The permanent left superior vena cava is the result of abnormal degeneration of the left anterior main vein, which begins at the junction of the left jugular vein with the left subclavian vein, travels anterior to the aortic arch and left pulmonary artery and to the lateral border of the left atrium, and drains into the coronary sinus in most cases and into the left atrium in rare cases because the coronary sinus has no apex. Embryonic development: In the seventh week of embryonic development, the anterior main veins on the left and right sides are joined by a vascular plexus to form an oblique vessel, the innominate vein. As the left innominate vein develops and forms, the left anterior main vein and Cuvier’s canal gradually occlude, the left superior vena cava degenerates, leaving a fibrous ligament, the Marshall’s ligament, and the right anterior main vein forever develops with Cuvier’s canal into the normal right superior vena cava. If the left anterior main vein degenerates abnormally and does not close, a permanent left superior vena cava is formed. Pathological typing: The permanent left superior vena cava is classified according to the entrance location: a. It drains into the coronary sinus, which is the most common type, accounting for 80-90% of cases; b. The permanent left superior vena cava drains into the left atrium. Morbidity, combined malformations and prognosis: The permanent left superior vena cava is the most common variant of the thoracic venous system, with a reported incidence of 0.3%-0.5% in the population and a high incidence of 5%-9% in infants with combined congenital heart disease and 9% in fetuses with congenital heart malformations. Common combined malformations of the permanent left superior vena cava include left ventricular outflow tract obstruction, conus arteriosus truncus malformation, and visceral ectopic syndrome. Absence of the right superior vena cava can occur in conjunction with a permanent left superior vena cava. The prognosis of the left superior vena cava alone is good, and most of them have no specific clinical manifestations after birth. When the left superior vena cava drains into the left atrium, clinical cyanosis may develop due to decreased oxygen saturation.