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Refers to a congenital variation of the upper lobe of the right lung.
It is seen in 1% of the population.
Arises embryologically from an anomalous lateral course of the azygos vein in a pleural septum within the apical segment of the right upper lobe.
The azygos fissure forms when the embryologic azygos vein migrates through the apical portion of the right lung instead of medial to it.
As the vein invaginates along, it creates a fissure with 4 pleural layers, 2 parietal And 2 visceral.
An azygos lobe is formed when the right posterior cardinal vein, one of the precursors of the azygos vein, fails to migrate over the apex of the lung and penetrates it instead, carrying along two pleural layers that invaginates into the upper portion of the right upper lobe.
Normally the azygous arch lies superior to the origin of the right main bronchus, however in patients with an azygous fissure the azygos arch is more cephalad.
It has no bronchi, veins or arteries of its own or corresponding alteration in the segmental architecture of the lung.
The lung medial to the fissure is called the azygos lobe although it does not have a separate bronchial or vascular supply.
It is not a true, or even, accessory, pulmonary lobe, but rather an anatomically separated part of the upper lobe.
Usually an incidental finding on chest x-ray or computed tomography and is as such not associated with any morbidity but can cause technical problems in thoracoscopic procedures.
On a PA chest x-ray, the azygos fissure may be identified as a curvilinear line extending from the apex of the right lung down to the level of the azygos arch .
The above complex is called the reverse comma sign.
A small triangular opacity known as the trigonum pareitale, in the apex of the origin of the azygos fissure sometimes seen.
CT of the thorax can easily identify the azygos fissure, lobe, and arch.
Azygos lobe demonstrated in 0.4% of chest x-rays and 1% of autopsy studies.
Azygos lobe more common in males and familial incidence has been noted.
Even more rare is a left azygos or hemiazygos lobe associated with migration of the left the superior intercostal vein through the left apex of the lung.
Generally has no clinical significance except it can be confused with the other conditions such as bullae, apical scarring and displaced fissures.
May complicate surgical procedures such as video assisted thorascopic surgery for the treatment of pneumothorax and thorascopic sympathectomy.