What causes the cupola sign on the supine abdominal radiograph?
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Answer:
The cupola sign is an arcuate, dome-shaped lucency projected over the lower thoracic vertebral bodies on a supine abdominal or chest radiograph, produced by free intraperitoneal gas collecting in the midline beneath the central tendon of the diaphragm. It is a marker of pneumoperitoneum in the patient who cannot be imaged erect, where the classic subdiaphragmatic crescent on an upright film is unavailable. It is visible because, with the patient supine, the central tendon is the highest point of the peritoneal cavity, so non-dependent gas rises to and is trapped under it, outlining the undersurface as a smooth dome. It should be distinguished from the Rigler sign (gas on both the luminal and serosal sides of the bowel wall), the falciform ligament sign (the ligament outlined as a linear density in the right upper quadrant) and the football sign (the whole peritoneal cavity outlined as a large oval lucency). Note the spelling: cupola, not copula.
Why is it called so?
The smooth arcuate lucency beneath the central tendon resembles a cupola, the small dome that crowns a roof; the word comes through Italian from the Latin for a small cask.
Pathophysiology
Perforation of a hollow viscus releases gas into the peritoneal cavity. Gas is buoyant and collects at whatever point is highest for the patient’s position. In the erect patient that point is under the hemidiaphragms laterally, producing the familiar subdiaphragmatic crescent. In the supine patient the anterior midline central tendon becomes the highest point, so gas migrates there and is confined by the tendon above and the liver and stomach below, generating a sharply defined dome-shaped lucency projected over the lower thoracic spine.
Alternative names: Cupola sign of pneumoperitoneum, saddlebag sign, moustache sign
Other associated named signs: Rigler sign, falciform ligament sign, football sign, triangle sign, and the continuous diaphragm sign
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
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