The azygos vein arises from the junction of the right ascending lumbar and right subcostal veins, ascends in the right paravertebral gutter, and arches anteriorly over the right hilum to drain into the superior vena cava at the level of T4. It receives the right posterior intercostal veins, the hemiazygos and accessory hemiazygos veins from the left side, the right bronchial veins, and the right superior intercostal vein.
The azygos system provides the critical collateral venous pathway between the SVC and IVC territories, allowing decompression of lower body venous hypertension when either great vein is obstructed. Azygos vein engorgement on chest radiograph is a sign of right heart failure, cardiac tamponade, or SVC obstruction. An azygos lobe is a common anatomical variant where the azygos vein fails to migrate, creating a mesentery (azygos mesentery) that indents the right upper lobe, visible as an inverted teardrop shadow on CXR. The azygos arch is a key landmark during thoracic surgery and right-sided thoracoscopic procedures.
The azygos arch, encountered during right thoracotomy and posterior mediastinal dissection, is at risk of injury during lobectomy and oesophagectomy, where accidental division causes significant haemorrhage requiring urgent suture control.
A rare congenital anomaly where the infrarenal IVC is absent and venous return from the lower body travels via an enlarged azygos vein to the SVC, important to identify before cardiac surgery and associated with polysplenia and complex congenital heart disease.
