Pneumobilia

Pneumobilia, also known as aerobilia, is the accumulation of gas 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

Etiology

• recent biliary instrumentation
  • ERCP
  • common bile duct stent placement (normal finding, indicating patency of the stent)
  • percutaneous transhepatic or intraoperative cholangiography (small amount of gas only)
• incompetent sphincter of Oddi
  • sphincterotomy (~50% pneumobilia at 1 year)
  • following passage of a gallstone
  • scarring e.g. chronic pancreatitis
  • drugs e.g. atropine
  • congenital
• biliary-enteric surgical anastomosis
  • cholecystoenterostomy
  • choledochoduodenostomy (with or without bile sump syndrome ²)
  • Whipple procedure
• spontaneous biliary-enteric fistula (cholecystoduodenal accounts for ~70% ³)
  • gallstone ileus
  • peptic ulcer disease
  • traumatic
  • neoplasm, eg. cholangiocarcinoma, ampullary cancer
• infection (rare)
  • cholangitis
  • emphysematous cholecystitis (usually gallbladder gas only, ~20% will have gas in the biliary tree also)
  • liver abscess (if contains gas and communicates with the biliary tree)
  •  ruptured hydatid cyst ⁶
• biliary-bronchopleural fistula (rare)

Radiographic features

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

Plain radiograph

Supine radiographs often demonstrate a sword-shaped lucency in the right paraspinal region representing gas from the common 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 gas-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 (carried along by the blood). Also, biliary gas is anti-dependent, and typically fills the left lobe of the liver.

MRI

Differentiating intrahepatic calculi from pneumobilia can be difficult on standard MRCP as often only T2 sequences are used. The addition of T1 GRE sequences can be helpful as gas tends to demonstrate susceptibility artifact in contradistinction to most calculi which do not ⁸. Nevertheless, a CT remains the first-line modality for differentiating intrahepatic calculi from gas.

Differential diagnosis

The differential of pneumobilia is very limited:

• portal venous gas
  • patients usually ill (e.g. ischemic bowel)
  • gas more peripheral in liver
  • Doppler imaging may help
• hepatic artery calcification (on ultrasound ⁵)
  • often seen in those with chronic renal failure
• small stones in the biliary tree (on ultrasound)
  • usually only one duct
  • cf. pneumobilia is multiductal

More importantly every attempt should be made at assessing the cause of pneumobilia, as a number of causative entities require prompt management.

The existence of pneumobilia is a useful sign to look for when trying to determine the patency of a common bile duct stent. Resolution or reduction of pneumobilia after stent placement is highly specific and quite accurate for predicting biliary stent occlusion ⁷.

See also

• abnormal intra-abdominal gas
• emphysematous cholecystitis
• pneumobilia vs portal venous gas (mnemonic)