The azygos vein transports deoxygenated blood from the posterior walls of the thorax and abdomen into the superior vena cava vein. The anatomy of this blood vessel can be quite variable. In some rare variations, for example, it also drains thoracic veins, bronchial veins, and even gonadal veins. The vein is so named because it has no symmetrically equivalent vein on the left side of the body. The azygos system is considered to be the azygos vein located from rib number 2 to rib number 4, while on the left part of the body, the hemiazygos vein and the accessory hemiazygos vein together form the analogous venous system. 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 in the posterior mediastinum, and arching over the right main bronchus posteriorly at the root of the right lung to join the superior vena cava. This "arch of the azygos vein" is an important anatomic landmark. As an anatomical variation in 1-2% of the population, the arch can be displaced laterally, thereby creating a pleural septum separating an azygos lobe from the upper lobe of the right lung. A major tributary is the hemiazygos vein, a similar structure on the opposite side of the vertebral column. Other tributaries include the bronchial veins, pericardial veins, and posterior right intercostal veins. It communicates with the vertebral venous plexuses. The origin and anatomical course of the azygos vein are quite variable. Usually, there is a singular azygos vein on the right side of the body. However, the azygos vein is occasionally located in the midline or two independent veins may be present like in early embryonic development. The azygos vein generally begins at the level of the lumbar vertebrae but may originate above the point in some cases. The lumbar aspect of the azygos vein can ascend anteriorly to the lumbar vertebrae and pass behind the right crus of the diaphragm or cross the aortic hiatus to the right of the cisterna chyli, a dilated lymph sac. The common trunk of the right ascending lumbar vein and the right subcostal vein join the azygos vein anterior to the body of T12. However, if the lumbar segment is absent, this trunk may form the azygos vein. It has been proposed that the azygos vein develops by originally draining to the posterior cardinal vein and then to the longitudinal venous channel. Following retrogression of the left common cardinal vein, the left azygos vein loses contact with the posterior cardinal vein. Thus, the blood drains into the right azygos line.
Function
The azygos system of veins is considered to be the azygos vein, along with its left-sided counterparts, the hemiazygos vein and the accessory hemiazygos vein. It also creates a cavo-caval anastomosis by offering an alternative, collateral blood flow from the lower half of the body to the superior vena cava, bypassing the inferior vena cava. This can have clinical significance in any blood flow restriction of the inferior vena cava.
Clinical significance
Azygos vein abnormalities can be suggested on chest radiograph by enlargement of the azygos shadow to greater than 1 cm. False positives can occur in heart failure causing increased pressures on the right side of the heart, or adjacent lymphadenopathy. Azygos and hemiazygos continuation of the inferior vena cava was not common in daily life. It is very hard to observe, particularly when it is not associated with congenital heart disease or deep venous thrombosis. 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, and congenital pulmonary venolobar syndrome. Azygos and hemiazygos continuation of the IVC is rare especially when it is not associated with congenital heart disease.
History
The Greek rootzyg refers to a pair. 'A-' means not. Thus, azygos means unpaired. The azygos vein is unpaired in that there is only one in the body, mostly on the right side. While there is the hemiazygos vein and its accessory on the left side of the body, they are considered tributaries of the azygos vein rather than its left-side equivalent. This terminology is only accurate in some species, such as the human, dog, and cat. Ruminants have paired azygos veins.
