Patau syndrome

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Overview
Patau syndrome, also known as trisomy 13, is a chromosomal abnormality, a syndrome in which a patient has an additional chromosome 13 due to a non-disjunction of chromosomes during meiosis. The extra chromosome 13 disrupts the normal course of development, causing the characteristic features of Patau syndrome. Like all non-disjunction diseases (Down syndrome, Edwards syndrome, etc...) the risk of disease in the offspring increases with maternal age at pregnancy. Patau syndrome affects approximately 1 in 15,000 live births.

History
Trisomy 13 was first observed by Erasmus Bartholin in 1657, but the chromosomal nature of the disease was ascertained by Dr.Klaus Patau in 1960. The disease is named in his honor. Patau syndrome was also described in Pacific island tribes. These reports were thought to have been caused by radiation from atomic bomb tests. The tribes were temporarily moved before and during the test by an "x" amount of distance. They were then put back where they had been taken; all of this occurred before it was known how long, or even if, radiation still lingered on after a nuclear explosion.

Etiology
Most cases of Patau's 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.

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 non-disjunction 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.

Manifestations and physical findings
Of those embryos that do survive to gestation and subsequent birth, common anomalies include:
 * mental & motor retardation
 * polydactyly (extra digits)
 * microcephaly
 * low-set ears
 * holoprosencephaly (failure of the forebrain to divide properly).
 * heart defects
 * structural eye defects, including microphthalmia, Peters anomaly, cataract, iris and/or fundus (coloboma), retinal dysplasia or retinal detachment, sensory nystagmus, cortical visual loss, and optic nerve hypoplasia
 * cleft palate or hare lip
 * meningomyelocele (a spinal defect)
 * omphalocele (abdominal defect)
 * abnormal genitalia
 * abnormal palm pattern
 * overlapping of fingers over thumb.

Recurrence risk
Unless one of the parents are carriers of a translocation the chances of a couple having another trisomy 13 affected child is less than 1% (less than that of Down Syndrome).

Prognosis
Most embryos with trisomy 13 do not survive gestation and are spontaneously aborted. Of those surviving to term gestation, approximately 82-85% do not survive past 1 month of age, and 85% do not survive past 1 year of age. Certain malformations, especially holoprosencephaly and other central nervous system malformations, yield a more grave prognosis. Of those infants that survive past 1 year, most have few major malformations, but the prognosis remains poor, owing to multiple factors including long term neurological disability, feeding difficulty, and frequent pneumonia and other respiratory infections. Currently there are over 66 survivors of some form or another of Trisomy 13. See.