Autosomal Trisomies: Down, Edwards, and Patau Syndromes
The autosomal trisomies result from an extra copy of chromosome 21, 18, or 13, most often due to meiotic nondisjunction whose risk rises with advancing maternal age. The three classic syndromes track together by incidence and survival: Down syndrome is both the most common and the most survivable, while Edwards and Patau syndromes are both rarer and lethal in infancy. High-yield differentiation rests on the karyotype, characteristic dysmorphic features, and the associated cardiac defect.
Down Syndrome (Trisomy 21)
Karyotype 47,XX/XY,+21. The most common autosomal trisomy at roughly 1:700 live births, and the most survivable — patients can live 50+ years with appropriate care. Congenital heart disease occurs in about 40–50% of patients, and the atrioventricular (AV) canal (endocardial cushion) defect is the most characteristic lesion, accounting for the largest share of those cardiac defects. Distinguishing features include hypotonia, flat facies, a single palmar (simian) crease, and duodenal atresia. Like the other trisomies, risk increases with maternal age through nondisjunction.
Edwards Syndrome (Trisomy 18)
Karyotype 47,XX/XY,+18. Incidence is about 1:5000, far less common than Down syndrome, and prognosis is poor — approximately 90% die within the first year of life. VSD and PDA are the commonly associated cardiac defects. The hallmark exam findings are clenched fists with overlapping fingers, rocker-bottom feet, micrognathia, and prominent occiput, and affected infants are typically small and growth-restricted. Memory aid: "Election age 18" — Edwards is trisomy 18.
Patau Syndrome (Trisomy 13)
Karyotype 47,XX/XY,+13. The rarest of the three at about 1:15,000, and like Edwards syndrome roughly 90% die within the first year. Cardiac involvement takes the form of various complex defects. Distinguishing features center on midline and structural anomalies: holoprosencephaly, cleft lip/palate, polydactyly, and cutis aplasia. Memory aid: "Un-lucky 13" — cleft lip, polydactyly (extra digits), and midline defects.
High-yield
- All three arise from nondisjunction; risk of nondisjunction (especially trisomies) rises with maternal age.
- Incidence order: Down (1:700) > Edwards (1:5000) > Patau (1:15,000).
- Down syndrome — congenital heart disease in ~40–50%; AV canal (endocardial cushion) defect is the most characteristic lesion.
- Edwards syndrome — VSD and PDA are common.
- Patau syndrome — various complex cardiac defects.
- Down = flat facies, hypotonia, single palmar crease, duodenal atresia; survives 50+ years.
- Edwards = clenched fists/overlapping fingers, rocker-bottom feet, micrognathia, prominent occiput; ~90% die in first year.
- Patau = holoprosencephaly, cleft lip/palate, polydactyly, cutis aplasia; ~90% die in first year.
- Mnemonic: "Election age 18" = Edwards; Patau is "un-lucky 13" with midline defects.
Pitfalls
- Confusing which trisomy is which number — Edwards is 18, Patau is 13; do not swap them.
- Assigning the AV canal defect to the wrong syndrome — it is the signature lesion of Down syndrome, not Edwards or Patau.
- Mixing up rocker-bottom feet (Edwards) with polydactyly (Patau); both are limb findings but belong to different trisomies.
- Assuming all trisomies are uniformly lethal — Down syndrome patients commonly survive into adulthood (50+ years), whereas Edwards and Patau are lethal in infancy.
- Forgetting that incidence and survival move together here: the more common trisomy (Down) is also the most survivable.
Clinical pearls
- Newborn with overlapping clenched fingers and rocker-bottom feet — think Edwards (trisomy 18).
- Newborn with cleft lip/palate, polydactyly, and holoprosencephaly — think Patau (trisomy 13).
- Hypotonic infant with flat facies, single palmar crease, and duodenal atresia — think Down (trisomy 21) and an AV canal defect.
- Advancing maternal age is the classic risk factor tying all three trisomies together via nondisjunction.
Frequently asked
Which cardiac defect is most classic for Down syndrome?
The atrioventricular (AV) canal (endocardial cushion) defect. Congenital heart disease affects about 40–50% of Down syndrome patients, and the AV canal defect is the single most characteristic lesion among them.
How do I quickly distinguish Edwards from Patau syndrome on a vignette?
Edwards (trisomy 18) features clenched fists with overlapping fingers, rocker-bottom feet, micrognathia, and a prominent occiput. Patau (trisomy 13) features holoprosencephaly, cleft lip/palate, polydactyly, and cutis aplasia.
What is the underlying mechanism of the autosomal trisomies?
Meiotic nondisjunction, in which chromosomes fail to separate. The risk of nondisjunction — especially for trisomies — increases with advancing maternal age.
Which trisomy has the best prognosis?
Down syndrome (trisomy 21); patients can survive 50+ years with appropriate care. In contrast, roughly 90% of infants with Edwards or Patau syndrome die within the first year.
What are the karyotypes for the three autosomal trisomies?
Down is 47,XX/XY,+21; Edwards is 47,XX/XY,+18; Patau is 47,XX/XY,+13.
Which trisomy is the most common, and which is the rarest?
Down syndrome is the most common at about 1:700 live births; Patau syndrome is the rarest at about 1:15,000, with Edwards syndrome intermediate at about 1:5000. Note that incidence and survival move in the same direction — Down is both the most common and the most survivable.
What cardiac defects are associated with Edwards and Patau syndromes?
Edwards syndrome commonly has VSD and PDA. Patau syndrome is associated with various complex cardiac defects.
Turn this into reasoning you can use on exam day — practice Autosomal Trisomies on branching cases where your decisions shape the patient.