Absent azygos vein

Last revised by Travis Fahrenhorst-Jones on 22 Aug 2021

An absent azygos vein is a very uncommon variant in which the azygos vein fails to develop. In cases of agenesis of the azygos vein, the hemiazygos and accessory hemiazygos veins play an important role in venous drainage, accounting for drainage of both the right and left intercostal veins 1-3.

The variant is exceedingly rare with few cases documented 2

Presentation is asymptomatic as drainage of the intercostal veins is ensured by the hemiazygos and accessory hemiazygos veins 3

Absence of the azygos vein occurs due to failure of the superior segment of the right supracardinal vein to develop 3

Absence of the azygos vein has been documented in the literature to demonstrate aortic nipple on frontal chest radiograph 1,4,5. This is due to enlargement of the left superior intercostal vein owing to increased pressure ​1. The shadow of the azygos vein will also be absent 1.

CT will demonstrate an absence of the azygos vein 4

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