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

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