Trisomy 18

Trisomy 18 is a genetic chromosomal disorder characterized by an extra chromosome that causes abnormalities in various parts of the body. Also known as Edwards’ syndrome, it is the second most common trisomy behind Trisomy 21 (Down Syndrome).

Trisomy 18 occurs when chromosome cells fail to properly divide during gestation. Usually, the chromosomes that make up your body come in sets of two, with a healthy genome being made up of 23 pairs. In cases of trisomy 18, a person develops a third chromosome in the 18th pair. The extra chromosome, which will be present in every cell in the body, causes severe and often fatal birth defects.

Three types of trisomy can occur:

  • Full trisomy 18: The most common variant of the condition (about 95% of all cases). In the case of full trisomy 18, the extra chromosome is found in every cell in the baby’s body. This form is not hereditary.
  • Partial trisomy 18: A very rare form of trisomy 18 that occurs when an extra chromosome is only partially present in the cells. It occurs when a piece of chromosome 18 attaches itself to another chromosome, which can happen before or after conception. In this instance, the syndrome could be caused by hereditary factors.
  • Mosaic trisomy 18: Also very rare, mosaic trisomy occurs when an extra chromosome 18 is present in only some of the body’s cells instead of all of them. Similarly to full trisomy 18, mosaic trisomy is a random occurrence during cell division and is not inherited from the parents.
How is Trisomy 18 Diagnosed?

Trisomy 18 is usually diagnosed by a health care professional prenatally, in other words, while the baby is still in the womb. However, the process of testing for the condition is the same whether it is done before or after birth. To test for trisomy 18, a DNA sample is extracted from either the baby’s blood or body tissue and cultured so that the doctor can examine a picture of the chromosomes known as a karyotype.

To successfully obtain a karyotype of the baby’s DNA, the chromosomes are isolated, stained, and then examined under a microscope that can then take a photo to be examined. This karyotype should be able to show your doctor if there is indeed a third, extra chromosome 18 present in your baby’s cells.

Another common method of testing for trisomy 18 and other types of abnormalities is a maternal serum screening, also known as a multiple marker test. This prenatal screening is administered either during the first trimester (a Combined First Trimester Screening, or “CFTS”), or during the second trimester (a Second Trimester Maternal Serum Screening, or “2TMSS”).

These screenings measure several substances in a pregnant woman’s blood. With this information, doctors can estimate the chance that the pregnancy will be affected by a chromosomal condition or certain other developmental problems by separating women in to “high chance” or “low chance” categories.

What are the Symptoms of Trisomy 18?

Much like Down syndrome, the effects of trisomy 18 can range from mild to severe, meaning that it will manifest in every child differently. No matter how mild or severe the case may be, the presence of trisomy 18 causes a host of problems.

Common problems associated with Trisomy 18 include:

  • Heart defects
    • Ventricular Septal Defect (VSD): A hole between the lower chambers
    • Atrial Septal Defect (ASD): A hole between the upper chambers
    • Coarctation of the aorta: A narrowed exit vessel from the heart
  • Kidney defects
  • Omphalocele: When a portion of the intestinal tract is outside of the stomach
  • Esophageal atresia: When the esophagus and stomach don’t connect
  • Polyhydramnios: Excess amounts of amniotic fluids
  • Choroid plexus cysts: Pockets of fluid in the brain
  • Mycrognathia (small jaw) and microcephaly (small head)
  • Spina bifida: A spinal cord that is not fully formed
  • Thin, frail body and small size, even at full term
  • Craniofacial abnormalities: Abnormalities of the jaw, skull, ears, and neck)
  • “Rocker bottom” feet: Legs and feet that are stuck in a curved position
  • Inability to extend fingers fully
  • Shortened breastbone
How is Trisomy 18 Treated? What is the Prognosis for Trisomy 18?

Unfortunately, studies show that there is a very high mortality rate associated with the condition. The average lifespan for infants that are born with this condition is between 3 days – 2 weeks. Studies have shown a 5-10% chance of surviving the first year.

Due to this, most care for babies born with Edwards’s syndrome is more focused on palliative care and optimizing quality of life. Treatment for complications varies depending on the affected organ and severity of the condition. The management of Edward’s syndrome will depend entirely on the severity of symptoms present in the child.

Conclusion

Sadly, a diagnosis of trisomy 18 means that you as a parent will be faced with some extremely difficult decisions regarding the care of your baby. There are many resources available to help you in this process, from intervention services, hospice care, social workers, and genetic counselors. There are also many support groups created for families that are dealing with the same problems, which are excellent sources of support as you work to cope with this diagnosis.

Further Readings
  1. The trisomy 18 syndrome” by Anna Cereda & John C. Carey, Orphanet Journal of Rare Diseases, 2012.
    An overview of Trisomy 18 syndrome, its causes, and its symptoms.
  2. Using Patient-Centered Care After a Prenatal Diagnosis of Trisomy 18 or Trisomy 13” by Shelly Haug, M.D. et al. JAMA Pediatr, 2017.
    A study on the effectiveness of Patient-Centered Care (PCC) in cases of prenatal diagnosis of Trisomy 18, as recommended by the Institute of Medicine to improve health care in the United States. A PCC approach has the potential to reduce harm when inadequate care and counseling strategies create conflict between parents and caregivers over the treatment of infants with Trisomy 18.
  3. First-trimester combined test and integrated tests for screening for Downs Syndrome and Trisomy 18” by Geralyn M. Messerlian, PhD et al., 2019.
    The various tests for Trisomy 18 and other types of genetic abnormalities that are available to new mothers during the first and second trimesters of pregnancy.
  4. Long Survival of a Patient with Trisomy 18 and Dandy-Walker Syndrome” by Ferreira de Souza LM et al., Medicina, 2019.
    Study of a patient diagnosed with full trisomy 18, Dandy-Walker Syndrome, and congenital heart defects on long-term survival. Patient is a 12-year-old girl that received cardiac surgery at 1 year and 7 months, suggesting that early surgical intervention and multidisciplinary follow-ups may change the clinical outcome of an otherwise fatal condition. However, further studies are required to properly evaluate the benefit of invasive procedures such as cardiac surgery.
  5. Congenital Heart Surgery on In-Hospital Mortality in Trisomy 18” by Katherine A. Kosiv et al., Journal of the American Academy of Pediatrics, 2017.
    A study of congenital heart disease (CHD), a common symptom of trisomy 18 (t18), to determine the impact of congenital heart surgery (CHS) on in-hospital mortality. Results showed that CHD was present in 91% of infants diagnosed with t18 and in 81% of infants diagnosed with Trisomy 13 (t13). In-hospital mortality was shown to have decreased in patients who underwent CHS (64% lower in t18 patients) and remained so in the following 24 months of follow-up. These results suggest that CHS may be beneficial in some cases.