A wide variety of condition can cause superior vena cava obstruction including thrombosis, external compression, and tumor invasion. The development of a collateral pathway is necessary to maintain venous flow. Stanford’s classification describes the relationship between the degree of SVC obstruction and the subsequent collateral flow:

• type I: partial SVC obstruction up to 90%, the blood drains via the right superior intercostal and left accessory hemizygous veins into the azygos and hemizygous systems where it flows antegrade into the right atrium.
• type II: nearly complete blockage 90-100-% with antegrade azygos flow, blood drains via the right superior intercostal and left accessory hemizygous veins into the azygos and hemizygous systems where it flows antegrade into the right atrium.
• type III: nearly complete blockage 90-100-% with retrograde azygos flow, blood flows retrograde in the azygos system into the iliac veins and inferior vena cava, internal mammary veins, paravertebral and chest-wall channels.
• type IV: complete blockage of the superior vena cava and the azygos system, blood flows through the internal mammary vein and lateral thoracic veins into inferior and superficial epigastric veins and reaches inferior vena cava.