CORONARY OSTIAL STENOSIS AND ATRESIA (COSA) COSA is a rare congenital coronary anomaly which affects the left coronary artery more frequently than the right coronary artery. The aortic ostium in a case of COSA can be in the normal location or at an ectopic site. Atresia of the left main coronary artery is characterised by the congenital absence of the left main coronary ostium and left main trunk with normally connected left anterior descending and circumflex arteries which proximally end blindly. These patients are able to survive via congenitally developed collateral circulation from either the conus branch artery (Vieussens’ anastomotic ring) or through the right coronary artery (retroaortic Kugel’s ring), anterior ventricular branches of the right coronary artery or through the terminal ramifications of the posterior descending branch with retrograde flow to the terminal branch of the left anterior descending artery at the apex of the left ventricle.
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