An azygos lobe is created when a laterally displaced azygos vein creates a deep pleural fissure into the apical segment of the right upper lobe during embryological development. It is a normal anatomic variant of the right upper lobe due to the invagination of the azygos vein and pleura during development in the fetus. It is not a true accessory lobe as it does not have its own bronchus.
An azygos lobe is found in 1% of anatomical specimens and is twice as common in males as females .
An azygos lobe forms when the right posterior cardinal vein, one of the precursors of the azygos vein, fails to migrate over the apex of the lung and penetrates it instead, carrying along with two pleural layers that invaginate into the upper portion of the right upper lobe .
The azygos fissure extends from the lateral aspect of the vertebral body posteriorly, to the right brachiocephalic vein and SVC anteriorly. This consists of two layers of parietal pleura and two layers of visceral pleura .
The azygos vein is seen as a thicker structure following the same path as the fissure. The position of the arch is higher than when it follows an intramediastinal course
The azygos vein usually ends in the SVC, but occasionally in the right brachiocephalic vein.