Giant left main coronary artery aneurysm with fistulous drainage to the right atrium: a rare case report and surgical management approach.

Coronary artery aneurysm (CAA) is defined as a focal dilation that exceeds 1.5 times the diameter of adjacent normal coronary segments. Involvement of the left main coronary artery (LMCA), particularly when associated with a fistulous communication to the right atrium (RA), is exceedingly rare. We report a case of a 55-year-old male with a giant LMCA aneurysm and a coronary artery fistula (CAF) draining into the RA, diagnosed via coronary angiography and contrast-enhanced CT angiography. The patient underwent surgical closure of the LMCA-RA fistula. Following discharge, he developed worsening respiratory symptoms, and electrocardiography revealed atrial fibrillation, which later progressed to symptomatic bradycardia. Sick sinus syndrome was diagnosed, and a permanent pacemaker was successfully implanted. This case underscores the critical importance of early diagnosis and definitive surgical repair of LMCA aneurysms with fistulous connections, as well as the need for vigilant monitoring of potential postoperative complications such as arrhythmias, given the hemodynamic implications of left-to-right shunting and subsequent atrial stretch.