We report the case of a 77-year-old patient admitted for worsening dyspnea, palpitations and dizziness. The electrocardiogram (ECG) showed atrial fibrillation (AF) and left bundle branch block (LBBB). Clinical examination showed hypoxemia (SaO 87% room air), jugular venous distension, pitting edema, arrhythmic heart sounds, a splitting of the second heart sound and systolic murmur (III–IV/VI) in the tricuspid area. Transthoracic echocardiography (TTE) showed right heart dilation, moderate tricuspid regurgitation, biventricular hypertrophy, and a dilated coronary sinus (CS). Agitated saline injection in the left basilar vein raised a suspicion of persistent left superior vena cava (PLSVC) to coronary sinus fistula. Transesophgaeal ecocardiography (TOE) showed a superior sinus venosus defect, with bidirectional shunt, and persistence of LSVC. CT angiography confirmed and detalied the malformation features. A low suspicion for congenital defects in older adults presents a significant diagnostic challenge. Multimodality cardiac imaging is critical in determining the etiology of right heart dilation, and knowledge of the anatomy and physiology of various shunt lesions is essential for clinicians.