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Pneumobilia

Pneumobilia, also known as aerobilia, is accumulation of air in the biliary tree.  It is important to distinguish pneumobilia from portal venous gas, the other type of branching hepatic gas. There are many causes of pneumobilia and clinical context is often important to distinguish between these 3.

Pathology

Causes

Radiographic features

Pneumobilia is typically seen as linear branching air within the liver most prominent in central large calibre ducts as flow of bile pushes gas toward the hilum. This is in contrast to portal venous gas where peripheral small calibre branching air is usually seen due to flow of blood out from the hilum.

Plain film

Supine radiographs often demonstrate a sword-shaped lucency in the right paraspinal region representing gas from the common bile duct and the left hepatic duct.  This has been termed the saber sign and is present in ~ 50% of patients with pneumobilia 4.

Ultrasound

Ultrasound is very sensitive in detecting gas within the liver as it causes artifact, specifically regions of high echogenicity with prominent shadowing or reverberation. The liver has been described as having a 'striped appearance'.

CT

Branching hepatic gas is easily appreciable on CT as branching air-density regions within the liver.

Differentiating between biliary and portal venous gas is usually achievable especially when intravenous contrast is used.  Gas within the biliary tree tends to be more central, whereas gas within the portal venous system tends to be peripheral (pushed along by the blood). Also, biliary gas is ante-dependent, and typically fills the left lobe of the liver.

Differential diagnosis

The differential of pneumobilia is very limited :

  • portal venous gas
    • patients usually ill (e.g. ischaemic bowel)
    • gas more peripheral in liver
    • Doppler imaging may help
  • hepatic artery calcification (on ultrasound)
    • often seen in those with chronic renal failure
    • mimic pneumobilia on ultrasound 5

More importantly every attempt should be made at assessing the cause of pneumobilia, as a number of causative entities require prompt management (e.g. intestinal ischaemia).

See also

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