Chromosome 18, trisomy

Chromosome 18, Trisomy

Chromosome 18, trisomy, also known as Trisomy 18 or Edwards syndrome, is a genetic disorder caused by the presence of an extra chromosome 18. This condition is associated with a high rate of mortality and a wide range of severe developmental and physical abnormalities.

Genetics

Trisomy 18 occurs when there is an extra copy of chromosome 18 in some or all of the body's cells. Normally, humans have 23 pairs of chromosomes, for a total of 46. In individuals with trisomy 18, there are three copies of chromosome 18, leading to a total of 47 chromosomes. This extra genetic material disrupts normal development, causing the characteristic features of the disorder.

The condition can occur as a result of nondisjunction during meiosis, which is the process by which gametes (sperm and egg cells) are formed. Nondisjunction is an error in cell division that results in reproductive cells with an abnormal number of chromosomes.

Clinical Features

Individuals with trisomy 18 often have:

• Severe intellectual disability
• Congenital heart defects
• Kidney problems
• Esophageal atresia
• Omphalocele
• Microcephaly (small head size)
• Micrognathia (small jaw)
• Clenched fists with overlapping fingers
• Rocker-bottom feet

Diagnosis

Trisomy 18 can be diagnosed before birth through prenatal testing. Common methods include:

• Amniocentesis
• Chorionic villus sampling (CVS)
• Non-invasive prenatal testing (NIPT)

After birth, a diagnosis can be confirmed through a karyotype test, which analyzes the number and structure of chromosomes.

Prognosis

The prognosis for individuals with trisomy 18 is generally poor. Many affected fetuses do not survive to term, and of those born alive, a significant number die within the first year of life. However, some children with trisomy 18 do survive beyond infancy, although they typically have severe developmental and physical challenges.

Management

There is no cure for trisomy 18, and treatment focuses on managing symptoms and complications. This may include:

• Surgical interventions for heart defects and other anomalies
• Supportive care for feeding difficulties
• Physical therapy
• Palliative care

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.

Also see

• Trisomy 21
• Trisomy 13
• Genetic counseling
• Prenatal diagnosis