is poor. Approximately 95% of pregnancies affected by trisomy 18 do not result in a live birth. Major causes of death include hypoxia and heart abnormalities. Half of the infants who are born alive do not survive beyond the first week. The median lifespan for those who survive past birth is five to 15 days, with only about 8-12% surviving longer than one year. Some studies have shown that with surgical intervention, infant survival rates can improve, and rare cases have been documented of individuals living into their twenties and thirties.

Epidemiology

Trisomy 18 occurs in approximately 1 in 5,000 live births. The risk of having a child with trisomy 18 increases with maternal age, with the average maternal age for conceiving a child with this disorder being 32.5 years. Although more common in female offspring, trisomy 18 affects all human populations.

History

John Hilton Edwards first identified trisomy 18 in 1960, although he initially believed it to be caused by a trisomy of chromosome 17. Subsequent research by Klaus Patau and Eeva Therman identified the correct chromosomal abnormality and helped to further characterise the syndrome.

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Self-assessment MCQs (single best answer)

What is another name for Trisomy 18?

Which characteristic is commonly observed in children with Trisomy 18?

How many chromosomes do individuals with Trisomy 18 typically have?

Which prenatal diagnostic test can definitively diagnose Trisomy 18?

Which heart defect is commonly seen in infants with Trisomy 18?

What is the median lifespan for infants born with Trisomy 18 who survive past birth?

What is the approximate occurrence rate of Trisomy 18 in live births?

Which of the following is NOT a common physical feature of Trisomy 18?

Who first described Trisomy 18?

Which type of chromosomal abnormality leads to Trisomy 18?