hemiazygos

Hemiazygos refers to the left-sided component of the azygos venous system in the posterior mediastinum. It collects blood from the lower left posterior intercostal veins and sometimes from the subcostal veins, ascends along the left side of the thoracic vertebral column, and participates in the midline venous return to the heart.

Typically, the hemiazygos vein begins around the ninth or tenth thoracic vertebral level as a convergence of the left lower posterior intercostal veins. It ascends on the left of the vertebral bodies and crosses to the right to join the azygos vein, usually behind the esophagus or behind the aorta, at about the eighth to ninth thoracic vertebral level. Through this anastomosis, blood reaches the superior vena cava via the azygos system. There is also an accessory or left superior hemiazygos vein that drains the upper left posterior intercostal veins (often the fourth to eighth spaces) and typically crosses to the right to join the azygos vein at a higher level than the main hemiazygos.

Variations are common. A persistent left superior intercostal vein may serve as an accessory channel, and the precise level at which the hemiazygos and accessory hemiazygos join the azygos vein can differ among individuals. The hemiazygos system provides collateral drainage when the primary pathways to the superior vena cava are obstructed, such as in SVC or caval obstruction. Abnormal enlargement or unusual courses may be revealed on imaging and can influence interpretation of chest radiographs or planning of intrathoracic procedures.

Developmentally, the hemiazygos vein and the azygos system arise from the left and right supracardinal veins, respectively, as the thoracic venous network remodels during embryogenesis.