Azygos vein

At a glance

English

Azygos vein

Latin (TA2)

Vena azyga

System

Vein

Region

Trunk

Side

Single, on the midline

About the azygos vein

The azygos vein is the main longitudinal channel draining the posterior wall of the thorax. It rises on the front right of the thoracic vertebral bodies, in the posterior mediastinum, and ends by arching forwards into the superior vena cava. Its name, from the Greek for 'unpaired', reflects the fact that the left side has no single matching vessel; instead the hemi-azygos and accessory hemi-azygos veins do that job and cross over to feed it.

It usually starts in the upper abdomen, around the twelfth thoracic vertebra, where the right ascending lumbar vein meets the right subcostal vein. A small lumbar root may also arise lower down from the back of the inferior vena cava or a lumbar vein. The vessel enters the chest through the aortic hiatus, right of the cisterna chyli, or else behind the right crus. In the thorax it ascends with the thoracic duct and aorta on its left. Near the fourth thoracic vertebra it turns forwards, hooking above the right main bronchus at the hilum of the lung, and drains into the back of the superior vena cava just before that vessel enters the pericardium. This curve, the arch of the azygos, is a recognised landmark on imaging and at surgery.

Along its way it collects most of the right posterior intercostal veins, the right superior intercostal vein, the hemi-azygos and accessory hemi-azygos veins from the left, and smaller oesophageal, mediastinal, pericardial and right bronchial veins. It also communicates freely with the venous plexuses of the spine. The pattern varies a good deal: the vein may lie in the midline, or be doubled.

Because it links the territory of the inferior vena cava with that of the superior vena cava, the azygos route can return blood from the lower body to the heart when either cava is obstructed. In some congenital anomalies the abdominal inferior vena cava is missing and the azygos carries its blood instead, appearing much widened. On a chest film an azygos shadow wider than about one centimetre suggests raised pressure or such a variant, and should not be mistaken for a paratracheal mass. Occasionally the arch sits further out and pulls a fold of pleura into the right upper lobe, marking off a small azygos lobe.

Course and drainage

Begins at

Begins near T12 from the union of the right ascending lumbar and right subcostal veins, sometimes with a lumbar root from the inferior vena cava; enters the thorax through the aortic hiatus or behind the right crus.

Drains into

Arches forwards above the right bronchus at about T4 to open into the posterior surface of the superior vena cava.

Drains

Drains the posterior thoracic and upper abdominal walls through the right intercostal veins and the hemi-azygos system, plus oesophageal, mediastinal, pericardial and bronchial veins; forms a collateral path between the two venae cavae.