The Interazygos Vein: A Case Report and Review of Preaortic Hemiazygos Anastomoses.

During routine dissection of the thoracic cavity, a rare variation of the azygos system of veins was observed. The hemiazygos vein and the accessory hemiazygos vein generally course posterior to the aorta to form anastomoses with the azygos vein. However, in this case, the hemiazygos vein and the accessory hemiazygos vein anastomosed together into a vessel that crossed anterior to the thoracic aorta before draining into the azygos vein on the right side of the thoracic cavity. This variation is also known as an "interazygos vein." While the venous system is recognized for its variability, a preaortic anastomosis is particularly rare. A literature review was conducted to investigate the morphology of previous reports of preaortic hemiazygos anastomoses and interazygos veins. It was determined that the term "interazygos vein" encompasses a range of morphological variations. Awareness of the interazygos vein is important clinically to avoid mediastinal misdiagnoses and surgical complications.