Persistent left superior vena cava Bilateral superior vena cava 2020-12-23T14:20:56Z The thoracic embryonic venous system is composed of two large veins (the superior cardinal veins) which return blood from cranial aspect of embryo, and the inferior cardinal vein, which returns blood from the caudal aspect. Both pairs of veins join to form right and left common cardinal veins before entering the embryological heart. The left common cardinal vein persists to form coronary sinus and oblique vein of left atrium. During the 8th week of gestation, an anastomosis forms between right and left superior cardinal veins resulting in the innominate (or brachiocephalic) vein. The cephalic portion of superior cardinal veins form the internal jugular veins. The caudal portion of right superior vein forms the normal right-sided superior vena cava, while the portion of the left superior cardinal vein caudal to the innominate vein normally regresses to become the ligament of Marshall. If this normal regression of the left superior cardinal vein fails to occur, a persistent left-sided vascular structure that empties into the coronary sinus, results (Persistent left superior vena cava or PLSVC). The innominate vein may or may not degenerate in these cases leading to variations in anatomy. The most common subtype of PLSVC results in the presence of both left and right SVCs (Bilateral SVC). A bridging innominate vein may or may not be present. Bilateral SVC The presence of a left and a right superior vena cava.