: 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
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 accessory hemiazygos vein
The azygos system is notoriously highly variable . Common variants include: : If the vein is underdeveloped or absent,