Down Syndrome (Trisomy 21)
The most common chromosomal abnormality and the most frequent genetic cause of intellectual disability. The majority of cases are due to nondisjunction (47, XX or XY, +21).
1. Clinical Features
| System | Characteristic Findings |
|---|---|
| Craniofacial | Flat facial profile, upward-slanting palpebral fissures, epicanthic folds, Brushfield spots. |
| Extremities | Single palmar crease (Simian crease), clinodactyly (incurved 5th finger), wide gap between 1st/2nd toes. |
2. Associated Pathologies
- Cardiac: Endocardial cushion defects (AV canal defect) are the most common congenital heart disease.
- Gastrointestinal: Duodenal atresia (“double bubble” sign), Hirschsprung disease, and annular pancreas.
- Hematological: Increased risk of leukemia (ALL and AML).
3. Screening and Diagnosis
- Antenatal Screening: Nuchal translucency (first trimester), Quad screen (second trimester).
- Gold Standard: Karyotyping (amniocentesis or chorionic villus sampling).
NEET PG Hint: Remember that Atlantoaxial instability is a serious complication; children should be screened before participating in contact sports. Always keep Duodenal atresia in mind if a newborn with suspected Down syndrome presents with bilious vomiting.