A<unk> <unk>  is the most common variation of the thoracic venous system, is  prevalent in 0.3% of the population, and an embryologic reminant that results from a failure to involute.
In<unk> , 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.
Persistent left superior vena cava
Persistent left inferior vena cava
Double inferior vena cava
Congenital stenosis of vena cava
Pruned pulmonary vasculature
Aberrant subclavian artery
Persistent thyroglossal duct
Interrupted aortic arch