An azygos lobe is a rare, anatomical variant of the upper lobe of the right lung found in approximately 1% of anatomic specimens and 0.4% of chest radiographs [1]. It forms during embryological development when the posterior cardinal vein, the precursor of the upper thoracic segment of the azygos vein, erroneously migrates through the upper lobe of the lung. Normally, the posterior cardinal vein migrates over the apex of the right lung to its final position in the mediastinum. Awareness of this anatomical variant and its characteristic x-ray features is important among clinicians because its presence may be confused for processes such as bulla, masses, or abscesses.

An azygos lobe is not a true accessory lobe as it does not have a bronchus or blood supply. It can be more accurately described as an anatomically separated segment of the right upper lobe. It does not serve any known physiologic purpose aside from gas exchange.

An azygos lobe is formed during embryological development when the right posterior cardinal vein aberrantly migrates through the upper lobe of the right lung rather than over the apex. As a result, two pleural layers are carried through the right upper lobe creating a fissure known as the azygos fissure (See Figure 1). While most of the posterior cardinal veins regress during embryological development, the right-sided supracardinal veins form a portion of the inferior vena cava, the intercostal veins, the hemiazygos vein, and the azygos vein. Interestingly, the failure of regression of left-sided supracardinal veins has been reported. This failure leads to such anomalies as the persistence of left superior and inferior vena cava [2].

An azygos lobe is not a true accessory lobe as it does not have a unique bronchus or blood supply. There are few reports describing bronchial and vascular supply. One report of an autopsy of a 43-year-old man found that the azygos lobe was supplied by apical sub-subsegmental branches of B1b (B1bi) and B1a (B1ai), and this was accompanied by the pulmonary artery of the same sub-subsegments, A1bi and A1ai. The lateral sub-subsegmental branches of B1aii and B1bii accompanied by the pulmonary artery did not supply the azygos lobe. Venous supply was from V1a and V2 but not V1b [3]. Another report described similar findings with the azygos lobe ventilated by the posterior branch of the apical segmental bronchus, B1a, and accompanied by two apical sub-segmental arteries, A1ai and A1aii, and the apical intersubsegmental vein, V1a [4].

The lymphatic supply to an azygos lobe does not differ from that of normal lung. Lymphatic vessels of the respiratory bronchioles merge and follow the bronchiole tree to the hilum.

The nerve supply of an azygos lobe is analogous to the nerve supply of any other segments of the lung. The hilum of each lung contains a pulmonary plexus composed of efferent and afferent nerve fibers from the vagus nerve as well as fibers from the sympathetic trunk and cardiac nerve plexus [5]. The pulmonary plexus innervates the bronchial tree and the visceral pleura. The pulmonary plexus is divided into the anterior and posterior divisions that lie in front of the lung and behind the lung, respectively. The majority of the supply is provided to the bronchi and bronchioles and control functions such as bronchodilation, bronchoconstriction, and glandular secretion. Afferent fibers relay impulses from stretch receptors in alveolar walls.