Corpus: Accessory hemiazygos vein

The accessory hemiazygos vein is an unpaired vein that runs along the left side of the thoracic vertebral bodies and is part of the azygos venous system. It drains blood from the superior intercostal veins.

The accessory hemiazygos vein typically collects blood from the posterior intercostal veins of the 4th to 7th intercostal spaces, situated between the left superior intercostal vein and the highest tributary of the hemiazygos vein. It often drains into the hemiazygos vein at the level of the 7th intercostal space or directly into the azygos vein. Cranially, it may anastomose with the superior intercostal vein, creating a connection to the left brachiocephalic vein.

There are numerous anatomical variations, with the size of the accessory hemiazygos vein inversely related to that of the superior intercostal vein. In some cases, when the accessory hemiazygos vein is smaller, the left superior intercostal vein may extend as far down as the 5th or 6th intercostal space.