The azygos lobe is a rare anomaly that developmentally arises when the right posterior cardinal vein penetrates the apex of the lung, instead of passing over it, and travels inferiorly taking pleural layers with it to entrap a portion of the right upper lobe. The two folds of pleura form the mesoazygos, a fissure visible on 0.4% of chest radiographs and 1.2% of high resolution computed tomography (CT) studies. The right azygos lobe is supplied by the medial segments of the apical and anterior or posterior branches of the apical segmental bronchial artery and vein. A true left azygos lobe has also been reported.
On chest radiography, the azygos lobe is usually distinguished by the azygos fissure, which superiorly has a triangular shape and inferiorly demonstrates the azygos vein as a tear-shaped shadow. The azygos fissure typically appears as a fine, convex line that crosses the apex of the right lung. The azygos lobe can appear opaque and be incorrectly interpreted as a pathologic mediastinal finding on PA chest radiographs.
When findings on traditional imaging are not clear, CT exams can be helpful in delineating relevant anatomy. Clinically, the knowledge of azygos lobe anatomy is important during thoracic surgical approaches. Partial obstruction of the thoracoscopic view during a bilateral sympathectomy was reported during attempted mobilization of the azygos lobe. Others reported difficulty reflecting the pleura during primary repair of the esophageal atresia in a pediatric patients. There are also reports of the phrenic nerve coursing within the azygos fissure. Finally, multiple authors have reported spontaneous pneumothorax associated with the azygos lobe in both the adult and the pediatric patient.