Case Report. A 32-year-old male suffered cardiac arrest out of hospital, was successfully resuscitated and transferred to our tertiary cardiac unit where we performed coronary angiography. Despite multiple attempts, the left main coronary artery (LMCA) could not be found in the corresponding cusp. The right coronary artery (RCA) was angiographically normal, but with multiple collaterals supplying the anterior septal and distal left anterior descending (LAD) artery territory (Figure 1).

Cardiac arrest was probably driven by severe ischemia of the anterior and posterior wall due to physical exertion. The patient subsequently underwent coronary artery bypass surgery using a left internal mammary artery (LIMA) graft to the LAD and a right internal mammary artery (RIMA) graft to the circumflex artery.
A separate ostium for the conus artery is relatively common, with that vessel then named the “third” coronary artery. Collaterals from the conus branch to the LAD are known as the “Vieussens ring”.¹ This phenomenon can also be observed in chronic occlusions of the proximal LAD. During embryologic development, the nonperfused, immature plexiform blood vessels covering the heart eventually connect with the aortic sinus, giving rise to an early functioning coronary circulation. Failure of this fusion process in the left coronary cusp may result in agenesis of the LMCA. Although LMCA atresia has been previously described,² agenesis is a very rare finding and, to our knowledge, there are no similar cases described in the literature.