An Anatomical Study on the Azygos Venous System

The azygos venous system under the diaphragm at the superior part of posterior abdominal wall varies in its origin. Commonly, the azygos vein begins by the union of the ascending lumbar and subcostal veins on the right side. It passes through the aortic opening or posterior to the right crus of diaphragm enter the thorax and lying on the right side of the vertebral bodies, it ascends on the vertebral bodies and then arches over the root of the right lung to enter the superior vena cava. It receives the right lower eight posterior intercostal veins and at its convexity the superior intercostal veins join it. The hemiazygos is formed on the left in a manner similar to that of the azygos and receives the lower posterior 9th to 11th intercostal veins.The accessory hemiazygos receives the 5 to 8 intercostal veins, these two veins enter the azygos vein.The upper posterior intercostals veins forms the left superior intercostal vein which drains into the left brachiocephalic vein. The aim of the present study was order to investigate the variation of the azygos venous system in human cadavers. For this purpose 10years of study was carried out on 40 male cadavers (with the proximate age of 40 to 80 years). In all cases the azygos vein lied on the right side of the vertebral column and also no bilateral azygos vein was observed. In the 5% of cases the hemiazygos was absent and the related intercostal veins drained directly into azygos. In 2.5% of the cases the accessory hemiazygos has not been formed and consequently the intercostal veins of 5 to 8 with a single trunk ran anterior to the vertebral column, then joined the azygos vein. Finally, in 25% of cases the hemiazygos and accessory hemiazygos communicated with each other and by a single channel at the level 8th vertebra enter the azygos vein. The network of azygos venous system is corresponded to the general patterns reported in Gray’s Anatomy, however the azygos vein was not observed neither on the left nor in front of vertebral column and in one case the hemiazygos was absent. The knowledge of these variations will be important in study of radiographic, dissection and surgery of the posterior mediastinum.