Patau syndrome is associated with severe mental retardation, small eyes that may exhibit a split in the iris (coloboma), a cleft lip and/or palate, weak muscle tone (hypotonia), an increased risk of heart defects, skeletal abnormalities, and other medical problems. Affected individuals rarely live past infancy because of the life threatening medical problems associated with this condition. Patau syndrome affects approximately 1 in 10,000 live births. The risk of having a child with Patau syndrome increases as a woman gets older.

People with Patau syndrome have additional DNA from chromosome 13 in some or all of their cells. The extra material disrupts the normal course of development, causing the characteristic features of Patau syndrome.

Most cases of Patau syndrome result from trisomy 13, which means each cell in the body has three copies of chromosome 13 instead of the usual two copies. A small percentage of cases occur when only some of the body's cells have an extra copy of chromosome 13, resulting in a mixed population of cells with a differing number of chromosomes, such cases are called mosaic Patau syndrome.

Patau syndrome can also occur when part of chromosome 13 becomes attached to another chromosome (translocated) before or at conception. Affected people have two copies of chromosome 13, plus extra material from chromosome 13 attached to another chromosome. With a translocation, the person has a partial trisomy for chromosome 13 and often the physical signs of the syndrome differ from the typical Patau syndrome.

Most cases of Patau syndrome are not inherited, but occur as random events during the formation of reproductive cells (eggs and sperm). An error in cell division called nondisjunction can result in reproductive cells with an abnormal number of chromosomes. For example, an egg or sperm cell may gain an extra copy of chromosome 13. If one of these atypical reproductive cells contributes to the genetic makeup of a child, the child will have an extra chromosome 13 in each of the body's cells.

Mosaic Patau syndrome is also not inherited. It occurs as a random error during cell division early in fetal development. As a result, some of the body's cells have the usual two copies of chromosome 13, and other cells have three copies of the chromosome.

Patau syndrome due to a translocation can be inherited. An unaffected person can carry a rearrangement of genetic material between chromosome 13 and another chromosome. This rearrangement is called a balanced translocation because there is no extra material from chromosome 13. Although they do not have signs of Patau syndrome, people who carry this type of balanced translocation are at an increased risk of having children with the condition.

Definition

Patau's syndrome, also called trisomy 13, occurs when a child is born with three copies of chromosome 13. Normally, two copies of the chromosome are inherited, one from each parent. The extra chromosome causes numerous physical and mental abnormalities. Owing mostly to heart defects, the lifespan of trisomy 13 babies is usually measured in days. Survivors have profound mental retardation.

Description

Individuals normally inherit 23 chromosomes from each parent, for a total of 46 chromosomes. However, genetic errors can occur before or after conception. In the case of Patau's syndrome, an embryo develops which has three copies of chromosome 13, rather than the normal two copies.

Trisomy 13 occurs in approximately 1 in 12,000 live births. In many cases, spontaneous abortion (miscarriage) occurs and the fetus does not survive. The risks of trisomy 13 seem to increase with the mother's age, particularly if she is older than her early 30s. Male and female children are equally affected, and the syndrome occurs in all races.

Causes & symptoms

Patau's syndrome is caused by the presence of three copies of chromosome 13. The presence of these three copies--rather then the normal two--is a random error and cannot be attributed to anything the parents did or did not do.

Newborns with trisomy 13 have numerous internal and external abnormalities. Commonly, the front of the brain fails to divide into lobes or hemispheres, and the entire brain is unusually small. Children who survive infancy have profound mental retardation.

Incomplete development of the optic (sight) and olfactory (smell) nerves often accompanies the brain defects, and the child may also be deaf. Frequently, a child with trisomy 13 has cleft lip, cleft palate, or both. Facial features are flattened and ears are malformed and lowset. Extra fingers or toes (polydactyly) may be present in addition to other hand and foot malformations.

In nearly all cases, trisomy 13 babies have respiratory difficulties and heart defects, including atrial and ventricular septal defects, patent ductus arteriosus, and defects of the pulmonary and aortic valves. Other organ systems may also be affected. The organ defects are frequently severe and life-threatening.

Diagnosis

A newborn's numerous malformations indicate a possible chromosomal abnormality. Trisomy 13 is confirmed by examining the infant's chromosomal pattern through karyotyping or another procedure. Trisomy 13 is detectable during pregnancy through the use of ultrasonography, amniocentesis, and chorionic villus sampling.

Treatment

Patau's syndrome cannot be cured. Some structural abnormalities can be treated through surgery, but malformations are often numerous and severe. Decisions regarding measures to prolong life are best made on an individual basis by the parents and the doctors. Medical treatment may simply focus on making the infant comfortable, rather than prolonging life.

Children who survive infancy require medical treatment to correct structural abnormalities and associated complications. Physical therapy, speech therapy, and other types of developmental therapy will help the child reach his or her potential.

Prognosis

Approximately 82% of trisomy 13 babies die within their first month of life; only 5-10% survive to one year. Survival to adulthood is very rare. Only one adult is known to have survived to age 33.

Survivors have profound mental and physical disabilities; however, trisomy 13 children do have some capacity for learning. Older children may be able to walk with or without a walker. They may also be able to understand words and phrases, follow simple commands, use a few words or signs, and recognize and interact with others.

Prevention

Patau's syndrome--trisomy 13--is not preventable.

Key Terms

Aminocentesis: A procedure in which a needle is inserted through a pregnant woman's abdomen and into her uterus to withdraw a small sample of amniotic fluid. The amniotic fluid can be examined for signs of disease or other problems afflicting the fetus.
Chorionic villus sampling: A medical procedure done during weeks 10-12 of a pregnancy. The procedure involves inserting a needle into the placenta and withdrawing a small amount for analysis.
Chromosome: A structure composed of DNA contained within a cell's nucleus. The DNA condenses into these readily recognizable structures only at certain times during cell growth. In humans, DNA is bundled into 23 pairs of chromosomes, each of which has recognizable characteristics--such as length and staining patterns--that allow individual chromosomes to be identified. Identification is assigned by number (1-22) or letter (X or Y).
Karyotyping: A laboratory procedure in which chromosomes are separated from cells and stained. The stained chromosomes are examined under a microscope and identified as chromosomes 1-22 and X or Y. There should be two copies each of chromosomes 1-22. The X/Y pair depends on gender. Females have two X chromosomes, and males have both an X and a Y chromosome.
Spontaneous abortion: The uninduced delivery of a fetus before survival outside the mother is possible. Also referred to as a miscarriage.
Trisomy: A condition in which a third copy of a chromosome is inherited. Normally only two copies should be inherited.
Ultrasonography: A medical test in which sound waves are directed against internal structures in the body. As sound waves bounce off the internal structure, they create an image on a video screen. An ultrasound of a fetus at weeks 16-20 of a pregnancy can be used to determine structural abnormalities.

For Your Information

Books

Gardner, R.J. McKinlay, and Grant R. Sutherland. Chromosome Abnormalities and Genetic Counseling. New York: Oxford University Press, 1996.
Jones, Kenneth Lyons. Smith's Recognizable Patterns of Human Malformation. 5th ed. Philadelphia: W.B. Saunders Company, 1997.
Periodicals
Baty, Bonnie J., Brent L. Blackburn, and John C. Carey. "Natural History of Trisomy 18 and Trisomy 13: I. Growth, Physical Assessment, Medical Histories, Survival, and Recurrence Risk." American Journal of Medical Genetic 49 (1994): 175-187.
Baty, Bonnie J., et al. "Natural History of Trisomy 18 and Trisomy 13: II. Psychomotor Development." American Journal of Medical Genetics 49 (1994): 189-194.
Organizations
Support Organization for Trisomy 18, 13, and Related Disorders (SOFT). 2982 South Union Street, Rochester, NY 14624. (800) 716-SOFT. http://www.trisomy.org/.

Gale Encyclopedia of Medicine. Gale Research, 1999.

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