Similar to extrahepatic duct dilatation, a mechanical biliary obstruction is the most common cause of intrahep-atic bile duct dilatation. Intrahepatic biliary dilatation in mechanical obstruction is generally tubular and lacks focal stricture formation. Caroli disease, recurrent pyogenic cholangitis, polycystic liver disease, primary sclerosing cholangitis, choledochal cyst, and peribiliary cysts should be included in the differential diagnosis of cystic intrahepatic biliary dilatation. Caroli disease is suggested by focal or diffuse biliary dilatation that is cystic or fusiform in character (Fig. 3) [10, 11]. When diffuse involvement is present, the bile ducts converge
Fig. 3. Caroli disease in a 23-year-old man who complained of abdominal pain. a Sagittal sonogram of the liver shows multiple cystic spaces in the posterior right lobe. b MRCP shows high signal intensity within these cysts. c Photograph of the resected liver shows focal cystic dilatation of the intrahepatic bile ducts
Fig. 3. Caroli disease in a 23-year-old man who complained of abdominal pain. a Sagittal sonogram of the liver shows multiple cystic spaces in the posterior right lobe. b MRCP shows high signal intensity within these cysts. c Photograph of the resected liver shows focal cystic dilatation of the intrahepatic bile ducts toward the porta hepatis. Echogenic intraductal sludge or inflammatory debris may be present, as well as echogenic stones with posterior acoustic shadowing. The most important differential diagnosis for patients with suspected Caroli disease is recurrent pyogenic cholangi-tis, which is characterized by biliary dilatation with intrahepatic stone formation. The left hepatic lobe is more commonly involved than the right in recurrent pyogenic cholangitis. Polycystic liver disease may mimic Caroli disease. However, in most cases, the bile ducts in poly-cystic liver disease are intrinsically normal; only rarely do cysts communicate with the bile ducts.
Although intrahepatic bile duct dilatation is a feature of primary sclerosing cholangitis, the duct dilatation is typically fusiform and isolated. The degree and extent of duct dilatation in primary sclerosing cholangitis is not as severe as that in obstructive biliary dilatation, Caroli disease, or recurrent pyogenic cholangitis because fibrosis, stricture formation, and secondary cirrhosis are the major features of primary sclerosing cholangitis.
Choledochal cyst should be considered in the differential diagnosis when both intrahepatic and extrahepatic duct dilatations are present. In general, patients with choledochal cyst will have more severe extrahepatic dilatation when compared to the degree of intrahepatic dilatation.
Multiple peribiliary cysts in sequence may simulate bile duct dilatation that has a beaded or saccular appearance . The bile ducts adjacent to peribiliary cysts are normal. Therefore, correct diagnosis depends upon visualization of a normal bile duct. Peribiliary cysts are usually associated with various hepatic diseases such as cirrhosis, polycystic liver disease, portal hypertension, portal vein obstruction, and metastatic disease.
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