The azygos vein (azygos: from the Greek meaning “which is not even”), also called the great azygos vein, is a vein located in the thorax.

Position. The azygos vein and its branches are located at the level of the upper lumbar region, as well as at the level of the chest wall.

Structure. The azygos vein is the main vein of the azygos venous system. The latter is divided into two parts:

• a straight part comprising the azygos vein or great azygos vein;
• a left part comprising the small azygos or hemiazygous veins, composed of the hemiazygous vein, or lower hemiazygous vein, and the accessory hemiazygous vein, or upper hemiazygous vein. (1) (2)

Vveine azygos

Origin. The azygos vein takes its origin at the height of the 11th right intercostal space, and from two sources:

• a source consisting of the union of the right ascending lumbar vein and the 12th right intercostal vein;
• a source formed either by the posterior surface of the inferior vena cava, or by the right renal vein.

Path. The azygos vein rises along the front face of the vertebral bodies. At the level of the fourth dorsal vertebra, the azygos vein curves and forms an arch to join the superior vena cava.

Branches. The azygos vein has several collateral branches which will join it during its journey: the last eight right posterior intercostal veins, the right superior intercostal vein, the bronchial and esophageal veins, as well as the two hemiazygous veins. (1) (2)

Hemiazygous vein

Origin. The hemiazygous vein arises at the height of the 11th left intercostal space, and from two sources:

• a source consisting of the union of the left ascending lumbar vein and the 12th left intercostal vein;
• a source consisting of the left renal vein.

Route. The hemiazygous vein travels up the left side of the spine. It then joins the azygos vein at the level of the 8th dorsal vertebra.

Branches. The hemiazygous vein has collateral branches which will join it during its journey: the last 4 or 5 left intercostal veins. (1) (2)

Accessory hemiazygous vein

Origin. The accessory hemiazygous vein drains from the 5th to the 8th left posterior intercostal vein.

Path. It descends on the left face of the vertebral bodies. It joins the azygos vein at the level of the 8th dorsal vertebra.

Branches. Along the route, collateral branches join the accessory hemiazygous vein: bronchial veins and middle esophageal veins.1,2

The azygos venous system is used to drain venous blood, poor in oxygen, from the back, chest walls, as well as abdominal walls (1) (2).

Phlebitis. Also called venous thrombosis, this pathology corresponds to the formation of a blood clot, or thrombus, in the veins. This pathology can lead to various conditions such as venous insufficiency (3).

Venous insufficiency. This condition corresponds to a dysfunction of a venous network. When this occurs in the azygos venous system, the venous blood is then poorly drained and can impact the entire blood circulation (3).

Medical treatment. Depending on the pathology diagnosed, certain drugs may be prescribed such as anticoagulants, or even antiaggregants.

Thrombolyse. This test consists of breaking up the thrombi, or blood clots, using drugs. This treatment is used during myocardial infarction.

Physical examination. First, a clinical examination is performed to assess the symptoms perceived by the patient.

Medical imaging exam. In order to establish or confirm a diagnosis, a Doppler ultrasound or a CT scan can be performed.

Description of the azygos vein. Bartolomeo Eustachi, 16th century Italian anatomist and physician, described many anatomical structures including the azygos vein. (4)