Patau Syndrome (Trisomy 13)

Definition and Epidemiology

Severe autosomal chromosomal disorder caused by the presence of an extra copy of chromosome 13.
Recognized as the third most common viable autosomal trisomy, following trisomy 21 and trisomy 18.
Incidence is estimated at 1 in 10,000 to 1 in 16,000 live births.
High rate of in utero lethality is noted, with the vast majority of affected conceptuses resulting in spontaneous abortion.
Risk of nondisjunction trisomy 13 increases with advancing maternal age.

Mechanism	Prevalence	Pathogenesis Description
Full Trisomy 13	75-80%	Caused by meiotic nondisjunction (predominantly maternal meiosis I) yielding 47 chromosomes in all somatic cells.
Unbalanced Translocations	20%	Extra chromosome 13 material attached to another chromosome, most frequently a Robertsonian translocation t(13;14).
Mosaic Trisomy 13	<5%	Arises from a post-zygotic mitotic error resulting in two distinct cell lines, yielding a highly variable phenotype.

Gene dosage imbalance primarily disrupts early embryonic development and affects the prechordal mesoderm.
This disruption leads to a failure of the prosencephalon to divide, culminating in holoprosencephaly and midline facial anomalies.

System	Characteristic Manifestations
Central Nervous System	Holoprosencephaly (hallmark malformation), microcephaly, profound global developmental delay, severe intellectual disability.
Craniofacial	Bilateral cleft lip and palate, microphthalmia, iris coloboma, retinal dysplasia, low-set malformed ears, sloping forehead.
Dermatological	Cutis aplasia congenita (punched-out scalp defects), prominent capillary hemangiomas over the forehead and nape.
Musculoskeletal	Postaxial polydactyly of hands and feet, rocker-bottom feet, clenched hands, severe hypotonia.
Cardiovascular	Congenital heart defects in 80% of cases, primarily VSD, ASD, and PDA.
Gastrointestinal and Genitourinary	Omphalocele, echogenic polycystic kidneys, cryptorchidism in males, bicornuate uterus in females.

First-trimester maternal serum screening typically reveals decreased free beta-hCG, decreased PAPP-A, and increased fetal NT.
Fetal ultrasound detects holoprosencephaly, midline facial defects, postaxial polydactyly, echogenic kidneys, and omphalocele.
NIPT utilizing cffDNA provides high sensitivity and specificity for advanced screening.

Standard G-banded karyotyping of peripheral blood lymphocytes remains the gold standard for definitive diagnosis.
FISH on uncultured blood allows for rapid preliminary detection within 24 to 48 hours.
CMA is useful for clarifying complex chromosomal rearrangements or partial trisomies.
Baseline evaluation requires urgent echocardiography, cranial imaging, and renal ultrasonography to delineate internal malformations.

Care paradigms emphasize a patient-centered approach involving shared decision-making regarding palliative care versus intensive neonatal interventions.
System-specific management includes airway support, medical management of heart failure, and pharmacological control of clinical seizures.
Prognosis remains exceptionally poor with a median survival of 7 to 10 days.
Approximately 50% of infants succumb within the first week, and 90% mortality is observed within the first year.
Long-term survivors experience severe intractable seizures, sensory impairments, and require total assistance for daily living.
Recurrence risk for full nondisjunction trisomy 13 is approximately 1%.
Parental karyotyping is mandatory for translocation cases; parental balanced translocation carriers face significantly elevated recurrence risks and require options like PGT or invasive prenatal diagnosis for future pregnancies.