Article # 446. Trisomy 13 Patau Syndrome

What is a trisomy?

The term trisomy (say: TRY-so-mee) means that there are three chromosomes, rather than the usual pair of chromosomes. For example, a baby with Down syndrome usually has three copies of chromosome 21, rather than the usual pair. The condition is thus called "trisomy 21."

What is trisomy 13?

Trisomy 13 is a rare chromosome abnormality that affects approximately one in every 5,000 live births. Babies with trisomy 13 have many abnormalities, involving nearly every organ system in the body, as well as developmental delay. Trisomy 13 is also called Patau syndrome, after the physician who first described the disorder. Patau syndrome, Trisomy 13, is the least common of the autosomal trisomies, after Down syndrome (Trisomy 21) and Edwards syndrome (Trisomy 18). The extra copy of chromosome 13 in Patau syndrome causes severe neurological and heart defects which make it difficult for infants to survive. The exact incidence of Patau syndrome is not known, although it appears to affect females more than males, most likely because male fetuses do not survive until birth. Patau syndrome, like Down syndrome, is associated with increased age of the mother. It may affect individuals of all ethnic backgrounds.

Signs and symptoms of trisomy 13

Babies with trisomy 13 often have a normal birth weight, a small head and a sloping forehead. Noses are usually large ("bulbous"), ears are low-set and unusual in shape, eye defects occur frequently, and cleft lip and palate as well as heart defects are very common. Many babies with trisomy 13 are born with small areas of missing skin on the scalp (cutis aplasia), which resemble ulcers.

The brains in babies with trisomy 13 usually have major structural problems and often, the brain does not divide properly into two hemispheres, resulting in a condition called holoprosencephaly. Many babies with trisomy 13 have extra fingers and toes (polydactyly). Some present with a sac attached to the abdomen in the area of the umbilical cord (omphalocele), which contains some of the abdominal organs, as well as spina bifida. Girls may have an abnormally shaped uterus, called a bicornuate uterus. In boys, the testes sometimes fail to descend into the scrotum.

Life expectancy of a baby with trisomy 13

Unfortunately, 90% of babies born with trisomy 13 die by age 1. It is important to note that 5% to 10% of babies with trisomy 13 do survive the first year of life. It is difficult to predict the life expectancy of a baby with trisomy 13 if the baby does not have any immediate life-threatening problems. There are a few reports of babies with trisomy 13 surviving to their teens; however, this is unusual.

What causes trisomy 13?

Usually each egg and sperm cell contains 23 chromosomes. At the time of fertilization, the sperm and egg unite and create a cell with 23 chromosome pairs, or 46 chromosomes. In this manner, a child receives exactly half of their genetic material from each parent.

Sometimes, an error occurs when the egg or sperm cell is forming, causing it to have an extra chromosome. When this cell contributes the extra chromosome 13 to the embryo, trisomy 13 results. The extra chromosome 13 can come from either the mother's egg cell or the father's sperm cell. The abnormalities seen in babies with trisomy 13 result from having this extra chromosome 13 in each of the body's cells.

Occasionally, the extra chromosome 13 is attached to another chromosome in the egg or sperm; this is called a translocation. This is the only form of trisomy 13 that can be inherited in a family. Sometimes, a parent can carry a "balanced" rearrangement where chromosome 13 is attached to another chromosome. However, since the parent does not have any extra or missing chromosome material, they are said to have a "balanced translocation" since they are normal and healthy.

Rarely, mosaic trisomy 13 may occur when the error in cell division occurs after fertilization. These babies have some cells with an extra chromosome 13 and others with the normal number. These cases usually have milder manifestations.

Diagnosis of trisomy 13

Babies with trisomy 13 have a unique group of characteristics and can be diagnosed by physical examination. To confirm the diagnosis, a small blood sample can be taken and the chromosomes can be analyzed to determine the presence of an extra chromosome 13.

Chromosomal abnormalities can also be diagnosed before birth by analyzing cells in the amniotic fluid obtained by amniocentesis or from the placenta, obtained by chorionic villus sampling (CVS). The diagnosis of trisomy 13 can also be suggested by detailed fetal ultrasound; however, ultrasound is not 100% accurate, since not all abnormalities can be seen on ultrasound. In contrast, a chromosome analysis, whether performed on a blood sample, cells from the amniotic fluid, or placenta, is over 99.9% accurate.

There is a small chance of having another child with trisomy 13

The risk of having a baby with trisomy 13 increases slightly with the mother’s age. However, the average age of the mother at delivery of a baby with trisomy 13 is 32 years. In general, in each subsequent pregnancy, the chance of having another baby with trisomy 13 is no greater than 1%.

Genetic counselling is recommended

Parents of a baby with trisomy 13 are encouraged to seek genetic counselling. This can help parents to:

·         understand the results of chromosomal tests in detail

·         understand the chances that other babies will be affected

·         understand the diagnosis options for other pregnancies

Key points

·         Trisomy 13 is a rare condition caused by an extra copy of chromosome 13.

·         Babies with trisomy 13 have many abnormalities, involving nearly every organ system in the body, as well as developmental delay.

·         It is difficult to predict the life expectancy of a baby with trisomy 13.

·         Parents of a baby with trisomy 13 are encouraged to seek genetic counselling.