A 5-year-old girl referred for a murmur has wide splitting of S2 and a soft ejection systolic murmur at the left upper sternal border. ECG shows left axis deviation with an incomplete right bundle branch block pattern. Echocardiography reveals an atrial level defect adjacent to the mitral valve with a cleft anterior mitral leaflet. The MOST likely diagnosis is:

• A Secundum atrial septal defect
• B Ostium primum atrial septal defect (partial atrioventricular septal defect) ✓
• C Sinus venosus atrial septal defect
• D Unroofed coronary sinus defect

Explanation

Ostium primum defect lies in the lower atrial septum near the atrioventricular valves, is part of the atrioventricular septal defect spectrum, and characteristically shows a cleft anterior mitral leaflet. Its hallmark ECG is left axis deviation, whereas secundum ASD shows normal or rightward axis. Sinus venosus defects sit near the SVC entry and associate with partial anomalous pulmonary venous connection. Left axis deviation is the single feature that discriminates primum from all other ASD types.

Reference: Park's Pediatric Cardiology for Practitioners, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.