Patau syndrome (Trisomy 13) is one of those Step 1 genetics diagnoses you can often spot from across the vignette: severe congenital anomalies, midline defects, and a grim prognosis. The test loves it because it ties together nondisjunction, chromosome analysis, and a set of “classic” physical findings that show up again and again.
Where Patau Syndrome Fits (Big Picture)
Patau syndrome = Trisomy 13, a chromosomal aneuploidy (extra autosome) most commonly due to meiotic nondisjunction.
Quick comparison (HY triad to differentiate trisomies)
| Syndrome | Karyotype | Classic clues | Common step buzzwords |
|---|---|---|---|
| Down | Trisomy 21 | Upslanting palpebral fissures, single palmar crease, duodenal atresia | AV septal defect, Alzheimer risk |
| Edwards | Trisomy 18 | Clenched fists, rocker-bottom feet | Prominent occiput |
| Patau | Trisomy 13 | Cleft lip/palate, polydactyly, holoprosencephaly | “Midline defects,” cutis aplasia |
First Aid cross-reference: Genetics → Chromosomal Disorders (Autosomal trisomies); craniofacial and congenital anomaly associations.
Definition (What It Is)
Patau syndrome is the presence of an extra copy of chromosome 13 in somatic cells, leading to severe multisystem congenital anomalies and high neonatal mortality.
Karyotype notation examples:
- Full trisomy: 47,XX,+13 or 47,XY,+13
- Robertsonian translocation form (less common): translocation involving chromosome 13 (e.g., t(13;14))
Pathophysiology (Why It Happens)
1) Most common mechanism: Meiotic nondisjunction
- Usually occurs in maternal meiosis
- Risk increases with advanced maternal age (same concept as other trisomies)
- Leads to a zygote with three copies of chromosome 13
2) Less common: Robertsonian translocation
- Classically involves acrocentric chromosomes: 13, 14, 15, 21, 22
- Can cause familial recurrence (important counseling point)
- Parent may be a balanced carrier (phenotypically normal)
3) Mosaic trisomy 13 (rare)
- Post-zygotic nondisjunction
- Phenotype can be less severe depending on mosaicism burden
HY principle:
- Nondisjunction → sporadic, maternal age effect
- Balanced translocation carrier in a parent → recurrence risk and “why this family has repeated affected pregnancies”
First Aid cross-reference: Genetics → Nondisjunction vs Robertsonian translocation; acrocentric chromosomes list.
Clinical Presentation (What You See in Vignettes)
Patau syndrome is all about midline developmental defects and severe malformations.
High-yield hallmark findings (memorize these)
- Cleft lip and/or cleft palate
- Polydactyly (often postaxial)
- Holoprosencephaly (failure of forebrain division)
- Can present with severe neurodevelopmental impairment
- May be associated with facial anomalies (e.g., hypotelorism)
Other common congenital anomalies
Cardiac
- Congenital heart disease is common (often VSD/ASD, PDA)
- Many questions just say “congenital heart defect” without specifying—still think trisomy
Neurologic / head
- Microcephaly
- Severe intellectual disability (if survival beyond neonatal period)
- Seizures may occur
Skin
- Cutis aplasia (classically scalp defects—areas of absent skin)
Eyes
- Microphthalmia, coloboma can occur
Renal / GU
- Renal malformations (e.g., cystic kidneys)
- Omphalocele can be seen
“Classic cluster” memory hook
If you see cleft + polydactyly + severe neuro defects, trisomy 13 should pop up immediately.
First Aid cross-reference: Genetics → Autosomal trisomies; embryology/anatomy tie-ins (clefting, cardiac defects).
Diagnosis (How It’s Confirmed)
Prenatal screening (risk assessment, not confirmatory)
- Cell-free fetal DNA (cfDNA/NIPT): high sensitivity for common trisomies (including 13)
- First trimester combined screening and second trimester quad screen can suggest aneuploidy, but Step questions increasingly favor cfDNA as the “best screening test.”
Prenatal diagnostic tests (confirmatory)
- Chorionic villus sampling (CVS): earlier (around 10–13 weeks)
- Amniocentesis: later (≥15 weeks)
- Confirmation via:
- Karyotype
- FISH (rapid detection)
- Chromosomal microarray (can detect copy number changes; classic trisomy visible)
Postnatal diagnosis
- Physical findings raise suspicion → confirm with karyotype (or rapid FISH while awaiting full karyotype)
HY exam move:
- Screening abnormal → confirm with CVS/amniocentesis
- Definitive answer is karyotype showing +13 (unless they’re testing translocation genetics)
First Aid cross-reference: Genetics → Prenatal testing: cfDNA, CVS, amniocentesis; karyotype/FISH.
Management & Prognosis (What You Do)
Management
There is no curative therapy for the chromosomal abnormality. Care is generally:
- Supportive and multidisciplinary
- Address life-threatening defects:
- Cardiac stabilization/surgery in selected cases
- Feeding support (cleft palate issues, aspiration risk)
- Seizure management if present
- Palliative care is frequently appropriate and tested as a realistic counseling outcome
Prognosis
- High neonatal mortality
- Many infants do not survive beyond the first weeks to months of life
- Survivors typically have profound developmental impairment
HY counseling angle:
- If question asks about “expected outcome,” Patau is associated with very poor survival.
Genetics & Counseling Pearls (Testable Details)
Recurrence risk depends on the mechanism
- Nondisjunction (sporadic): recurrence risk is generally low, but maternal age remains a factor
- Robertsonian translocation: recurrence risk can be significantly higher
- Evaluate parents with karyotyping to assess balanced translocation carrier status
Acrocentric chromosomes (Robertsonian favorites)
- 13, 14, 15, 21, 22
If you see a parent with a balanced translocation involving 13, you should think about Trisomy 13 risk (and vice versa).
High-Yield Associations & Differential (How Step 1 Tries to Trick You)
Patau vs Edwards vs Down (rapid differentiation)
- Polydactyly + cleft lip/palate + holoprosencephaly → Patau (13)
- Clenched fists + rocker-bottom feet + prominent occiput → Edwards (18)
- Endocardial cushion defects + duodenal atresia + upslanting eyes → Down (21)
Midline defect tie-in (repeatedly tested concept)
- Holoprosencephaly = midline developmental failure
- Often paired with clefting and severe neurologic deficits
If the vignette emphasizes “midline defects,” Trisomy 13 should be near the top.
Table: Patau Syndrome “Must-Know” Summary
| Category | High-yield points |
|---|---|
| Definition | Trisomy 13 (extra chromosome 13) |
| Most common cause | Maternal meiotic nondisjunction (risk ↑ with maternal age) |
| Key findings | Cleft lip/palate, polydactyly, holoprosencephaly, cutis aplasia, congenital heart defects |
| Diagnosis | Screening: cfDNA; Confirmation: CVS/amniocentesis with karyotype/FISH |
| Prognosis | Poor; high neonatal mortality |
| Counseling | Consider parental karyotype if Robertsonian translocation suspected |
USMLE-Style Mini Drill (What They Might Ask)
- Newborn with cleft lip, polydactyly, and severe neurologic abnormalities → which chromosome? 13
- Prenatal cfDNA suggests Trisomy 13 → next step to confirm? CVS or amniocentesis with karyotype
- Multiple affected pregnancies; one parent is phenotypically normal but has a chromosomal rearrangement → mechanism? Robertsonian translocation, check acrocentric chromosomes
First Aid Cross-References (Where to Review)
- Genetics: Autosomal trisomies (Down, Edwards, Patau)
- Genetics: Nondisjunction vs Robertsonian translocations; acrocentric chromosomes (13, 14, 15, 21, 22)
- Repro/OB: Prenatal screening vs diagnostic testing (cfDNA, CVS, amniocentesis)
- Embryology: Cleft lip/palate and midline development concepts