- Normal variant, found in 1% of anatomic specimen and about 0.4% of chest radiographs
- Right posterior cardinal vein (precursor of azygos vein) fails to migrate over lung apex, instead penetrating the lung and carrying pleural layers with it.
- Two folds of parietal and visceral pleural layers create a "fissure"
- Enclosed lung is a part of apical or posterior segment of the right upper lobe
- Fine, convex line crosses the apex of the right lung.
- Azygos vein visible in the lowermost portion of the fissure "teardrop" shape.
- Absence of azygos vein at the right tracheobronchial angle.
Mimics of Azygos Fissure
- Walls of bullae
- Displaced fissures
- Supernumerary fissures
Mata J, Caceres J, Alegret X, Coscojuela P, De Marcos JA. Imaging of the azygos lobe: normal anatomy and variations. AJR 1991;156:931-937.