Free air in the chest: pneumomediastinum and subcutaneous emphysema
- Finding
Air escapes into the middle of the chest or under the skin. On the X-ray it draws thin black lines around the heart, vessels and muscles.
What it is
Pneumomediastinum is air in the mediastinum; subcutaneous emphysema is air in the soft tissues of the neck and chest wall, and the two often appear together. On the chest X-ray, the air shows as thin dark streaks that outline structures normally invisible against each other: the edge of the aorta, the pulmonary arteries, the trachea and the heart border, lifted away by a black stripe. In the neck and chest wall the air dissects between muscle fibres and gives a streaky, feathery pattern. Most cases in young people are spontaneous, after asthma, coughing, vomiting or straining, and settle on their own, but the same picture can come from oesophageal rupture or an airway injury, which are emergencies.
What it looks like
- Thin dark lines running parallel to the heart border, aortic knob and great vessels, with a white line (the mediastinal pleura) lifted off them.
- Continuous diaphragm sign: air under the heart joins the two hemidiaphragms into one continuous line.
- Ring around the artery: on the lateral view, air encircles the right pulmonary artery.
- In infants, air can lift the thymus off the heart, so it looks like a raised sail.
- Subcutaneous emphysema: streaky lucencies in the neck and chest wall that outline the pectoral muscle fibres.
- Air may extend up into the neck and down into the retroperitoneum.
Why it looks that way
- Raised pressure in the alveoli (asthma, coughing, vomiting, straining, ventilation) ruptures them; the air tracks along the bronchovascular sheaths to the hilum and mediastinum.
- The mediastinum is continuous with the neck spaces and the retroperitoneum, so the air spreads up into the neck and chest wall and down into the abdomen.
- Air can also come straight from a torn oesophagus, trachea or bronchus, or from the neck after surgery or trauma.
Differential and pitfalls
- Medial pneumothorax: air lies against the lung and moves with position; pneumomediastinum outlines mediastinal structures and extends into the neck.
- Pneumopericardium: air stays inside the pericardial sac around the heart and does not rise above the root of the great vessels.
- A thin dark band next to a bright border (Mach effect) is an optical illusion, not air; there is no separate pleural line.
- Vomiting followed by chest pain and pneumomediastinum suggests oesophageal rupture: a left-sided effusion and air near the left lower mediastinum raise concern.
- A crunching sound with each heartbeat on auscultation may accompany pneumomediastinum.
Next step
- Young patient, mild symptoms, clear trigger (asthma, coughing): observe, treat the cause and repeat the chest X-ray if symptoms change.
- Small or uncertain air: CT of the chest is more sensitive and shows where the air is and where it may come from.
- Vomiting, severe pain, fever or a pleural effusion: suspect oesophageal rupture; CT and a water-soluble contrast swallow, and call surgery at once.
- After trauma or with a persistent air leak: suspect airway injury; CT and bronchoscopy, discuss with a senior.
- On a ventilator: look for an associated pneumothorax and review the ventilator pressures with the team.
Test yourself
Think of the answer, then tap the question to see it.
What is the continuous diaphragm sign?
Air in the mediastinum under the heart outlines the central diaphragm, so both hemidiaphragms appear as one unbroken line: a sign of pneumomediastinum.
How does air from ruptured alveoli reach the mediastinum?
It tracks along the sheaths around the bronchi and vessels toward the hilum and into the mediastinum, and from there into the neck and chest wall.
Which history with pneumomediastinum should make you suspect oesophageal rupture?
Forceful vomiting followed by severe chest pain, especially with fever, a left-sided pleural effusion or air near the left lower mediastinum.
How do you tell pneumopericardium from pneumomediastinum?
Pneumopericardium is confined to the pericardial sac around the heart and does not rise above the great vessels; pneumomediastinum outlines the vessels and extends into the neck.
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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Pneumomediastinum (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.