Trisomy 13 (Patau Syndrome)

A trisomy is a type of aneuploidy (chromosomal abnormality) in which the cells of the body contain an extra copy of a chromosome. While most aneuploidies result in miscarriage, fetal development can progress to live birth in the case of trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), and a few other, less common chromosomal abnormalities. Of the three most common trisomies, Patau syndrome is the rarest; it also has the most severe clinical presentation.

What Is Trisomy 13?

Trisomy 13 is an aneuploidy in which the body cells contain excess genetic material from chromosome 13. Typically, an entire third copy of chromosome 13 is present in each cell. In some cases, however, the aneuploidy is due to chromosomal translocation, and in even fewer cases it is caused by mosaicism, whereby only some of the body cells are affected.

The presence of additional genetic material from chromosome 13 strongly impacts development, causing multiple and often severe organ defects, as well as mental disabilities (see Clinical Presentation below). In the case of mosaic trisomy 13, the extent and severity of these defects depend on the type and number of cells that have an additional chromosome but are typically milder than the abnormalities seen with full trisomy 13.

What Causes Trisomy 13?

The majority of trisomy 13 cases are not inherited. Whereas full trisomy 13 is a result of nondisjunction (an error of cell division) of chromosomes during meiosis (cell division of sex cells), mosaic trisomy 13 is caused by nondisjunction during mitosis (cell division in early fetal development).

Translocation trisomy 13 can be inherited. A parent may carry a rearrangement of genetic material between chromosome 13 and another chromosome without being affected himself/herself. Such parent, however, faces an increased risk of having a child with this condition.

In any case, even though trisomy 13 is seen in only 1 out of 10,000–20,000 live births and is typically due to random events occurring during egg or sperm formation in healthy parents, a strong correlation exists between maternal age and the likelihood of an aneuploidy to occur. Racial or geographical correlations have not been observed.

Clinical Presentation

Trisomy 13 can present with multiple (and typically severe) physical and mental abnormalities. The most common characteristics include

• heart defects (> 80% of cases)
• holoprosencephaly (failure of the forebrain to develop into two hemispheres)
• polydactyly (extra fingers and toes)
• rocker-bottom feet
• cleft lip (abnormal groove in the upper lip)
• cleft palate (incomplete closure of the roof of the mouth)
• microphthalmia (unusually small eyes)
• kidney malformations
• hypotonia (weak muscle tone)
• mental retardation
• developmental delays

Other possible characteristics include

• apnea
• neural tube defects
• cryptorchidism (undescended testes in boys)
• meningomyelocele (hernial protrusion of the spinal cord)
• dextrocardia
• omphalocele (abdominal wall defect)
• single umbilical artery
• vision problems due to eye defects
• cyclopia (single eye)
• proboscis (projecting tissue just above the eye)
• relatively small head (microcephaly) with a sloping forehead
• widely set eyes
• low-set ears
• wide and short hands with short fingers
• congenital trigger digits
• overlapping of fingers over thumb
• prominent heel
• broad, flat nose
• scalp defects
• cutis aplasia (missing portion of skin/hair)

Survival Rate

Most fetuses with trisomy 13 are spontaneously miscarried. Of those who make it to birth, more than 80% die within the first year of life. In fact, due to the severity of their condition, many neonates die within their first days or weeks of life. The median length of survival is about one week. Reports of adults (or even teenagers) with trisomy 13 are rare.

The most common causes of death are cardiopulmonary arrest, congenital heart disease, and pneumonia. Survivors exhibit severe mental retardation and developmental delays. Interestingly, females have greater rate of survival than males with trisomy 13.

Treatment

There is no cure or fully effective treatment for trisomy 13. Due to the severity and multiplicity of conditions affecting neonates with trisomy 13, clinical management normally focuses on enabling feeding and minimizing discomfort. Beyond that, if the baby survives, treatment is customized based on the particular problems with which the child is born. Usually, surgeries are necessary to repair heart defects, cleft lip, and cleft palate. Physical and speech therapies are also necessary to help improve motor skills as well as receptive and expressive communication skills.