The accessory hemiazygos vein, also called the superior hemiazygous vein,[1] is a vein on the left side of the vertebral column that generally drains the fourth through eighth intercostal spaces on the left side of the body.

[2] The accessory hemiazygos vein varies inversely in size with the left superior intercostal vein.

The vein usually crosses the body of the eighth thoracic vertebra to join the azygos vein.

Alternatively, it ends in the hemiazygos vein.

[3] When this vein is small, or altogether absent, the left superior intercostal vein may extend as low as the fifth or sixth intercostal space.