: The vein descends along the left side of the 4th to 8th thoracic vertebrae. At approximately the level of the 7th or 8th thoracic vertebra (T7-T8) , it crosses the midline to the right, passing posterior to the aorta and esophagus, to drain into the azygos vein. Tributaries : Left 4th to 8th posterior intercostal veins. Left bronchial veins (variable). Some esophageal and mediastinal veins. Anatomical Variants
The azygos system is notoriously highly variable . Common variants include:
: It is formed by the confluence of the 4th through 8th left posterior intercostal veins .
: In rare cases (1–2%), it may drain directly into the left brachiocephalic 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
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.
