From Incidental Cardiomegaly to a Rare Right Coronary Artery-to-Coronary Sinus Fistula: Unravelling the Cause of Left Ventricular Dilatation

We present the case of an asymptomatic 68-year-old woman in whom mild cardiomegaly identified incidentally on chest radiography initiated a stepwise investigation for otherwise unexplained left ventricular dilatation.Transthoracic echocardiography demonstrated mild left ventricular dilatation with preserved systolic function, a markedly dilated right coronary artery, a prominent coronary sinus, and abnormal continuous left-to-right flow.However, the anatomical origin of the suspected shunt remained uncertain.Cardiac magnetic resonance imaging showed borderline increased left ventricular end-diastolic volume with preserved systolic function and a pulmonary-to-systemic flow ratio (Qp: Qs) of 1.7:1, confirming a haemodynamically relevant left-to-right shunt but failing to identify its anatomical source.Persistent investigation with cardiac computed tomography angiography ultimately demonstrated a markedly dilated and tortuous dominant right coronary artery draining through a narrow aperture into a dilated coronary sinus, establishing the diagnosis of a right coronary artery-to-coronary sinus fistula.Despite the shunt and mild left ventricular dilatation, the patient remained asymptomatic with preserved biventricular function and no evidence of myocardial fibrosis or ischaemia.Following specialist congenital cardiac multidisciplinary review, conservative management with interval imaging surveillance was recommended.This rare case highlights the importance of pursuing the underlying cause of otherwise unexplained cardiac chamber dilatation.It demonstrates the complementary roles of multimodality cardiovascular imaging when the physiological significance of a shunt is apparent, but its anatomical source remains uncertain.