Dilated unroofed coronary sinus mimicking cor triatriatum in cardiac-type total anomalous pulmonary venous connection

We present a rare case of a patient with a dilated unroofed coronary sinus mimicking cor triatriatum that altered mitral inflow in a cardiac-type total anomalous pulmonary venous connection. The membranous tissue in the left atrium, thought to be cor triatriatum tissue preoperatively, was actually a remnant of a dilated coronary sinus tissue. Although this tissue did not create a significant pressure gradient and merely induced a turbulent mitral inflow, we resected it to avoid future problems with mitral leaflet function.