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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.
An azygos lobe is found in 1% of anatomical specimens 2 and is twice as common in males as females 3.
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.
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.
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.
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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- 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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- 4. Assane Ndiaye, Nafi Ba Ndiaye, Aïnina Ndiaye, Mamadou Diop, Jean Marc Ndoye, Abdarahmane Dia. The azygos lobe: an unusual anatomical observation with pathological and surgical implications. (2012) Anatomical Science International. 87 (3): 174. doi:10.1007/s12565-011-0119-5
- 5. Caceres J, Mata J, Andreu J. The Azygos Lobe: Normal Variants That May Simulate Disease. European Journal of Radiology. 1998;27(1):15-20. doi:10.1016/s0720-048x(97)00146-0
- 6. Assane Ndiaye, Nafi Ba Ndiaye, Aïnina Ndiaye, Mamadou Diop, Jean Marc Ndoye, Abdarahmane Dia. The azygos lobe: an unusual anatomical observation with pathological and surgical implications. (2012) Anatomical Science International. 87 (3): 174. doi:10.1007/s12565-011-0119-5