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| azygos lobe | |
|---|---|
| Name | Azygos lobe |
| Latin | Lobus azygos |
| Caption | Azygos lobe in right lung specimen |
| System | Respiratory system |
| Location | Right upper pulmonary field |
| Artery | Right intercostal arteries |
| Vein | Azygos vein tributaries |
| Nerve | Pulmonary plexus |
azygos lobe
The azygos lobe is an anatomical variant of the right upper lung characterized by an invaginated pleural fold created by an aberrant trajectory of the azygos vein within the right hemithorax. It presents as an accessory subdivision bounded by a distinct fissure and pleural fold and is encountered in thoracic surgery, radiology, and anatomical descriptions. Recognition of this variant is important for differentiating normal anatomy from pathological processes during evaluation by clinicians from institutions such as Mayo Clinic, Johns Hopkins Hospital, Cleveland Clinic, and in textbooks authored by figures like Henry Gray and contributors to Netter Atlas of Human Anatomy.
The azygos lobe is formed when the azygos vein courses medially over the apex of the right lung, carrying a double-layered pleural septum (the mesoazygos) that demarcates a small portion of lung parenchyma. The lobe is bounded by the azygos fissure, which contains the mesoazygos investing the aberrant vascular structure; it does not have its own bronchus or separate pulmonary artery branch, differentiating it from true lobes described by authorities at Royal College of Surgeons of England and in editions of Gray's Anatomy. Anatomical relationships include proximity to the right apical pleura, the right subclavian vessels near Thoracic inlet, and mediastinal structures such as the superior vena cava and the right brachiocephalic vein, as catalogued in atlases used by trainees at Massachusetts General Hospital and Charité – Universitätsmedizin Berlin.
Embryologically, the variant results from an anomalous migration of the right posterior cardinal or supracardinal venous channel during the stage of venous remodeling in the embryo, a process discussed in works by researchers at University of Cambridge and Harvard Medical School. Normally the azygos vein arches over the right lung apex to join the superior vena cava; failure to assume the typical course leads to encasement by pleura and formation of the fissure. Developmental explanations draw on comparative studies referencing embryologic series published by laboratories at Karolinska Institutet and developmental models used at Stanford University School of Medicine.
Clinically, the azygos lobe is usually asymptomatic but has implications in differential diagnosis and procedural planning for physicians from centers like Memorial Sloan Kettering Cancer Center and Royal Brompton Hospital. It may mimic pathological lines on chest imaging, such as pneumothorax or pleural thickening, leading to consultation with specialists affiliated with American Thoracic Society and European Respiratory Society. Rarely, localized infections, neoplasms, or bronchogenic cysts may involve the azygos lobe, prompting multidisciplinary discussion among teams at MD Anderson Cancer Center, UCLA Medical Center, and oncology groups collaborating with NCI. Historical case series documented at institutions including Guy's Hospital and Toronto General Hospital describe incidental findings during thoracotomy and postmortem examinations.
On chest radiography and computed tomography, the azygos fissure appears as a fine curvilinear density converging toward the right mediastinum, often terminating at a teardrop-shaped density representing the entrapped azygos vein. Radiologic identification is emphasized in protocols from Radiological Society of North America and imaging texts by authors at Mayo Clinic School of Medicine and University College London Hospitals. High-resolution CT and multiplanar reconstructions used at Mount Sinai Health System and Karolinska University Hospital enhance visualization, distinguishing the azygos fissure from extrapulmonary lines associated with prior surgery or pleural disease; nuclear medicine assessments by groups such as Johns Hopkins Radiology further document perfusion characteristics.
Surgeons and interventionalists must anticipate the presence of an azygos lobe during procedures including right apical thoracotomy, video-assisted thoracoscopic surgery, central venous catheterization, and mediastinal dissection. Centers with thoracic programs at Guy's and St Thomas' NHS Foundation Trust and Royal Marsden Hospital report modified approaches to avoid injuring the mesoazygos and the azygos vein, minimizing hemorrhagic complications. Anatomic knowledge is relevant for placement of right-sided subclavian or jugular catheters, as described in guidelines from American College of Surgeons and case reports emerging from Singapore General Hospital and Sainte-Anne Hospital Center.
Incidence estimates for the azygos lobe vary by population and detection method, with reported prevalence ranging from roughly 0.2% to 1.2% in chest radiograph and CT series compiled in studies from Seoul National University Hospital, Hospital Clínic de Barcelona, and multicenter analyses by researchers at University of Tokyo. The variant is more commonly unilateral on the right; left-sided counterparts are exceptionally rare and have been described in isolated case reports from institutions like Charité – Universitätsmedizin Berlin and Hôpitaux Universitaires de Genève. Variations include size of the entrapped segment, depth of the fissure, and relation to adjacent mediastinal anatomy, documented across anatomical collections at Smithsonian Institution and museum specimens studied by departments at University of Oxford.
Category:Lung anatomy