In PLSVC, the left brachiocephalic vein does not develop fully and the left upper limb and head & neck drain into the right atrium via the coronary sinus. The variation, in insolation, is considered benign, but is very frequently associated with cardiac abnormalities (e.g. ventricular septal defect, atrioventricular septal defect) that have a significant mortality and morbidity. It is more frequent in patients with congenital heart defects.