Patau syndrome (Trisomy 13)

There are three mechanisms (similar to Down syndrome) that can lead to trisomy 13

1. Free trisomy 13 (~75%): Karyotype 47,XX,+13 or 47,XY,+13

   • Maternal meiotic nondisjunction (most common mechanism)
   • Usually sporadic (de novo)

2. Robertsonian translocation (~20%): Most commonly rob(13;14), also rob(13;13)

   • ~75% de novo
   • ~25% inherited from balanced carrier parent
   • Carrier recurrence risks:
     • Either parent carrier of rob(13;14): ~1% recurrence of a viable affected liveborn (most trisomy 13 conceptions miscarry, so empiric risk is much lower than the rob(14;21) figures taught for Down syndrome)
     • Either parent carrier of rob(13;13): 100% recurrence (all viable offspring affected)
   • Always obtain parental karyotypes

3. Mosaicism (~5%): Variable phenotype, may be milder

• Midline defects:
  • Holoprosencephaly (cyclopia to mild hypotelorism)
  • Midline cleft lip/palate
  • Omphalocele
• Polydactyly (postaxial)
• Microphthalmia/anophthalmia
• Cutis aplasia (scalp defects)
• Cardiac defects (80%)

• Prenatal: abnormal ultrasound (holoprosencephaly, midline facial clefts, polydactyly, cardiac defects) and/or positive cfDNA → CVS or amniocentesis → karyotype
• Postnatal: clinical features confirmed by karyotype (also identifies Robertsonian translocations and mosaicism)
• Parental karyotypes are indicated when a translocation is found

• Care planning is guided by goals and prognosis; options range from comfort-focused care to selected interventions
• Brain imaging for holoprosencephaly and cardiology evaluation; surgery considered case by case after family and multidisciplinary discussion
• Feeding support and management of apnea, seizures, and reflux
• Surveillance for and management of associated anomalies (renal, ophthalmologic)
• Genetic counseling, including parental karyotypes if a translocation is identified

• Median survival: 7-10 days
• ~10% survive to 1 year