Azygos venous system

Last revised by Yoshi Yu on 8 Apr 2023

The azygos venous system , also known as just the azygos system, is a collective term given to the H-shaped configuration of the azygos, hemiazygosaccessory hemiazygos and left superior intercostal veins.

It is responsible for draining the thoracic wall and upper lumbar region via the lumbar veins and posterior intercostal veins 1. It also provides important collateral circulation between the superior and inferior venae cavae should they become obstructed 2

See also: venous drainage of the thoracic wall.

Spelling it "azygous" when referring to the vein is incorrect, regardless of whether British or American English 6. Azygous is a word in English meaning 'without pair', but in the context of anatomy, see Terminologia Anatomica, azygos vein is the sole correct spelling.

The azygos vein initially arises at the union of the right ascending lumbar vein and the right subcostal vein around T12. It enters the thorax via the aortic hiatus in the diaphragm and ascends posteriorly alongside the vertebral bodies, arching over the right main bronchus at T5-T6 and enters the superior vena cava (SVC) at T4 2. 

The azygos vein receives the lower eight right-sided posterior intercostal veins, as well as the bronchial veins from the right lung. It is joined by the right superior intercostal vein superiorly. Although there is much variability, the azygos vein receives the accessory hemiazygos vein and the hemiazygos vein at the level of T8 and T9 respectively.

The azygos vein contains a valve halfway along the azygos arch (between the vertical azygos vein and the point where the azygos vein enters the SVC). This valve is a common place for the intravenously injected contrast agent to accumulate, which may simulate pathology on CT 3.  

The accessory hemiazygos vein drains the superior left hemithorax. It arises from the 4th to 8th left posterior intercostal veins and lies longitudinally on the left side of the vertebral bodies. It also drains the left bronchial vein and some veins from the esophagus. It joins the azygos vein behind the esophagus at the level of T8.

The hemiazygos vein usually arises from the left ascending lumbar vein and passes through the left crus of the diaphragm. It drains the 9th to 11th left posterior intercostal veins and left subcostal vein, alongside some esophageal veins. The hemiazygos vein ascends the thorax to the left of the vertebral bodies and passes behind the esophagus at the level of T9 to join the azygos vein. 

The azygos venous system arises from the supracardinal veins embryologically. The right supracardinal vein becomes the azygos vein, while the left supracardinal vein becomes the hemiazygos vein 4.

Variants of the azygos system anatomy occur frequently, especially the configuration of the hemiazygos and accessory hemiazygos veins. In some cases, the hemiazygos and accessory hemiazygos vein form a common trunk instead of draining separately into the azygos vein.

The azygos vein may rarely drain into the right brachiocephalic vein, right subclavian vein, intrapericardial SVC or directly into the right atrium 5.

Variation in the azygos venous system may also be acquired. Superior or inferior vena cava obstruction will enlarge the azygos veins due to collateralisation of blood flow. Fibrosing mediastinitis may also cause variable engorgement of collateral veins 2.

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