The azygos venous system is located on either side of the vertebral column and drains the viscera within the mediastinum, as well as the back and thoracoabdominal walls. This system consists of the azygos vein and its two main tributaries: the hemiazygos vein and the accessory hemiazygos vein. This article will discuss the anatomical course, relations and variations of the azygos vein as well as its tributaries. This will be followed by a discussion of any related clinical pathology.
The azygos vein usually originates from the posterior aspect of the inferior vena cava, at the level of the renal veins. It ascends within the posterior mediastinum to the level of T4 before it arches above the right pulmonary hilum. It drains into the superior vena cava just before it pierces the pericardium.
Relationships of the azygos vein to other structures within the mediastinum can be important in clinical practice. The following structures are located posterior to the azygos vein:
- the anterior longitudinal ligament
- right posterior intercostal arteries
- the bodies of T4-T12
In the posterior thorax, the azygos vein is located posterior to the recess of the right pleural sac and the oesophagus.
Medial to the vein one can find:
- the thoracic duct
- right vagus nerve
The following structures are located laterally to the axygos vein:
- the right greater splanchnic nerve
The azygos vein is also in close proximity to the right posterolateral aspect of the descending thoracic aorta. Pulsations within the aorta may aid venous return in the azygos vein, as well as the hemiazygous vein.
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 usually originates at the level of the lumbar vertebrae, but may originate further up. The lumbar aspect of the azygos vein can ascend anterior to the lumbar vertebrae and pass behind the right crus of the diaphragm or cross the aortic hiatus (where the aorta pierces the diaphragm) 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 joins the azygos vein anterior to the body of T12. However, if the lumbar segment is absent, this trunk may form the azygos vein.
The azygos vein has two tributaries, which are referred to as the hemiazygos vein and the accessory hemiazygos vein.
The hemiazygos vein is often connected to the left renal vein. It is formed by the oesophageal and mediastinal tributaries, the common trunk of the left ascending lumbar vein and left subcostal vein, and by the lower three posterior intercostal veins. It ascends anterior to the vertebral column before crossing the column posterior to the aorta, oesophagus and thoracic duct at the level of T8.
The accessory hemiazygos vein is formed by veins from the fourth to eighth intercostal spaces and sometimes by the left bronchial veins. It descends to the left of the vertebral column before crossing T7, where it joins with the azygos vein. Sometimes it joins the hemiazygos vein and, in this case, their common trunk drains into the azygos vein.
Laceration of the azygos vein may occur in blunt trauma to the thorax as a result of a motor vehicle accident or a fall from a height. Rupture of the vein usually occurs at its arch, just proximal to where it joins the superior vena cava. Damage to the vein can result in a haemothorax, a collection of blood in the pleural space, which is usually detected by an X-ray. If this occurs, a thoracotomy is usually indicated, which involves an incision to the chest wall to allow drainage of the blood within the pleural space.
An aneurysm of the azygos vein is very rare and usually causes no symptoms. They are usually detected on a chest X-ray as an abnormal mediastinal mass. They can occur as a result of inferior vena cava obstruction, portal hypertension and congestive heart failure. Surgical intervention is indicated when there is a risk of rupture or pulmonary embolism.
Rarely, the hepatic portion of the inferior vena cava does not form during embryonic development and as a result the inferior vena cava continues as the azygos vein. This is referred to as azygos continuation of the inferior vena cava or absence of the hepatic segment of the inferior vena cava with azygos continuation. To compensate for this congenital abnormality, the azygos vein dilates in order to accommodate for the increase in blood flow. It occurs in 0.3% of the population and is associated with congenital heart disease or asplenia syndromes (syndromes with an absence of normal spleen function).