Which clinical feature best distinguishes Down syndrome (trisomy 21) from Edwards syndrome (trisomy 18)?

Explanation

## Distinguishing Down Syndrome from Edwards Syndrome ### Key Discriminating Features **Key Point:** Down syndrome (trisomy 21) presents with characteristic facial dysmorphism including flat facies, upslanting palpebral fissures, and a single palmar crease — features that are relatively **absent or mild** in Edwards syndrome. ### Comparative Table: Trisomy 21 vs Trisomy 18 | Feature | Down Syndrome (Trisomy 21) | Edwards Syndrome (Trisomy 18) | | --- | --- | --- | | **Facial features** | Flat facies, upslanting palpebral fissures, low-set ears | Prominent occiput, micrognathia, low-set ears | | **Hand findings** | Single palmar crease, sandal gap | Clenched fists, overlapping fingers (index over third) | | **Feet** | Sandal gap (wide space between 1st and 2nd toe) | Rocker-bottom feet (convex sole) | | **Cardiac defects** | AV canal defect (40%), VSD | VSD, PDA, complex lesions | | **Survival** | Many survive to adulthood | Most die in infancy (median ~5 days) | | **Intellectual disability** | Mild to moderate | Severe | | **Growth** | Normal birth weight, later growth delay | Severe intrauterine growth restriction | **High-Yield:** The **single palmar crease** and **upslanting palpebral fissures** are pathognomonic for Down syndrome and are rarely seen in Edwards syndrome. These two features together form a strong discriminator. ### Clinical Pearl Edwards syndrome (trisomy 18) is characterized by a distinctive **"clenched fist" appearance with overlapping fingers** (index finger overlaps the third finger) and **rocker-bottom feet** — these are NOT typical of Down syndrome, where the hand posture is more relaxed and the feet show a sandal gap. ### Why Down Syndrome Facial Features Stand Out The combination of: 1. Flat facies (reduced nasal bridge projection) 2. Upslanting palpebral fissures (lateral canthi higher than medial) 3. Single palmar crease (instead of two) ...is virtually **pathognomonic for trisomy 21** and allows rapid clinical differentiation from other aneuploidies.