Accessory Hemiazygos Vein - Location, Function

The accessory hemiazygos vein is a vertically oriented vein on the left side of the posterior thoracic wall. It is part of the azygos venous system and complements the hemiazygos vein by draining the mid-thoracic intercostal spaces. Like its counterpart, it plays a role in venous drainage and collateral circulation between the superior and inferior vena cava.

Origin

The accessory hemiazygos vein typically begins as a continuation of the 4th or 5th left posterior intercostal vein. It descends vertically along the left side of the thoracic vertebral bodies, situated between the aortic arch above and the hemiazygos vein below.

Course

The vein runs inferiorly on the left side of the vertebral column from approximately T4 to T8. It may cross to the right side posterior to the thoracic aorta, esophagus, and thoracic duct to join the azygos vein directly, or it may first join the hemiazygos vein.

Termination

The accessory hemiazygos vein usually crosses the vertebral column at the level of T7 or T8 and drains into the azygos vein. In some individuals, it may merge with the hemiazygos vein before the crossover.

Tributaries

4th to 8th left posterior intercostal veins
Left bronchial veins (occasionally)
Esophageal veins (minor contributions)

Relations

Anterior: Descending thoracic aorta and esophagus
Posterior: Vertebral bodies
Medial: Thoracic duct (at crossover)
Lateral: Left pleura and lung

Function

The accessory hemiazygos vein drains the upper and middle portions of the left posterior thoracic wall. It contributes to the collateral circulation between the superior and inferior vena cava via its connections with the azygos and hemiazygos veins.

Anatomical Variations

Sometimes absent or fused with the hemiazygos vein, forming a single hemiazygos trunk
May have communication with the left superior intercostal vein or with thoracic wall venous plexuses

Histological Structure

Like other systemic veins, it has a large lumen and a relatively thin wall composed of:

Tunica intima: Endothelium and sparse connective tissue
Tunica media: Thin smooth muscle layer
Tunica adventitia: Thick connective tissue layer with vasa vasorum

Clinical Significance

Collateral drainage: It serves as an important route if there is obstruction of the superior vena cava or inferior vena cava
Imaging: Can become enlarged and visualized on CT scans in certain vascular disorders or mediastinal masses

Topographic Summary

Origin: Continuation of upper left posterior intercostal veins (4th–5th)
Course: Descends along vertebral column (T4–T8)
Termination: Joins azygos vein or hemiazygos vein around T7–T8
Drainage: Mid-left thoracic wall, posterior intercostal spaces