Eluding normal variant.

BACKGROUND

A 70-year-old male patient, farmer by occupation, presented with complaints of dry cough for the last one year, with no complaint of hemoptysis. The patient had no history of smoking or tuberculosis. The hemogram findings were unremarkable. The respiratory system examination delineated insignificant findings. A chest x-ray, posteroanterior (PA) view, followed by high-resolution computed tomography (HRCT) thorax were done for final diagnosis.

QUESTIONS

Q1. What are the imaging findings and likely diagnosis based on chest x-ray and HRCT thorax?

Q2. Define and describe the clinical importance of the entity described above.

Q3. Delineate the embryogenesis of the diagnosed entity with a diagram.

Q4. What are the differential diagnoses?

ANSWERS

Answer 1: Figure 1: The chest x-ray, erect, PA view demonstrates a right-sided upper paramediastinal soft opacity, with a smooth convex outline, up to the right hilum (Red arrow). The hilum appears normal. The rest of the lung fields and bony thorax appear normal. Both the costophrenic angles and domes of the diaphragm appear normal. A differential of the right mediastinal mass with a computed tomography (CT) scan for further evaluation may be advised.

Figure 1

Chest X-ray, erect, PA view demonstrates a right-sided upper paramediastinal soft opacity, with a smooth convex outline, up to the right hilum (Red arrow). Both the hila appear normal. The rest of the lung fields and bony thorax appear normal. Both the costophrenic angles and domes of diaphragm appear normal

Figure 2(a): The HRCT of the thorax, axial section, delineates a well-defined, convex-shaped, fold extending from the superior vena cava (SVC) and merging with the right hilum. There is an incidental azygos fissure (Blue arrow). The lung fields and bony thorax appear clear. No lymphadenopathy is noted. The teardrop-shaped density at the bottom of the fissure (Green arrow) is the azygos vein.

Figure 2

(a) HRCT thorax—the axial section delineates a well-defined, convex-shaped fold extending from the superior vena cava and merging with the right hilum (Blue arrow). The lung fields and bony thorax appear clear. No lymphadenopathy is noted. A teardrop-shaped density is noted at the bottom of the fold (Green arrow); (b) HRCT thorax — the axial section reveals a prominent structure (Orange arrow) in the posterior right upper lobe, within the convex fold, coursing anteriorly to join the superior vena cava

Figure 2(b): The HRCT of the thorax, axial section, reveals a prominent azygos vein in the posterior right upper lobe, coursing anteriorly, to join the superior vena cava (Orange arrow).

FINAL DIAGNOSIS

An azygos lobe masquerading as a right mediastinal mass.

Answer 2: Definition: An azygos lobe is created when a laterally displaced azygos vein makes a deep fissure in the upper part of the lung during embryological development. It is, therefore, not a true accessory lobe, but rather a normal variant appearance of the right upper lobe, which results from invagination of the azygos vein.[1]

Synonym: Adam's lobe[2]

Incidence: 0.4% in clinical cases.[3]

Clinical importance

The position of the azygos vein may be important in thoracic surgery, as an unknown migrant azygos vein can potentially be the source of a bleeding complication

Empty azygos fissure (vanishing azygos lobe): Displacement of the azygos vein following atelectasis[1]

Mimicker of a mass, scar, pneumothorax, esophageal dilatation[23]

Presence of the azygos vein in the pulmonary parenchyma can complicate the evolution of some erosive or infiltrative processes from the superior lobe of the right lung. Any surgical procedure in this region has to consider the possibility of this anatomical variation, to avoid possible injury to the azygos vein or its tributaries

The discovery, during necropsy, of a hemorrhage, made by accidental injury to the azygos vein, in its aberrant trajectory, suggests a preoperative misdiagnosis, which calls for the attention of the clinician as well as the radiologist.[4]

Treatment

No treatment is required. Preoperative awareness of a migrating azygos vein is useful for a safe eventual surgery.[23]

Answer 3: Embryology: The azygos lobe forms 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 with it pleural layers that entrap a portion of the right upper lobe [Figure 3].

Figure 3

Embryogenesis of azygos lobe

Total layers-four, two visceral and two parietal (mezoazygos).

Etiopathogenesis

Increased intrathoracic pressure

Spontaneous or iatrogenic pneumothorax

Sudden development of kyphosis.[15]

Answer 4: Differential diagnosis with radiographic appearances

Mediastinal mass - A CT scan is confirmatory to rule out a superimposed thymic shadow or supra-aortic vessels

Pulmonary scar - Appears as a coarse linear opacity

Bubble - Appears as a rounded focal lucency of more than 1 cm in size or as an area of decreased attenuation, surrounded by a thin wall (less than 1 mm). Multiple bubbles are often present and are associated with other signs of pulmonary emphysema[5]

Pulmonary fissure - Appears as a linear opacity, normally 1 mm or less in thickness, corresponding in position and extent to the anatomic fissural separation of pulmonary lobes or segments. These include minor, major, horizontal, oblique, accessory, anomalous, and azygos fissures[5]

Pneumothorax — A visceral pleural edge is visible with no pulmonary parenchyma visible on the external side of this edge

A tension pneumothorax may be associated with considerable shift of the mediastinum and/or depression of the hemidiaphragm[5]

Esophageal dilatation-may be visible on a chest x-ray in severe disease as a retrocardiac clarity displacing the paraesophageal line, with the air-fluid level at the aortic arch or above. On chest CT, the esophageal dilatation is visible as a dilated luminal structure with retained debris and narrowing at the level where it enters the stomach.[45]