Accessory Hemiazygos - 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

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

: It is formed by the confluence of the 4th through 8th left posterior intercostal veins . : If the vein is underdeveloped or absent,

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. parallel to the descending thoracic aorta.