The azygos vein pathway: an overview from anatomical variations to pathological changes.

The azygos venous system represents an accessory venous pathway supplying an important collateral circulation between the superior and inferior vena cava. The aim of this article is to revise the wide spectrum of changes ranging from normal to pathological conditions involving the azygos system. Teaching points • The azygos vein is a collateral venous pathway, becoming a vital shunt if major pathways of venous return are obstructed. • In azygos continuation, the azygos vein becomes significantly enlarged due to inferior vena cava interruption. • Fibrosing mediastinitis is an underestimated acquired disorder. • Fibrosing mediastinitis induces a variable engorgement of collateral veins. • Fibrosing mediastinitis leads to superior vena cava syndrome.