Patau syndrome is the least common and most severe of the viable autosomal trisomies. Median survival is fewer than 3 days. First identified as a cytogenetic syndrome in 1960, Patau syndrome is caused by an extra copy of chromosome 13, a medium-length acrocentric chromosome.
Many of the clinical features widely vary; however, severe mental deficiency is a consistent feature in children born with Patau syndrome. Holoprosencephaly, polydactyly, flexion of the fingers, rocker-bottom feet, facial clefting, neural tube defects, and heart defects are also frequent clinical features. Patau syndrome is generally recognized at birth by the presence of structural birth defects and poor neurologic performance.
Patau syndrome is caused by the presence of an extra copy of chromosome 13, generally present at conception and transmitted to every cell in the body. Although the exact mechanisms by which chromosomal trisomies disrupt development are unknown, considerable attention has been paid to trisomy 21 as a model system for the autosomal trisomies. 
Normal development requires 2 (and only 2) copies of most of the human autosomal genome; the presence of a third copy of an autosome is generally lethal to the developing embryo. Therefore, trisomy 13 is distinctive in that it is one of only 3 autosomal trisomies for which development can proceed to live birth. In fact, trisomy 13 is the largest autosomal imbalance that can be sustained by the embryo and yet allow survival to term. Complex physiologic structures, such as those found in the CNS and heart, appear to be particularly sensitive to chromosomal imbalance, either through the actions of individual genes or by the destabilization of developmental processes involving many genes in concert.
Incidence of Patau syndrome is approximately 1 case per 8,000-12,000 live births. Significant racial or geographic differences in frequency are not evident, although a well-known association is recognized between Patau syndrome and increased maternal age, an association common to all autosomal trisomies in fetuses that survive to term.
A study by Springett et al of 25 population-based registries in 16 European countries found the incidence of trisomy 13 to be 1.9 per 10,000 total births. 
Median survival age for children with Patau syndrome is 2.5 days, with only one in 20 children surviving longer than 6 months. However, some children survive into their teens and seem to fare better than might be expected based on reports from those who die in the perinatal period. A multi-state, population-based study by Meyer et al reported the 5-year survival rate for trisomy 13 to be 9.7%, with the lowest mortality rates found among females and the offspring of non-Hispanic black mothers.  Reports of adults with Patau syndrome are rare.
Holoprosencephaly, a frequent brain malformation associated with Patau syndrome, is associated with severe neurological impairment; development of the structural features of the midface is disrupted when holoprosencephaly is present. Serious cardiac anomalies are often present.
In the aforementioned study by Springett and colleagues, of 240 live-born infants with trisomy 13, the prevalence of anomalies was as follows  :
Cardiac anomalies: 57%
Nervous system anomalies: 39%
Eye anomalies: 30%
Orofacial cleft: 45%
The most common causes of death in Patau syndrome are cardiopulmonary arrest (69%),  congenital heart disease (13%), and pneumonia (4%). Survivors with Patau syndrome exhibit severe mental retardation and developmental delays and are at increased risk for malignancy. Infants who survive the neonatal period have an average length of stay in a neonatal ICU of 10.8 days.
The sex ratio at birth is slightly skewed toward females, presumably because of decreased survival among males, with continued skewing of the ratio further toward females as these children age.
Patau syndrome is expressed prenatally and is fully evident at birth. A significant number of cases that are trisomic for chromosome 13 end in spontaneous abortion, fetal demise, or stillbirth.  The mortality rate is very high among neonates. Children who survive the neonatal period continue to express developmental delays and exhibit a declining developmental quotient over time. This decline does not result from loss of developmental milestones but instead reflects a worsening developmental lag compared with other children. A report on a group of 21 individuals with Patau syndrome (3 mosaic and 18 nonmosaic) who survived past age 5 years showed the oldest to be aged 21 years.
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