A case of coronary sinus ostium atresia with muscular obstruction in a complex univentricular heart managed in stages.

We report a rare case of coronary sinus ostium atresia diagnosed due to impaired cardiac function caused by Marshall vein stenosis, which occurred during surgical treatment. Transcatheter perforation of the coronary sinus ostium and stenting of the Marshall vein stenosis improved ventricular function, enabling subsequent surgical enlargement. This staged strategy minimised surgical risks and demonstrated a reliable and effective treatment for such challenging conditions.