Obstruction of the superior vena cava (SVC) may lead to the formation of portosystemic venous shunting leading to increased blood flow in the quadrate lobe (segment IV) of the liver as seen on contrast-enhanced CT (CECT) and also classically seen as the “hotquadrate lobe” sign on 99mTc–sulfur colloid scans of the liver. Collateral flow from the subclavian vein goes to the superior epigastric vein via the internal mammary vein into the recanalized paraumbilical vein, which in turn drains into the left portal vein creating a characteristic wedge-shaped area of enhancement in the quadrate lobe during the arterial phase of the CT (Figures 94.1–94.5). Blood is shunted to the IVC via the hepatic veins ultimately reaching the right atrium. The internal mammary vein also drains into inferior phrenic vein and subcapsular hepatic veins, which in turn drain into the intrahepatic portal tributaries. This pathway causes subdiaphragmatic focal enhancement near the bare area of the liver (Figures 94.4, 94.5, and 94.6). Additionally, the azygos and the hemiazygos veins may also become dilated and tortuous as a result of diversion of blood from the superior to the inferior vena cava (Figure 94.6).
Other collateral pathways for SVC obstruction include the lateral thoracic and superficial thoracoabdominal venous route, and the vertebral venous plexus route.
Causes of obstruction of the SVC include compression by a mediastinal neoplasm, post-radiation fibrosis, mediastinitis, or thrombosis and stenosis due to presence of indwelling catheters. Cavoportal collaterals due to SVC obstruction may be seen as a hypervascular lesion on a contrast-enhanced CT and can be mistaken for a tumor. This could expose the patient to risks of unnecessary imaging and/or invasive testing such as a biopsy. Presence of collateral vessels seen on CT is a highly accurate predictor of SVC obstruction with a sensitivity of 96% and a specificity of 92%.
Typical clinical scenario
SVC obstruction may be seen in patients with mediastinal masses, post-radiation fibrosis, or long-term indwelling catheters. A typical CT would show absence of contrast in the superior vena cava distal to the site of obstruction with luminal thrombus or stenosis, combined with enhancement of the collateral vessels.