Where there is blood, there is a way: unusual collateral vessels in superior and inferior vena cava obstruction.

Obstruction of the superior vena cava (SVC) or inferior vena cava (IVC) is most commonly an acquired condition, typically caused by malignancy, benign conditions such as mediastinal fibrosis, and iatrogenic causes such as venous catheterization. In the event of chronic occlusion, collateral pathways must develop to maintain venous drainage. The major collateral pathways seen with SVC or IVC obstruction are well described and include the azygos-hemiazygos, internal and external mammary, lateral thoracic, and vertebral pathways. In addition, several unusual collateral pathways may be seen with SVC or IVC obstruction; these include systemic-to-pulmonary venous, cavoportal, and intrahepatic collateral pathways. In patients with systemic-to-pulmonary venous collateral vessels, the systemic veins drain directly into the left side of the heart, resulting in a right-to-left shunt. The collateral veins consist of mediastinal connections between the innominate veins and the superior pulmonary veins through bronchial venous plexuses around the airways, hilar vessels, and pleura. The cavoportal collateral pathways consist of collateral formation between the SVC or IVC and a tributary to the portal system. They include the caval-superficial-umbilical-portal pathway, caval-mammary-phrenic-hepatic capsule-portal pathway, caval-mesenteric-portal pathway, caval-renal-portal pathway, caval-retroperitoneal-portal pathway, and intrahepatic cavoportal pathway. These types of collateral pathways may result in unusual enhancement patterns in the liver. An understanding of these unusual collateral pathways is essential in a patient with caval occlusion who presents with signs and symptoms of a right-to-left shunt or has unusual enhancing lesions in the liver.