Overview
A venous system found in the posterior mediastinum consisting of the azygous vein on the right and the hemiazygous and accessory hemiazygous system on the left. Drains thoracic wall and viscera within the mediastinum. It also serves as a major collateral pathway between inferior vena cava and superior vena cava.
Azygous Vein
Origin
Originates variably at the level of the renal veins (L1 – L2), either as;
- Branch of the inferior vena cava or
- The confluence of right ascending lumbar vein and right subcostal vein
Course
- Enters thoracic cavity passing behind right crus, throught the aortic hiatus of the diaphragm at the level of T12, together with the aorta
- Ascends within the posterior mediastinum on the anterolateral surface of the vertebral column (T12 → T5), right of the midline, aorta and thoracic duct
- At level of T4, it arches anteriorly over hilum of right lung
- Terminates as it drains into the superior vena cava at upper limit of the pericardium
Relations
Anterior
- Right hilum: bronchus, pulmonary arteries and veins, hilar lymph nodes
Posterior
- Anterior longitudinal ligament
- Right posterior intercostal arteries
- Vertebral bodies of T4 – T12
Medial
- Thoracic duct – lies between azygous vein and thoracic aorta
- Thoracic aorta
- Oesophagus
- Trachea
- Right vagus nerve
Laterally
- Right greater splanchnic nerve
- Right lung and pleura
Tributaries
- Receives all but the first intercostal vein – supreme intercostal vein (drains into the brachiocephalic vein or the vertebral vein).
- Second, third and fourth intercostal vein drain via the right superior intercostal vein.
- Hemiazygous and accessory azygous vein drain into azygos vein at mid-thoracic level.
- Right bronchial veins drain into azygous vein near its terminaion
- Right ascending lumbar
- Right subcostal
- Other tributitaries: oesophagus, pericardium, mediastinum
Variants
- Different configuration of the hemiazygous and accessory hemiazygous veins.
- Common trunk of hemiazygos and accessory hemiazygos vein
- Drainage of azygous vein into the right brachiocephalic vein, right subclavian vein, intrapericardial SVC or directly into the right atrium
Hemiazygous
Origin
- Commences in a similar manner to azygos vein, on the left of the abdominal aorta.
- Origin often communicates with the left renal vein
Course
- Ascends anterior to the vertebral column to the midthoracic level (T9) where it courses to the right, crossing the midline posterior to the descending aorta to drain into the azygos vein
Tributaries
- Left ascending lumbar
- Lowest four posterior intercostal veins
- Left subcostal vein
- Oesophageal veins
- Medastinal veins
Variants
- Hemiazygous and accessory hemiazygos may drain into azygous
Accessory Hemiazygous
Tributaries
- The accessory hemiazygos vein receives the fourth to eighth posterior left intercostal veins
- First left intercostal vien drains into left brachiocephalic vein distally
- The second and third left intercostal vein drains into the left brachiocephalic vein proximally
- May also receive left bronchial veins
Course
Descends on the left side of the vertebral column, crossing the midline posterior to the aorta at midthoracic level (approximately at the inferior border of T8) to drain into the azygous vein.