What is Patau syndrome?
The only consistently reported risk factor is advanced maternal age, since the extra copy of chromosome 13 most commonly arises from an error in meiosis during egg cell maturation. Sex does not appear to be a significant risk factor, with the incidence in female fetuses being 52 percent as compared to 48 percent in males, as reported in a 2012 Pediatricsstudy by K. E. Nelson, K. R. Hexem, and C. Feudtner.
The presence of an extra chromosome 13 in the cells of a developing fetus results from a type of error called nondisjunction, which can occur during gamete (sperm or egg) production in either parent. Either the failure of homologous chromosomes to separate from each other during the first meiotic division or the failure of sister chromatids to separate from each other during the second meiotic division will result in mature sperm cells or egg cells that have either one extra or one missing chromosome. Since each chromosome contains thousands of genes, it is not surprising that individuals with extra or missing chromosomes in all cells would have a severe imbalance of genetic information and suffer from multiple developmental anomalies. In fact, only three autosomal trisomies (conditions in which each cell has three copies of a nonsex chromosome) are generally known to be consistent with full-term delivery, and Patau syndrome is the least common and most severe of these. The most common and least severe is Down syndrome (trisomy 21). Edwards syndrome (trisomy 18), like Patau syndrome, results in affected newborns with multiple structural and developmental problems, and survival beyond the first year is rare.
Very occasionally, a case of trisomy 13 occurs in which the extra copy of chromosome 13 does not appear as a separate chromosome but rather is physically attached onto the end of another chromosome; such cases are known as translocation Patau syndrome. While the clinical features of the affected newborn do not differ from the usual form of the syndrome, it is particularly important to identify this variety of Patau, since it may be transmitted with high frequency by a normal-appearing parent who carries the translocation chromosome.
One additional variant that is infrequently encountered is known as mosaic trisomy 13. The bodies of mosaic individuals are composed of two distinctly different cell lines, in which only some of the cells have the extra chromosome 13, while the remainder have a normal chromosome complement. The severity of the clinical presentation in these cases depends on the type and number of cells that carry the extra chromosome, but in almost all cases a less severe form of the syndrome is manifested.
The most consistent symptoms present at birth include microcephaly (small head), cleft lip and/or palate, and polydactyly (extra fingers or toes). Ears are often low-set and malformed, and the nose can be oddly shaped or occasionally altogether absent. Most affected individuals are presumed to be hearing impaired, and many are blind as well. Other neurological problems are common, including profound intellectual disability and failure of the brain to divide into its proper hemispheres during gestation. About 80 percent of affected newborns are reported to have moderate to severe heart defects, as reported by the National Organization for Rare Diseases.
According to the University of Chicago, Patau syndrome occurs in about 1 out of 5,000 live births, and diagnosis is most often immediately apparent, although there is some overlap of symptoms with Edwards syndrome. Genetic studies should be performed to confirm the diagnosis. Ultrasound examinations and imaging studies should be done to check for more extensive developmental problems. They include brain, heart, and kidney defects as well as an extra spleen, rotated intestines, and defects of the liver and pancreas. Males may have undescended testes, while females frequently have a divided uterus.
Because of the heterogeneous nature of each clinical presentation, treatment is usually specifically directed to the particular physical problems with which each affected child is born. As reported by the University of Chicago, about 44 percent of affected newborns die within the first month and more than 70 percent die within a year. Most die from serious heart defects or severe neurological problems. Historically, medical treatment has focused primarily on patient comfort and noninvasive symptom treatment rather than on prolonging life. Surgery may be performed to repair heart defects or cleft lip and palate. In those cases where survival extends beyond one or two years, additional surgeries and physical therapy are often undertaken to allow the affected child to reach his or her full developmental potential. Nelson, Hexem, and Feudtner's study indicates that children born with Patau syndrome are surviving for increasingly longer periods and that neonatal interventions may improve survival rates.
Except for the rare translocation form of Patau syndrome, there is no effective means of prevention. Genetic counseling should always be available for parents of an affected child, and amniocentesis is an option for older at-risk mothers. According to the University of Chicago, only up to 30 percent of affected newborns survive the first year of life and survival into the teenage years is exceedingly rare.
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