Azygos lobe
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At the time the article was created Behrang Amini had no recorded disclosures.
View Behrang Amini's current disclosuresAt the time the article was last revised Joachim Feger had no financial relationships to ineligible companies to disclose.
View Joachim Feger's current disclosures- Azygos fissure
- Azygos lobes
- Ayzgous lobe
- Ayzgous fissure
An azygos lobe is a normal variant that develops when a laterally displaced azygos vein creates a deep pleural fissure in 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 bronchopulmonary segment 1.
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Terminology
Azygos is Greek, meaning unpaired. Azygous also means unpaired and is a combination of the preferred Greek term with the English suffix -ous.
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 a chest radiograph, delineated by the azygos fissure crossing the right lung apex.
The superior portion of the azygos fissure is expanded by extrapleural areolar tissue, forming a triangular shape called the trigonum parietale 5. The malpositioned azygos vein curves around the inferior margin of the fissure instead of forming a shadow at the right tracheobronchial angle 1,5.
CT
CT shows the azygos fissure extending from the lateral aspect of a vertebral body to the right brachiocephalic vein and superior vena cava anteriorly. The fissure consists of two layers of parietal pleura and two layers of visceral pleura in contrast to normal fissures which contain only two layers of visceral pleura 4. The fissure may be straight or laterally convex if the lobe is larger 5.
The azygos vein is seen as a thicker structure curving around the inferior margin of the fissure. The arch is positioned more superiorly compared to the usual intramediastinal course 5. The azygos vein usually drains into the superior vena cava, but occasionally into the right brachiocephalic vein 5.
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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 double 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 not to cause pathology inherently, there are surgical considerations. The lobe potentially impedes an extrapleural approach to the oesophagus 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.
References
- 1. Akhtar J, Lal A, Martin K, Popkin J. Azygos Lobe: A Rare Cause of Right Paratracheal Opacity. Respir Med Case Rep. 2018;23:136-7. doi:10.1016/j.rmcr.2018.02.001 - Pubmed
- 2. Mata J, Cáceres J, Alegret X, Coscojuela P, De Marcos J. Imaging of the Azygos Lobe: Normal Anatomy and Variations. AJR Am J Roentgenol. 1991;156(5):931-7. doi:10.2214/ajr.156.5.2017954 - Pubmed
- 3. Paul Butler, Adam Mitchell, Jeremiah C. Healy et al. Applied Radiological Anatomy. (2012) ISBN: 9780521766661 - Google Books
- 4. Ndiaye A, Ndiaye N, Ndiaye A, Diop M, Ndoye J, Dia A. The Azygos Lobe: An Unusual Anatomical Observation with Pathological and Surgical Implications. Anat Sci Int. 2011;87(3):174-8. doi:10.1007/s12565-011-0119-5 - Pubmed
- 5. Caceres J, Mata J, Andreu J. The Azygos Lobe: Normal Variants That May Simulate Disease. Eur J Radiol. 1998;27(1):15-20. doi:10.1016/s0720-048x(97)00146-0 - Pubmed
- 6. Ndiaye A, Ndiaye N, Ndiaye A, Diop M, Ndoye J, Dia A. The Azygos Lobe: An Unusual Anatomical Observation with Pathological and Surgical Implications. Anat Sci Int. 2011;87(3):174-8. doi:10.1007/s12565-011-0119-5 - Pubmed
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