[2][3] The azygos vein transports deoxygenated blood from the posterior walls of the thorax and abdomen into the superior vena cava.

It is formed by the union of the ascending lumbar veins with the right subcostal veins at the level of the 12th thoracic vertebra, ascending to the right of the descending aorta and thoracic duct, passing behind the right crus of diaphragm, anterior to the vertebral bodies of T12 to T5 and right posterior intercostal arteries.

At the level of T4 vertebrae, it arches over the root of the right lung from behind to the front to join the superior vena cava.

This can have clinical significance in any blood flow restriction of the inferior vena cava.

[8] Azygos and hemiazygos continuation of the inferior vena cava (IVC) was not common in daily life.

Thus, it is crucial to diagnose the enlarged azygos vein at the confluence with the superior vena cava and in the retrocrural space to prevent misdiagnosis as a right-sided paratracheal mass.

The loss of the intrahepatic segment of IVC with azygous and hemiazygos continuation happens in 0.6% of patients diagnosed for congenital heart disease and usually occurs simultaneously with situs inversus, asplenia, or polysplenia, persistent left superior vena cava (SVC), and congenital pulmonary venolobar syndrome.