Fig. 16.

a Absent azygos vein schematic. Congenital absence of the azygos vein is associated with enlargement of the remaining paravertebral vessels, as the hemiazygos, that serve as a collateral pathway draining usually into the left brachiocephphalic vein. b Absent azygos vein imaging. Contrast-enhanced CT scan denoting an absent azygos vein (asterisk) with enlargement of the hemiazygos (triangle) which drains to the left brachiocephalic vein by an enlarged left superior intercostal vein (arrow)