Accessory hemiazygos vein

The accessory (or superior) hemiazygos vein forms part of the azygos system and along with the hemiazygos vein, it is partially analogous to the right-sided azygos vein. It drains the left superior hemithorax.

Gross anatomy
Origin and course
The accessory hemiazygos vein is formed by the confluence of the middle left posterior intercostal veins. It descends to the left of midline, adjacent to the thoracic vertebrae and crosses posteriorly to the aorta at the level of T7-8 to drain into the azygos vein. It normally anastomoses with the left superior intercostal vein.

Tributaries
left 4th-8th posterior intercostal veins
left bronchial veins (variable)
Variant anatomy
drains via a common trunk with the hemiazygos vein into the azygos vein
forms a common trunk with the hemiazygos vein that passes anterior to the aorta called the interazygos vein 2
drains directly into the left brachiocephalic vein (rare) 2
References
Related Radiopaedia articles
Anatomy: Thoracic
thoracic skeleton[+]
muscles of the thorax[+]
spaces of the thorax[+]
thoracic viscera[+]
blood supply of the thorax
arteries[+]
veins
superior vena cava (cavoatrial junction)
variant anatomy[+]
brachiocephalic veins (retro-aortic)[+]
azygos vein (azygos system)
hemiazygos vein
accessory hemiazygos vein
inferior vena cava
coronary veins[+]
pulmonary veins[+]
lymphatics[+]
innervation of the thorax[+]