Hemiazygos Vein

The hemiazygos vein is a prominent vein located on the left side of the posterior thoracic wall. It serves as part of the azygos venous system, which provides drainage for the thoracic wall and forms a crucial collateral pathway between the superior and inferior vena cava. The hemiazygos vein mainly drains the lower left intercostal spaces, esophagus, and mediastinal structures before crossing to join the azygos vein on the right side.

Origin

The hemiazygos vein typically originates in the abdomen from the confluence of the left ascending lumbar vein and the left subcostal vein. In some cases, it may arise directly from the posterior surface of the left renal vein.

Course

After forming in the abdomen, the hemiazygos vein enters the thoracic cavity by passing through the left crus of the diaphragm or the aortic hiatus. It ascends on the left side of the vertebral column from around the T12 to T8 or T9 vertebral levels. At this level, it typically crosses over to the right, posterior to the aorta, esophagus, and thoracic duct, and empties into the azygos vein.

Tributaries

The hemiazygos vein receives blood from the following sources:

• 9th to 11th left posterior intercostal veins
• Left subcostal vein
• Esophageal veins
• Mediastinal veins
• Inferior hemiazygos communicating veins (when present)

Termination

At approximately the level of the T8 or T9 vertebra, the hemiazygos vein crosses to the right side behind the aorta, thoracic duct, and esophagus to join the azygos vein, which then continues upward to drain into the superior vena cava.

Relations

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

Function

The hemiazygos vein drains the lower left posterior thoracic wall and serves as part of the collateral venous return to the superior vena cava, especially if there is obstruction or compression of the inferior vena cava.

Anatomical Variations

• The hemiazygos vein may be incomplete or absent in some individuals
• In some cases, it may communicate with the left renal vein or continue as a single trunk with the accessory hemiazygos vein

Histological Structure

Like other large veins, the hemiazygos vein has a thin wall with a large lumen. Its structure includes:

• Tunica intima: Endothelium with a thin subendothelial layer
• Tunica media: Sparse smooth muscle
• Tunica adventitia: Thick outer layer with collagen fibers and vasa vasorum

Clinical Significance

• Collateral circulation: In cases of inferior vena cava obstruction, the hemiazygos vein provides a vital alternate pathway to the superior vena cava via the azygos system
• Imaging: An enlarged hemiazygos vein may be visible on CT or MRI in cases of SVC/IVC obstruction or portal hypertension

Topographic Summary

• Origin: Left ascending lumbar vein and left subcostal vein
• Entry into thorax: Via left crus or aortic hiatus of diaphragm
• Termination: Joins azygos vein around T8–T9
• Drainage area: Lower left intercostal spaces, posterior thoracic wall, esophagus, mediastinum