Coronary aneurysm secondary to coronary fistula is a rare condition, with no existing report on its pathological examination. We report the case of a patient diagnosed with a right coronary artery fistula with coronary aneurysm during the fetal period. During follow-up after delivery, the aneurysm became larger, even though the shunt size decreased. We were afraid the aneurysm would rupture and therefore, planned elective catheter embolization. At the age of 4 years, the patient underwent surgery, which involved closing the fistula and making the lumen of the aneurysm smaller. However, the surgery was not catheter embolization as planned because segment 3 branched off from the largest aneurysm where we planned to embolize. Pathologically, the structure of the coronary artery differed from that of a healthy one, with thickened intima and media, fewer scattered smooth muscle cells, widely distributed elastic fibers, and mucoid degeneration in the media. The structure of the coronary artery suggested that the vessel wall was weak and that the aneurysm would rupture if not treated. Postoperative coronary angiography showed that segment 2 was obstructed, while the collaterals from the left coronary artery perfused the area. We could have treated the fistula with a catheter as scheduled.
Keywords: collateral; coronary aneurysm; coronary fistula; pathological examination.