Accessory: Hemiazygos Vein

: It may join the hemiazygos vein to form a common trunk before entering the azygos vein.

: If the vein is underdeveloped or absent, the left superior intercostal vein may extend further down to drain those levels. Clinical Significance Accessory Hemiazygos Vein | Complete Anatomy - Elsevier accessory hemiazygos vein

The accessory hemiazygos vein is an unpaired vessel that typically resides on the left side of the vertebral column, parallel to the descending thoracic aorta. : It may join the hemiazygos vein to

: It is formed by the confluence of the 4th through 8th left posterior intercostal veins . accessory hemiazygos vein

: In rare cases (1–2%), it may drain directly into the left brachiocephalic vein .