Trisomy 20 is a rare chromosomal condition that occurs when an individual has an extra copy of chromosome 20. Normally, humans have two copies of each chromosome, but in trisomy 20, there are three copies of chromosome 20 in some or all cells. This genetic change can lead to a range of physical and developmental effects.
Genetic Basis of Trisomy 20
A trisomy describes the presence of an extra chromosome in a cell, meaning there are three copies instead of the usual two. This chromosomal abnormality can manifest in two primary forms: full trisomy 20 and mosaic trisomy 20.
Full trisomy 20 occurs when every cell in the body contains the extra copy of chromosome 20. This complete duplication often results from an error during the formation of egg or sperm cells before fertilization. Pregnancies with full trisomy 20 frequently lead to spontaneous miscarriage.
Mosaic trisomy 20, the more common type, involves the extra chromosome 20 being present in only a portion of the body’s cells, while other cells maintain the typical two copies. This mosaic pattern usually arises from an error in cell division that happens shortly after fertilization, during early embryonic development. The varying proportion of cells with the extra chromosome contributes to a wide spectrum of possible symptoms and outcomes.
Recognizing Trisomy 20
The manifestations of trisomy 20 can vary considerably, influenced by whether the condition is full or mosaic, and the percentage of affected cells in mosaic cases. Full trisomy 20, though rarely seen in live births, is often linked to severe anomalies, particularly affecting the central nervous system and heart. In contrast, individuals with mosaic trisomy 20 can have outcomes ranging from a normal appearance to more noticeable health issues.
Common physical characteristics reported in individuals with mosaic trisomy 20 include spinal abnormalities such as stenosis, vertebral fusion, and kyphosis. Decreased muscle tone, known as hypotonia, and chronic constipation are also frequently observed. Some individuals may present with sloped shoulders or skin pigmentation changes, such as linear and whorled nevoid hypermelanosis.
Developmental delays, particularly in psychomotor skills and speech, can occur. While some individuals with mosaic trisomy 20 may have intellectual abilities within the normal range, others experience significant learning disabilities. More severe presentations can include mild facial differences, cardiac anomalies like ventricular septal defects or dysplastic tricuspid valves, and kidney abnormalities such as horseshoe kidneys.
Diagnosis and Detection
Diagnosing trisomy 20 involves genetic testing, which can be performed both before and after birth. Prenatal diagnostic methods are employed when screening tests, such as ultrasound or non-invasive prenatal testing (NIPT), suggest a potential chromosomal abnormality. NIPT analyzes cell-free DNA from the mother’s blood and can indicate the presence of an extra copy of chromosome 20.
For confirmation, invasive prenatal procedures like chorionic villus sampling (CVS) or amniocentesis are often recommended. CVS involves taking a small sample of placental cells, usually between 10 and 13 weeks of pregnancy, while amniocentesis collects amniotic fluid cells, typically between 15 and 20 weeks. These samples undergo specialized genetic tests, including karyotyping, fluorescence in situ hybridization (FISH), or microarray analysis, to confirm the presence and extent of trisomy 20.
Postnatal diagnosis involves analyzing tissue samples, such as blood or skin biopsies, from the infant or child. Genetic counseling plays an important role both before and after diagnosis, providing families with information about the condition, potential outcomes, and support resources. A normal ultrasound cannot rule out trisomy 20.
Outlook and Supportive Care
The prognosis for individuals with trisomy 20 varies significantly, largely depending on whether it is full or mosaic, and the proportion of affected cells in mosaic cases. Full trisomy 20 often results in spontaneous miscarriage, and live births are rare, typically associated with severe medical problems. In contrast, pregnancies with mosaic trisomy 20 have a higher chance of progressing to term, with a wide range of outcomes.
For mosaic trisomy 20, a normal outcome is observed in a high percentage of prenatally detected cases, estimated to be between 90-95%. However, a smaller proportion, around 5-10%, may experience adverse outcomes such as unexplained fetal demise, intrauterine growth restriction, or multiple congenital anomalies.
Supportive care for individuals with trisomy 20 focuses on managing specific symptoms and improving quality of life. This involves a multidisciplinary team of specialists addressing various organ systems, including:
Physical therapy for hypotonia and motor coordination.
Occupational therapy for daily living skills.
Speech therapy for communication development.
Medical specialists, such as cardiologists for heart defects or nephrologists for kidney issues, for targeted care.