Author(s)

Gori G, Iacconi C, Caramella D.

Patient

female, 43 year(s)

A 43 year old patient underwent a preoperative chest radiograph.

A pilonidal cyst had to be removed in this patient, situated near the coccyx, which caused to her pain and tumefaction. The preoperative standard chest radiograph did not show any pleuro-parenchymal lesion and the dimensions of the heart were normal. Incidentally, we describe the presence of an Azygos Lobe.

The Azygos Lobe is found in about 1% of autopsies and in 0.4% of radiograms. Usually in embryonic stages, the precursor of the azygos vein, the right posterior cardinal vein, first sets itself on the medial surface of the right upper lobe, and then descends in a position situated two centimetres cranially to the right superior lobar bronchus.
Two principal hypotheses about the origin of Azygos Lobe have been made. In some authors’ opinion the Azygos Lobe would derive from the precocious displacement of the heart from the cervical position to the thoracic one, before the azygos vein has reached its normal position. According to others authors, it would form because of an excessive development of the pulmonary apex so that the azygos vein would remain entrapped in it. In both cases, the azygos vein will be surrounded by two layers of parietal pleura and by two layers of visceral pleura. These layers link the azygos vein to the thoracic wall through a structure of mesenterial type.
On the chest radiograph, 4 structures can be noticed in the right upper pulmonary field: 1) the shadow of the azygos vein (unusually situated at the level of the pulmonary parenchyma); 2) a fine, convex line departing from the shadow of the azygos vein in cranial direction. This line is constituted by the apposition of the two layers of visceral pleura and by the two layers of parietal pleura (these two internal layers are known as mesoazygos); 3) the Azygos Lobe, which represents the portion of pulmonary parenchyma delimitated by the previously mentioned line; 4) a triangular area (trigonum parietale) near the thoracic wall, where the two internal layer of mesoazygos continue with costal pleura.
According to its size, the Azygos Lobe has been classified in three types: type A (the trigonum parietale is located in the lateral aspect of the pulmonary apex); type B (the mesoazygos has a vertical path); and type C (the trigonum is located medially).
Axial CT images give a more accurate representation of the anomalous course of the azygos vein, of the presence of the Azygos Lobe and of their relations with mediastinal organs. The Azygos Lobe is not a pathological finding, but it can create problems during surgery of the pulmonary apex or in sympaticectomy operations (e.g. for hyperhidrosis).