Trisomy 18 – Edwards Syndrome Explained
Trisomy 18 is a serious chromosomal disorder in which chromosome 18 is present three times instead of two, causing severe developmental abnormalities and often life-threatening complications.
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Trisomy 18 is a serious chromosomal disorder in which chromosome 18 is present three times instead of two, causing severe developmental abnormalities and often life-threatening complications.
What is Trisomy 18?
Trisomy 18, also known as Edwards syndrome, is a genetic condition in which a person has three copies of chromosome 18 in their cells instead of the usual two. In a typical human cell, there are 46 chromosomes arranged in 23 pairs. In trisomy 18, a third copy of chromosome 18 is present, resulting in 47 chromosomes in total. This extra genetic material disrupts normal development and leads to severe physical and intellectual disabilities.
Causes
Trisomy 18 is most commonly caused by a cell division error called non-disjunction, in which chromosomes fail to separate properly during the formation of egg or sperm cells. When such a cell is fertilized, the resulting embryo carries three copies of chromosome 18 in every cell.
- Full trisomy 18: Approximately 94% of cases involve an extra chromosome 18 in all cells of the body.
- Mosaic trisomy 18: Only some cells carry the extra chromosome. Symptoms may be less severe.
- Partial trisomy 18: Only a segment of the additional chromosome 18 is present, leading to variable presentations.
Advanced maternal age (particularly over 35) is associated with an increased risk, though trisomy 18 can occur in pregnancies of any age.
Symptoms
Trisomy 18 is associated with a wide range of serious physical abnormalities and developmental challenges:
- Low birth weight and growth retardation
- Congenital heart defects (in more than 90% of cases), such as ventricular septal defect
- Malformations of the brain, kidneys, and gastrointestinal tract
- Overlapping fingers (a characteristic feature)
- Small head size (microcephaly) and a prominent back of the skull
- Facial features such as low-set ears, small mouth, and a narrow palate
- Severe intellectual disability
- Breathing difficulties and feeding problems in newborns
Diagnosis
Trisomy 18 can be diagnosed both before birth (prenatally) and after birth (postnatally).
Prenatal Diagnosis
- First-trimester screening: Ultrasound and blood tests to assess the risk in the first trimester of pregnancy
- Non-invasive prenatal testing (NIPT): Analysis of fetal DNA from a maternal blood sample with high accuracy
- Amniocentesis: Chromosomal analysis from amniotic fluid to confirm the diagnosis
- Chorionic villus sampling (CVS): Tissue sampling from the placenta for chromosomal testing
Postnatal Diagnosis
- Clinical examination of the newborn
- Chromosomal analysis (karyotyping) from the child's blood sample
Treatment and Care
There is no cure for trisomy 18. Treatment focuses on managing symptoms and improving the quality of life for the affected child.
- Cardiac surgery: In selected cases, surgical repair of heart defects may be considered, though this is always an individualized decision.
- Intensive medical care: This may include assisted ventilation, tube feeding, and infection prevention.
- Palliative care: Many families choose a palliative approach focused on comfort and well-being.
- Early intervention therapies: For children who survive infancy, physical, occupational, and speech therapy can be beneficial.
- Psychosocial support: Counseling and support for affected families through healthcare professionals and support groups
Prognosis
The prognosis for trisomy 18 is serious. Approximately 50% of affected newborns do not survive beyond the first week of life, and only about 10% reach their first birthday. However, some individuals, particularly those with mosaic trisomy 18, have survived into adolescence or adulthood. Every child is unique, and medical decisions should always be made collaboratively with the medical team, taking into account the wishes and values of the family.
References
- Trisomy 18 Foundation – Edwards Syndrome Overview. Available at: https://www.trisomy18.org
- Cereda A, Carey JC. The trisomy 18 syndrome. Orphanet Journal of Rare Diseases. 2012;7:81. doi:10.1186/1750-1172-7-81
- National Institutes of Health (NIH), MedlinePlus – Trisomy 18. Available at: https://medlineplus.gov/genetics/condition/trisomy-18/
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