Hemi-azygos vein

At a glance

English

Hemi-azygos vein

Latin (TA2)

Vena hemiazyga

System

Vein

Region

Trunk

Side

Single, on the midline

About the hemi-azygos vein

The hemi-azygos vein is the lower of the two left-sided vessels that, together, mirror the azygos. Where the azygos collects most posterior intercostal veins of the right, the hemi-azygos and the accessory hemi-azygos do the same on the left, then hand their blood across to the azygos rather than running up to the superior vena cava themselves. The name means 'half of the unpaired vein', since it copies only the lower part of the azygos.

It usually arises in the upper abdomen from the left ascending lumbar vein, often joined by the left subcostal vein, and it may also have a root from the left renal vein. It enters the chest by piercing the diaphragm's left crus and climbs on the left front of the lower thoracic vertebral bodies. At about the eighth or ninth thoracic vertebra it turns to the right and crosses in front of the spine, passing behind the aorta, oesophagus and thoracic duct, and empties into the azygos.

Its tributaries are the lower three left posterior intercostal veins (from the ninth to the eleventh spaces), the left subcostal vein, and small oesophageal and mediastinal veins. Above, a link to the accessory vein may or may not exist; the arrangement of the whole left-sided system varies widely.

In some people the inferior vena cava fails to form its segment within the liver, and blood from the lower body returns instead by an enlarged azygos or hemi-azygos channel, the latter called hemi-azygos continuation. On plain films or scans this widened vein can mimic a mediastinal or retroperitoneal mass, enlarged lymph nodes or an aortic dissection. Similar widening follows blockage of either vena cava, right heart failure or portal hypertension. The variant matters for cardiac surgeons: with no proper caval trunk for a bypass cannula, the superior vena cava and right atrium have to be cannulated separately, and the hepatic veins may open straight into the atrium.

Course and drainage

Begins at

Arises in the upper abdomen from the left ascending lumbar vein, with the left subcostal vein and sometimes a branch of the left renal vein; passes through the left crus into the thorax.

Drains into

Crosses to the right at about T8 or T9, passing behind the aorta, oesophagus and thoracic duct, and empties into the azygos.

Drains

Drains the lower left posterior thoracic wall through the ninth to eleventh left posterior intercostal veins and the left subcostal vein, with small oesophageal and mediastinal veins.