An azygos lobe is a normal variant that develops 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 not a true accessory lobe as it does not have its own bronchus or corresponding specific bronchopulmonary segment 1.
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Epidemiology
An azygos lobe is found in 1% of anatomical specimens 2 and is twice as common in males as females 3.
Radiographic features
Plain radiograph
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.
The superior portion of the azygos fissure will demonstrate a triangular shape called the trigonum parietale - a small segment of extrapleural areolar tissue sandwiched between the two layers of parietal pleura 5. Additionally, the azygos vein itself will appear inferiorly as a tear-shaped opaque shadow.
Given the variant path of the azygos vein, the azygos arch will not be present at the usual location of the junction between the trachea and the right main bronchus 5. To rule out an azygos lobe identifying the azygos vein within this normal position can be helpful 1.
CT
CT shows the deep penetration of the lung behind the superior vena cava and the trachea.
The azygos fissure extends from the lateral aspect of the vertebral body posteriorly, to the right brachiocephalic vein and superior vena cava anteriorly. This consists of two layers of parietal pleura and two layers of visceral pleura 4. The fissure will appear C-shaped in larger lobes while adopting a straighter appearance in smaller lobes 5.
The azygos vein is seen as a thicker structure following the fissure in which it sits. The arch is positioned more superior than when it follows the traditional intramediastinal course 5. The azygos vein usually drains into the superior vena cava, but occasionally in the right brachiocephalic vein instead 5.
Development
An azygos lobe forms when the right posterior cardinal vein, one of the precursors of the azygos vein, fails to slide medially over the apex of the lung and instead penetrates it, carrying with it two pleural layers that invaginate into the superior portion of the right upper lobe 2. The upper lobe is therefore divided, with the azygos vein at the bottom of the fissure encased by pleura 4.
History and etymology
The German anatomist Heinrich Wrisberg (1739-1808) first described this anatomical variant of the right upper lobe in 1778 1.
Clinical importance
While the azygos lobe itself is thought to not inherently cause pathology, there are surgical considerations. The lobe potentially impedes an extrapleural approach to the esophagus via thoracotomy and increases the risk of neurovascular damage in thoracoscopic procedures, especially as the phrenic nerve may become trapped in the fissure 6.
Scarring, bullae, and displacement of fissures can all mimic an azygos fissure on chest radiographs 1.
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