Either luminal obstruction or extrinsic compression can impair venous flow to the right atrium. Luminal obstruction can be caused by neoplastic infiltration, or more commonly by thrombosis. Intravascular clot formation can be enhanced by turbulent rheology in the superior vena cava. Hypercoagulation and thrombosis are more likely to result in acute and complete obstructive SVC syndrome. In many cases some component of extrinsic compression is also present and post-mortem examinations frequently reveal that both mechanisms are involved.
Extrinsic compression is more likely to be a gradual process.
The mass may arise from any of the structures in close anatomic proximity to the superior vena cava. Enlargement of the lymph nodes, trachea and mainstem bronchus, aorta, and esophagus can impinge on the superior vena cava. Chronic extrinsic compression provides time for adequate collateral drainage to develop. There are four potential routes for venous drainage to circumvent the SVC from the upper torso and arms. The first available bypass is the azygous vein.