Distal coronary sinus-left atrial fistulous communication uncovered during atrial fibrillation ablation.

Background: Congenital anomalies of the coronary sinus (CS) are rare and often overlooked on standard imaging. Among these, the unroofed coronary sinus (URCS) is the most recognized, typically involving the proximal CS and frequently associated with a persistent left superior vena cava. In contrast, fistulous communications between the distal CS and the left atrium (LA) are exceptionally uncommon. Their recognition is important during atrial fibrillation (AF) ablation, where unanticipated channels may complicate catheter manipulation and lesion delivery. Case summary: A 76-year-old woman with recurrent paroxysmal AF underwent catheter ablation. A lumen-equipped multipolar catheter was positioned in the CS, and transseptal access to the LA was achieved. During electroanatomical mapping with a high-density catheter, entanglement with the CS catheter suggested an anomalous connection. Contrast injection through the CS catheter opacified the left atrial appendage (LAA), and selective angiography revealed simultaneous filling of the LAA and the left inferior pulmonary vein (LIPV), confirming a fistulous tract between the distal CS, LAA, and LIPV. Pulmonary vein isolation was completed without complication. Retrospective review of pre-procedural computed tomography demonstrated a connecting vein from the distal CS to the LA at the LAA-LIPV ridge, corroborating the procedural findings. Discussion: This case highlights a rare distal CS-LA fistulous communication involving the LAA and LIPV. It broadens the anatomical spectrum between URCS and partial anomalous pulmonary venous connection. Recognition of these anomalies is critical for procedural safety during AF ablation, underscoring the value of multimodality imaging and intra-procedural venography.