Anatomical variations of the azygos venous system

The azygos vein (gr. azygos – 'unpaired') typically starts from the posterior aspect of the inferior vena cava (IVC), at or below the level of the renal veins. The origin is not constant. If present, the lumbar azygos ascends anterior to the upper lumbar vertebrae. It may pass behind the right crus of the diaphragm or pierce it, or it may traverse the aortic hiatus to the right of the cisterna chyli. Anterior to the twelfth thoracic vertebral body, the azygos is joined by a large vessel formed by the right ascending lumbar and subcostal veins that passes forward and to the right of the twelfth thoracic vertebra behind the right crus. In the absence of a lumbar azygos this common trunk may form the azygos vein itself. Whatever its origin, the azygos vein ascends in the posterior mediastinum to the level of the fourth thoracic vertebra, where it arches forward above the right pulmonary hilum. It ends in the superior vena cava, before the latter pierces the pericardium. The hemiazygos vein is formed on the left side from the lower three posterior intercostal veins, a common trunk formed by the left ascending lumbar and subcostal veins, and by esophageal and mediastinal tributaries. It ascends anterior of the vertebral column to the eighth thoracic level then crosses the vertebral column posterior to the aorta, esophagus and thoracic duct and ends in the azygos vein. Its lower end is often connected to the left renal vein [1]. The azygos system of veins drains most of the posterior abdominal and thoracic wall. It also receives mediastinal, bronchial and esophageal veins. The azygos vein serves as an important channel of communication between superior and inferior vena-cava. The vein also communicates with the vertebral venous plexus. The azygos vein system functions as an additional drainage way when high pressure and obstruction occurs in most of the veins that inferior vena cava vein drains. In the obstruction of the IVC, it joins whole venous drainage below the diaphragm except the digestive system. This system connects to the cerebral vein system with the intercostal veins and vertebral venous plexuses. This connection is important because of venous metastatic pathways in breast and bronchial cancers. Also, in superior vena cava syndrome it joins with IVC [2-4]. Azygos veins are therefore important cava-caval and porto-caval junctions which form a collateral circulation in caval vein occlusion and in portal hypertension …