: In rare cases (1–2%), it may drain directly into the left brachiocephalic 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. : In rare cases (1–2%), it may drain
: It is formed by the confluence of the 4th through 8th left posterior intercostal veins . : It is formed by the confluence of
The (also known as the superior hemiazygos vein ) is a major component of the Azygos venous system . Located in the upper left posterior mediastinum, it serves as the left-sided counterpart to the superior portion of the azygos vein, primarily draining the mid-to-upper thoracic wall. Anatomical Overview