A Variation of Azygos Venous System with Absence of Hemiazygos and Accessory Hemiazygos Veins: A Case Report

Introduction: The structure of the azygos venous system often has different variations. There are many variations in origin, course, tributaries, anastomosis, and termination in the azygos vein. Embryologically, the azygos venous system originates from subcardinal veins, and changes in the azygos venous system development create several variations. 
Case Presentation: The azygos vein system was evaluated in the cadaver of a 55-year-old man whose hemiazygos and accessory hemiazygos veins could not be found. The right posterior intercostal veins directly opened into the azygos vein. There was no variation in the right side, while there was a variation in the drainage of the left posterior intercostal veins. The first left posterior intercostal vein was directly drained into the brachiocephalic vein, while the second, third, and fourth left posterior intercostal veins were drained to the azygos vein through the left superior intercostal vein. Besides, a connection was formed between the fifth and sixth left posterior intercostal veins before being drained to the azygos vein. There was also a short connection between the seventh and eighth left posterior intercostal veins. The connection between the tenth and eleventh left posterior intercostal veins was formed on one anastomotic circle and this common trunk was opened to the azygos vein near the entrance to the ninth left posterior intercostal vein.

 Conclusion: The present case was consistent with type III in the Anson classification system with the prevalence of 1-5%. Variations of the hemiazygos and accessory hemiazygos veins were confirmed by means of computerized tomography. This could precisely clarify one of the variations of the mediastinal vessels. The embryological pattern of the azygos system for this variation was discussed, as well.