Which of the following cardiac defects is MOST commonly associated with Down syndrome (Trisomy 21)?

Explanation

## Cardiac Defects in Down Syndrome **Key Point:** Endocardial cushion defects (also called atrioventricular septal defects or AVSD) occur in approximately 40–50% of children with Down syndrome who have congenital heart disease. ### Pathophysiology Endocardial cushion defects result from failure of fusion of the endocardial cushions during weeks 4–7 of embryogenesis. In Down syndrome, the trisomy 21 genotype disrupts normal mesenchymal cell migration and differentiation, predisposing to this specific malformation. ### Clinical Features of AVSD in Down Syndrome - Primum atrial septal defect (ASD) - Inlet ventricular septal defect (VSD) - Cleft mitral and/or tricuspid valve - Left-to-right shunt (acyanotic) - Early onset of pulmonary hypertension and heart failure ### Frequency of Cardiac Defects in Down Syndrome | Defect | Frequency in Down Syndrome | Frequency in General Population | | --- | --- | --- | | Endocardial cushion defect | 40–50% | 1–2% | | Ventricular septal defect | 30–35% | 20–25% | | Atrial septal defect | 10–15% | 10% | | Tetralogy of Fallot | 7–10% | 7–10% | | Patent ductus arteriosus | 5–10% | 5–10% | **High-Yield:** AVSD is so characteristic of Down syndrome that its presence in a newborn should prompt karyotyping or chromosomal microarray testing, even without other clinical features. **Clinical Pearl:** Children with Down syndrome and AVSD require early echocardiographic screening and often need surgical repair in the first 6–12 months of life to prevent irreversible pulmonary vascular disease.