Radiological signs
The radiographic appearance directly depends on which anatomical structures of the chest are outlined by the accumulated gas. Air from the mediastinum can highlight the medial contours of large vessels (while their lateral borders are outlined by aerated lung parenchyma).
Key radiological phenomena:
- Continuous diaphragm sign (continuous diaphragm sign). On a frontal chest radiograph, gas accumulated in the anterior mediastinum behind the pericardium forms a continuous radiolucent band directly above the diaphragm.
- Ring-around-the-artery sign (ring around the artery sign). Occurs when gas spreads into the root of the lung, with air encircling the pulmonary artery (most commonly the right one).
- Sail sign (sail sign). Characteristic when the thymus is elevated by air masses.
- Double bronchial wall sign (double bronchial wall sign). The wall of the central bronchus becomes clearly visible because air is present not only inside its lumen but also surrounds it externally.
- Tubular artery sign (tubular artery sign).
Computed tomography
CT is the most accurate diagnostic method for pneumomediastinum. Air is visualized as rounded or linear collections surrounding the vessels and structures of the mediastinum.
A specific CT sign of pneumomediastinum is soft-tissue strands (strands of soft tissue) running through the areas of gas accumulation. This is because air entering under pressure roughly dissects the mediastinal adipose tissue, leaving visible connective tissue fibers. Via perivascular spaces, this air can spread far beyond the thoracic cavity — superiorly (into the supraclavicular regions and neck) and inferiorly (into the retroperitoneal space, retroperitoneum).
Differential diagnosis on radiographs
When analyzing a standard chest radiograph, pneumomediastinum can easily be confused with other optical and pathological phenomena:
- Medial pneumothorax. Similar to pneumomediastinum, a linear collection of air may be observed, bounded laterally by a thin line (pleura). For accurate differentiation, one must look for other signs of pneumothorax in the pleural cavity or additional gas collections in the mediastinum.
- Mach effect (Mach effect). This is a purely optical illusion caused by a sharp contrast in densities (e.g., between the lung and the convex border of the heart). It appears as a radiolucent zone adjacent to a convex structure. Unlike true pneumomediastinum, the Mach effect lacks a distinct opaque line delimiting the "gas".
Pneumomediastinum vs. Pneumopericardium and Pneumothorax on CT
The main criterion for differentiation on CT is the presence of soft-tissue strands.
In pneumomediastinum, air dissects the adipose tissue, so structural strands will always be visible inside the gas pockets. In pneumopericardium or pneumothorax, such strands are absent. Air in these cases accumulates in "true" anatomical spaces (between the parietal and visceral layers of the pericardium or pleura) where connective tissue septa are lacking.
Briefly: answers to topical questions
With which conditions must pneumomediastinum be differentiated on a radiograph?
On a chest radiograph, pneumomediastinum must be differentiated from a medially located pneumothorax and the "Mach effect".
- Medially located pneumothorax — manifests as a linear collection of air bounded laterally by a thin pleural line. For differentiation, one should look for signs of pneumothorax in other parts of the thoracic cavity.
- "Mach effect" (Mach effect) — an optical illusion due to a sharp drop in radiological density between the lung and adjacent structures (heart, mediastinum). It appears as a radiolucent zone near convex surfaces, but lacks the opaque line typical of true pneumomediastinum.
What are the clinical symptoms of pneumomediastinum?
The main clinical symptom of pneumomediastinum is retrosternal chest pain.
Physical examination reveals the following signs:
- Subcutaneous emphysema — usually localized in the jugular notch extending to the neck.
- Hamman's sign — a crunching or clicking sound over the precordium, synchronous with the heartbeats.
In mediastinal emphysema, air accumulation can lead to vascular compression and circulatory impairment.
In which group of patients is the "sail sign" most commonly seen in pneumomediastinum?
In newborns and infants, pneumomediastinum usually outlines the contours of the thymus. The "sail sign" (sail sign) occurs as a result of thymic elevation by the accumulated air; it is most frequently visualized on the right side.
Pneumomediastinum can be a cause of respiratory distress in newborns, although it is often small in volume, transient, and requires no intervention.
To avoid confusing the CT appearance: the mediastinum is filled with "cotton-like" fat (adipose tissue), so air tears through it and leaves fibers — strands. The pleural and pericardial cavities are empty "sacs", so air accumulates there cleanly, without strands.
Class and exam questions
Why does the bronchial wall appear doubled in pneumomediastinum?
Normally, we only see the internal lumen of the bronchus because externally it merges with the soft tissues of the mediastinum. In pneumomediastinum, gas surrounds the bronchus externally, and the wall becomes contrasted on both sides (air inside + air outside).
How to distinguish pneumomediastinum from the Mach effect on X-ray?
The Mach effect is an optical illusion at the boundary of media with different densities. Unlike a true accumulation of air in the mediastinum, with the Mach effect you will not see a thin opaque line physically delimiting the air pocket.
Does pneumomediastinum have independent clinical significance?
Gas in the mediastinum itself causes only discomfort or retrosternal pain, but does not directly threaten life. It is critically important to find the cause of the pneumomediastinum — for example, an esophageal or bronchial rupture.
What else to study on this topic
- Anatomy and radiological evaluation of the pericardium
- Fibrosing mediastinitis: CT findings and complications
- Tubular artery sign
- Mach syndrome (optical illusions in diagnostic imaging)
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