An azygos lobe is created when a laterally displaced azygos vein makes a deep fissure in the upper part of the lung during embryological development. It is therefore not a true accessory lobe, but rather a normal variant appearance of the right upper lobe, which results from invagination of the azygos vein.
An azygos lobe is found in 1% of anatomic specimens2.
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 pleural layers that entrap a portion of the right upper lobe2.
The azygos lobe is usually well seen on the chest radiograph, where it is limited by the azygos fissure - a fine, convex (relative to the mediastinum) line that crosses the apex of the right lung.
CT shows the deep penetration of the lung behind the SVC and the trachea.
The azygous fissure extends from the lateral aspect of the vertebral body posteriorly, to the right brachiocephalic vein and SVC, and anteriorly.
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 azygous vein usually ends in the SVC, but occasionally in the right brachiocephalic vein.
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- 1. Shannon EH. THE AZYGOS LOBE OF THE LUNG. Can Med Assoc J. 1931;24 (4): 498-500. - Free text at pubmed - Pubmed citation
- 2. Mata J, Cáceres J, Alegret X et-al. Imaging of the azygos lobe: normal anatomy and variations. AJR Am J Roentgenol. 1991;156 (5): 931-7. AJR Am J Roentgenol (abstract) - Pubmed citation
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