The azygos lobe is a rare anatomical variant of the lung, first recognised by Heinrich August Wrisberg in 1777 [9]. It is encountered in approximately 1% of anatomical specimens, 0.4% of chest radiographs and 1.2% of high-resolution CT (HRCT) thorax scans [1, 2].
The azygos vein is a unilateral vein, formed by the right ascending lumbar veins and right subcoastal veins [10, 11]. It enters the thoracic cavity at the T12 vertebral level through the aortic hiatus [11]. It ascends anterolateral to the thoracic vertebral column and joins the superior vena cava at T4 vertebral level [10]. On chest radiograph, the normal azygos vein forms the right cardiomediastinal contour.
During embryological development, the posterior cardinal veins regress completely except for a small proximal segment on the right side [10, 11]. This precursor of the azygos vein normally migrates medially over the right lung apex [1, 10, 11]. If the right posterior cardinal vein fails to migrate medially and in turn penetrates through the upper lobe of right lung, it creates an azygos fissure [1, 2, 4]. The portion of the upper lobe superomedial to the azygos fissure is termed as the azygos lobe. It has 4 layers of pleura (2 folds of visceral pleura and 2 folds of parietal pleura) [1, 4, 11]. The azygos lobe is not considered a true accessory lobe as it does not have a separate bronchus or vascular supply [10, 12].
The azygos fissure can usually be identified on frontal chest radiographs [1]. On chest radiograph, the azygos fissure may be seen as a curvilinear opacity within the upper right lung zone with its concavity towards the upper mediastinum (Fig. 2a,b). A tear-drop shaped structure representing the azygos vein can be seen at the inferior aspect of the azygos fissure (Fig. 2a,b) [1, 13]. However, it can be misdiagnosed as on chest radiograph as bullae or displaced fissures are possible mimics [12].
Computed Tomography (CT) of the thorax has superior capability of characterizing the morphology as well as presence of any pathology within the azygos lobe [1, 2, 14]. Furthermore, it can differentiate between azygos lobe, bulla and lung cyst. The azygos fissure is distinctly visible as a curvilinear density in the right upper lobe (Fig. 3a). It is usually C-shaped, but its morphology depends on the size of the azygos lobe [1]. The azygos vein can be seen as a thick curved structure at the inferior aspect of the azygos fissure (Fig. 3b).
Tuberculosis of the azygos lobe is rare with only a few reports in literature [5]. The classical features of pulmonary TB on CT thorax are lung nodules, miliary opacities, mediastinal and/or hilar lymphadenopathy, pleural effusion and/or pleural thickening, cavities, and lung consolidation [8]. Chronic tuberculosis may show fibrotic changes along with bronchiectasis and may cause significant architectural distortion of the lung parenchyma [8]. Centrilobular nodules showing “tree-in-bud” appearance are a common occurrence in pulmonary TB [8, 15, 16].
Sputum smear microscopy for AFB is one of the most used investigations for detection of pulmonary TB in developing nations [17]. Culture & nucleic amplification methods for tuberculosis are further recommended owing to their superior sensitivity [18].
The azygos lobe can also have surgical implications as it hinders the extrapleural approach to the esophagus during thoracotomy procedure [19]. Hence, it is pertinent for a radiologist to be aware of this variant.