Superior vena cava (SVC) syndrome is a clinical condition that occurs as a result of obstruction of the SVC, leading to interrupted venous return from the head, thorax, and upper extremities to the right atrium. The increased venous pressure results in edema of the head, neck, and arms, often with cyanosis, plethora, and distended subcutaneous vessels.  It can be caused by either intraluminal obstruction of the SVC or extrinsic compression.
Preazygous or supra-azygous
Postazygous or infra-azygous
Obstruction below the entrance of azygos vein into the SVC results in retrograde flow through the azygos via collaterals to the inferior vena cava, resulting in not only the symptoms and signs of preazygous disease, but also dilation of the veins over the abdomen.
This is usually more severe and poorly tolerated than preazygous obstruction.
Supra- and infra-azygous obstruction leading to superior vena cava (SVC) syndrome. IVC: inferior vena cava
Reproduced with permission from Braunwald's Heart Disease, 8th ed (2008)
Luminal obstruction (e.g., pacemaker leads or catheter-related thrombosis).
Extrinsic compression (e.g., malignancy, fibrosing mediastinitis due to infection/radiation, aortic arch aneurysm, hematoma, or goiter).