Hemiazygos Vein

Origin

The hemiazygos vein is an unpaired vein that represents the left-sided equivalent of the inferior portion of the azygos vein. It is formed by the union of the left subcostal and ascending lumbar veins.

Course

The hemiazygos vein ascends in the posterior mediastinum, sitting on the left side of the lower four thoracic vertebral bodies. At the level of the eighth thoracic vertebra, the hemiazygos vein crosses posterior to the descending thoracic aorta and esophagus, and either anterior or posterior to the thoracic duct to join the azygos vein on the right side.

The origin, course, tributaries and termination of the azygos system is highly variable. Sometimes, the left side (the hemiazygos and accessory hemiazygos veins) is absent or underdeveloped, meaning that the azygos vein is responsible for drainage of the left posterior intercostal veins. If the accessory hemiazygos drains into the left brachiocephalic vein, the azygos vein assumes a median position in the posterior mediastinum.

Tributaries

The major tributaries of the hemiazygos vein include the ninth to eleventh left posterior intercostal veins, the left subcostal vein, left ascending lumbar vein, esophageal, and sometimes the mediastinal veins.

Structures Drained

Numerous structures are drained indirectly by the hemiazygos vein via its tributaries, including the thoracic and abdominal wall, esophagus and structures of the anterior mediastinum.