The hemiazygos vein is the asymmetric counterpart to the azygos vein and forms part of the azygos venous system.
The spelling hemiazygous when referring to the vein is incorrect, regardless of whether British or American English is used . In the context of anatomy, hemiazygos vein is the sole correct spelling (see Terminologia Anatomica).
The hemiazygos vein is formed by the confluence of the left ascending lumbar and left subcostal veins.
The hemiazygos vein enters the thorax either through the aortic hiatus or directly through the diaphragmatic crura. It then courses superiorly to the left of the midline in the posterior mediastinum, adjacent to the thoracic vertebrae until the level of T8 or T9 vertebral bodies, where it crosses the midline anteriorly to the vertebral column to drain into the azygos vein.
- left posterior 8-11 intercostal veins
- left superior phrenic vein
- left renal vein (occasionally)
- IVC (occasionally)
- hemiazygos continuation of the IVC: often occurs with duplicated IVCs
- interazygos vein: occurs when forming a common trunk with the accessory hemiazygos vein anterior to the aorta