trisomy 13
Summary
Trisomy 13, or Patau syndrome, is a chromosomal disorder caused by an extra copy of chromosome 13, resulting in severe congenital anomalies. It is characterized by microcephaly with holoprosencephaly, cleft lip/palate, polydactyly, and cutis aplasia, along with severe intellectual disability. Most affected infants die within the first year of life.
Detail
Trisomy 13 (Patau syndrome) results from nondisjunction during meiosis (most commonly maternal meiosis I), leading to an extra chromosome 13 in most cases (~75-80%); less commonly it results from Robertsonian translocation (often involving chromosomes 13 and 14) or mosaicism. Incidence increases with advanced maternal age, similar to other trisomies. Classic clinical features include the '3 C's and P's' mnemonic elements: severe midline facial defects due to failure of prosencephalon cleavage (holoprosencephaly), microphthalmia, cleft lip/palate, cutis aplasia (scalp defects), polydactyly, cardiac defects (VSD, PDA, ASD - present in >80%), rocker-bottom feet, and omphalocele. Renal anomalies (polycystic kidneys) are also common. Severe intellectual disability and seizures occur due to underlying brain malformations. Prenatal diagnosis can be made via first-trimester screening (elevated nuchal translucency, decreased PAPP-A, decreased beta-hCG), followed by confirmatory karyotype or chromosomal microarray via chorionic villus sampling or amniocentesis. Prognosis is poor, with median survival of days to weeks; less than 10% survive beyond the first year, typically due to cardiac and CNS complications. Management is largely palliative/supportive care, though some families pursue surgical interventions for specific defects. Differentiating from Trisomy 18 (Edwards syndrome) and Trisomy 21 (Down syndrome) is high-yield: Trisomy 13 emphasizes midline defects and polydactyly, Trisomy 18 emphasizes clenched fists with overlapping fingers, rocker-bottom feet, and micrognathia, while Trisomy 21 has more favorable prognosis with characteristic facial features and increased leukemia/Alzheimer's risk.
Sources
- First Aid for the USMLE Step 1
- Robbins Basic Pathology
- UpToDate: Patau syndrome (Trisomy 13)
- Nelson Textbook of Pediatrics
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