First and second year — the floor first, then every step
Pneumomediastinum is the finding; the question is where the air came from. Look at the airway wall, the endotracheal tube cuff, and the oesophagus, and remember the Macklin effect (alveolar rupture) explains most trauma pneumomediastinum.
Orient first
Most traumatic pneumomediastinum is from alveolar rupture tracking along the bronchovascular sheaths (Macklin effect), not from an airway or oesophageal tear.
Tracheobronchial injury clusters within about 2.5 cm of the carina, and persists as a pneumothorax that will not resolve with a drain.
Oesophageal injury in blunt trauma is rare; it is more often penetrating or iatrogenic, and mediastinitis follows if it is missed.
Acquire the study
Thin-section CT on lung and soft-tissue windows with multiplanar and minimum-intensity-projection reformats of the airway; oral water-soluble contrast CT when an oesophageal injury is suspected.
The manoeuvre
Lung window: distribution of mediastinal gas — perivascular and peribronchial streaks (Macklin) versus a focal collection at the airway or oesophagus.
Trachea and main bronchi on coronal and minIP reformats: wall discontinuity, abnormal contour, and the "fallen lung" collapsed away from the hilum.
Endotracheal tube: an overdistended cuff (wider than the trachea) or a cuff/tube outside the expected lumen.
Oesophagus: wall thickening, periesophageal fluid and gas, and oral contrast extravasation when given.
Pleural effusion and a persistent pneumothorax despite a well-placed drain.
What confirms it
A visible wall defect, extraluminal contrast, or a fallen lung.
What licenses you to exclude it
Pneumomediastinum with the Macklin pattern and an intact airway and oesophagus on CT is usually benign — but a persistent air leak or clinical deterioration needs bronchoscopy or endoscopy.
The classic misread
Calling every pneumomediastinum an airway injury.
Missing an overinflated endotracheal cuff causing tracheal injury.
Reporting the injury
Classification to use
Describe by site (trachea, main bronchus, oesophageal segment) and extent; Macklin-effect pneumomediastinum reported as such.
Measurements — and how to take them
Distance of a defect from the carina in mm; endotracheal cuff diameter against the tracheal diameter.
What to report
Pneumomediastinum pattern, the airway wall and tube position, oesophageal findings, pneumothorax behaviour, mediastinal fluid.
How to report it
CT: "Pneumomediastinum tracking along the bronchovascular sheaths with alveolar interstitial emphysema, in keeping with the Macklin effect. The tracheobronchial tree and oesophagus appear intact."
What not to report
Do not state an airway injury is "excluded" by CT when a persistent air leak is present.
Associated injuries to look for
Upper rib, sternal and cervical spine fractures; pneumothorax; subcutaneous emphysema.
What changes management
Tracheobronchial tear or oesophageal perforation → surgery or endoscopic management; Macklin-effect pneumomediastinum → usually conservative.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Endotracheal tube (cuff) · Cuff position relative to the vocal cords and carina
the cuff should sit below the vocal cords and the tip still obeys the carina-distance reference value; a cuff at the cords is too high, and a tip at the carina or in a bronchus is too low
Neck flexion advances the tube toward the carina and extension withdraws it — a single film is a snapshot. Right-main intubation is the classic malposition.
X-ray · CT
Trachea · Tracheal coronal diameter
approximately 13–25 mm in men and 10–21 mm in women
Axial CT about 2 cm above the aortic arch, in end-inspiration.
⚠️ SEX-SPECIFIC — a single combined range mislabels normal tracheae in both directions. Above the range suggests tracheobronchomegaly (Mounier-Kuhn); a sabre-sheath configuration indicates COPD. Expiratory collapse of more than about 50% suggests tracheomalacia and requires a dedicated expiratory acquisition to assess.
a named pathway (alveolar rupture → interstitial air → hilum → mediastinum); it is not an oesophageal perforation until that has been thought about
Boerhaave is the miss. Name whether the oesophagus was considered.
CT · X-ray
Trachea · Expiratory tracheal collapse for tracheomalacia
greater than 50% reduction in cross-sectional area on dynamic expiration is the classical criterion; over 70% is increasingly preferred because healthy subjects can exceed 50%
⚠️ THE THRESHOLD IS GENUINELY CONTESTED — state which criterion is being applied. A forced-expiratory or coughing acquisition collapses more than quiet expiration; the technique must accompany the percentage.
CT
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.