Author James Watts, Department of Anaesthesia and Critical Care Medicine, East Lancashire NHS Hospitals Trust, discusses a case of azygous lobe, an unusual thoracic anatomical variant.
Introduction
An elderly patient presented with recurrent pneumonia. During their stay on ITU they underwent a chest X ray which appeared to show a right upper lobe teardrop shaped mass. At first thought to be a tumour, it was later reported by a consultant radiologist to be an azygous lobe, a normal but uncommon anatomical variant.
Anatomy
The azygous (from the Greek word meaning ‘unpaired’) vein is a unilateral right sided vessel which drains deoxygenated blood from the posterior abdominal and thoracic walls into the Superior Vena Cava (SVC).
It has a variable development and anatomy, but generally it arises at about T12 from the the ascending lumbar vein and lower subcostal vein, and joins the SVC at about T4 level after passing over the apex of the right lung. The part of the vein that curves over the top of the right lung is referred to as the ‘arch of the azygous’. (1)
In approximately 0.3 to 1% of the population, the azygous vein is displaced laterally, in which case it impinges into the lung, creating what is described as an extra upper lobe. This is not a true lobe as there is no associated bronchus but instead describes the fact that the vein creates a fissure (the azygous fissure, lined by invaginated pleura and inside of which the vein itself runs) into, partially separating part of the upper lobe from the rest of the lung. This is referred to as an ‘azygous lobe’. (see figures 1 to 5) (2-4).
The Xray
The upper part of the azygous fissure is referred to radiologically as the trigonum parietale, formed by the extra-pleural alveolar tissue trapped in the folds of the pleura. The more lateral the trigonum, the larger the azygous lobe. The azygous vein sits in the lower part of the folds and is characterised on chest X ray as a radio-opaque tear shape (5-8).
This anatomical variant is reported to affect males twice as often as females. It is in itself benign, but can be mistaken by the unwary as a paratracheal mass; or can be misidentified during intrathoracic surgery with serious consequences. It can also be associated with other congenital abnormalities (eg intrapleural right brachiocephalic veins, oesophageal atresia etc).
Summary
The azygous lobe is an uncommon anatomical variant that in itself is benign but which can lead to misdiagnosis or surgical hazards for the unwary. Its significance in critical care practise is that it may be mistaken for a paratracheal mass. In anaesthetic practise, misinterpretation of the anatomy during surgical procedures may predispose to serious complications. As a result, when detected by radiologists, the anomaly should be reported appropriately.
References
1. Azygos vein – Anatomy, Location, Structure, Diagram, Function anatomy.co.uk accessed 08/04/2025
2. Yurasakpong, L; Yammine, K; Limpanuparb, T; et al (2021-05-07). “The prevalence of the azygos lobe: A meta‐analysis of 1,033,083 subjects”. Clinical Anatomy. 34 (6): 872–883
3. Akhtar J, Lal A, Martin KB, et al (2018) Azygos lobe: A rare cause of right paratracheal opacity. Respir Med Case Rep. Feb 3;23:136-137.
4. Al-Mnayyis A, Al-Alami Z, Altamimi N, et al . (2020) Azygos Lobe: Prevalence of an Anatomical Variant and Its Recognition among Postgraduate Physicians. Diagnostics Jul 10;10(7):470.
5. Caceres J, Mata J M, Andreu J, (1998) The azygos lobe: normal variants that may simulate disease, European Journal of Radiology, Volume 27, Issue 1, Pages 15-20
6. Copley, S, Hansell, D, & Kanne, J (2014), Thoracic Imaging : Illustrated Clinical Cases, Second Edition, Taylor & Francis Group, London.
7. Rivaud, Y. Maldjian, P. D. (2019). The Azygos Vein From A to Z. Journal of Thoracic Imaging, 34 (5), W100-W108
8. Cáceres J, Mata JM, Alegret X, et al (1993) Increased density of the azygos lobe on frontal chest radiographs simulating disease: CT findings in seven patients. AJR Am J Roentgenol. Feb;160(2):245-
Images: Submitted by the author
