The brachiocephalic veins, superior vena cava and azygos-hemiazygos system drain the upper extremities and the head and neck. They are connected by a rich network of smaller venous plexuses that are normally not or only barely opacified on chest CT. On superior vena cava thrombosis the blood flow diverts into this network of collateral channels. 

The development of rich collaterals protect this patient against developing the most severe symptoms of superior vena cava syndrome.