Restrictive Superior Vena Cava to Left Atrium Communication With a Large Left-to-Right Shunt: A Previously Undescribed Sinus Venosus Defect Variant.

A 40-year-old woman was referred for cardiac assessment following a syncopal event. Echocardiography showed a dilated right ventricle and subsequent cross-sectional imaging suggested partial anomalous pulmonary venous connection (PAPVC) and a sinus venosus defect (SVD). She reported exertional dyspnoea, fatigue, paroxysmal nocturnal dyspnoea, longstanding palpitations, and documented runs of atrial tachycardia. Transoesophageal echocardiography demonstrated a dilated right ventricle and pulmonary arteries, mild tricuspid regurgitation, anomalous connection of the right upper and right middle pulmonary veins to the right superior vena cava (SVC), and a persistent left SVC draining to a dilated coronary sinus. Unusually, the interatrial communication consisted of only a small, restrictive fenestration between the SVC and the left atrium. Invasive haemodynamics confirmed a large left-to-right shunt (Qp:Qs 3.3:1) with normal pulmonary artery pressure (mean 19 mmHg). Balloon interrogation demonstrated a restrictive communication and prolapse of the compliant balloon into the pulmonary veins with apparent obstruction, indicating unsuitability for transcatheter correction; the patient underwent surgical repair. We report a previously undescribed SVD in which the interatrial communication is restrictive despite a large shunt carried by the anomalous venous connection and discuss its implications for transcatheter versus surgical decision-making.