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 4^th-8^th 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