A 2-month-old Indian infant presents with severe developmental delay, seizures, and multiple congenital anomalies. Karyotype shows trisomy 13. Which finding best distinguishes Patau syndrome from Down syndrome?

Explanation

## Patau Syndrome (Trisomy 13) vs Down Syndrome: Key Discriminators ### Central Nervous System Malformations: The Critical Differentiator **Key Point:** **Holoprosencephaly** (failure of the forebrain to divide into two hemispheres) is the **pathognomonic CNS finding** of Patau syndrome and is **virtually absent in Down syndrome**. This single feature, combined with cleft lip/palate and polydactyly, is the best discriminator. ### Comparative Table: Trisomy 13 vs Trisomy 21 | Feature | Patau Syndrome (Trisomy 13) | Down Syndrome (Trisomy 21) | | --- | --- | --- | | **CNS malformations** | **Holoprosencephaly** (50–70%), microcephaly | Normal brain structure; developmental delay is functional | | **Facial clefts** | Cleft lip/palate (60–80%) | Rare; no cleft palate | | **Polydactyly** | **Postaxial polydactyly** (60–80%) | Absent | | **Cardiac defects** | VSD, PDA, complex lesions | AV canal defect (40%), VSD | | **Renal anomalies** | Cystic kidneys, hydronephrosis | Rare | | **Ocular defects** | Microphthalmia, coloboma, cyclopia (in holoprosencephaly) | Refractive errors, strabismus | | **Survival** | Median 7–10 days; 90% die by 1 year | Many survive to adulthood | | **Intellectual disability** | Severe (profound) | Mild to moderate | ### High-Yield: The "Holoprosencephaly Triad" of Trisomy 13 **Mnemonic: HCP** - **H**oloprosencephaly (CNS) - **C**left lip/palate (orofacial) - **P**olydactyly (limbs) When all three are present in a severely affected neonate, **trisomy 13 is the diagnosis**. ### Clinical Pearl Holoprosencephaly in Patau syndrome can manifest as: - **Alobar holoprosencephaly** (most severe): single ventricle, absent corpus callosum, cyclopia or proboscis - **Semilobar holoprosencephaly** (intermediate): partial separation of ventricles - **Lobar holoprosencephaly** (least severe): near-normal brain with midline defects This CNS malformation is **incompatible with prolonged survival** and explains the very short median lifespan in Patau syndrome. ### Why This Distinguishes Patau from Down Down syndrome infants have: - **Structurally normal brains** (no holoprosencephaly, no microcephaly) - **No cleft palate** (cleft lip is rare) - **No polydactyly** - **Functional developmental delay** due to intellectual disability, not structural brain malformation These differences make Patau syndrome clinically far more severe and rapidly fatal.