Cyst, choledochal

The wall of the cyst, which is lined by cylindrical or cuboidal epithelium supported by connective tissue and occasional elastic and smooth muscle fibres, varies from 210 mm in thickness. Type I choledocal cyst (Fig.1) refers to aneurysmal dilatation of the common bile duct often accompanied by distal narrowing. It accounts for 80% - 90% of cases.

Types II and III refer to choledocal diverticulum which occurs in 2% of cases and to choledochocoele which involves only the intraduodenal portion of the main bile duct and accounts for 1 - 5% of cases. Choledochocoele is a protrusion of a dilated intramural common bile duct into the duodenum. There are two anatomical types that can not always be differentiated by radiological imaging. In the first, the common bile duct terminates into the choledochocoele which drains into the duodenum via an aperture in its wall. The main pancreatic duct may also empty into the choledochocoele. The second type occurs when the common bile duct enters normally into the papilla and the choledococoele fills and empties directly into the common bile duct. In this variant the pancreatic duct usually enters into the main bile duct proximally to the choledochocoele. Type IV refers to multiple cysts. Type IVa refers to multiple intrahepatic cysts and an extrahepatic cyst and Type IVb refers to multiple extrahepatic cysts. Type V refers to Carolis disease. Classification schemes vary and the definition of Types IV and V are not universally accepted.

Clinical symptoms related to choledochal cysts may appear only at the adult age and are usually caused by complications. Complications of choledochal cysts include: recurrent pancreatitis, cholangitis, biliary stones, intrahepatic abscesses, biliary cirrhosis, portal hypertension and duodenal compression.

Radiological diagnosis

Sonographically the Type I cyst is visible as a fluid containing anechoic structure distinct from the gallbladder but in continuity with the common bile duct. Sonography will also detect associated stones, strictures or carcinoma.

Choledocal cysts are revealed on CT (Fig.2) and MRI as fluid containing, sharply delineated masses in continuity with the extrahepatic bile ducts mostly situated within the pancreatic head. The exact anatomical and topographic situation is better displayed in the coronal than in the transverse plane. This can be accomplished by CT using reformation in the coronal plane with or without the use of cholangiographic contrast medium or by direct coronal imaging with MRI. Optimal and complete anatomical visualization of the anomaly and its topographic relation to the rest of the intra- and extrahepatic bile duct system, particularly in a preoperative setting, requires direct cholangiography (Fig.3). 

However, MRCP (Fig.4) nowadays may be able to provide almost identical detailed information as cholangiography without the need of any invasive procedure or of contrast medium. MRCP will also contribute to the differential diagnosis between choledochal cysts and pancreatic cysts or pseudocysts, as well as with enteric duplication cysts. Choledochocoele has different features on cross-sectional imaging than the other types of choledochal cysts. The impression and protrusion of this fluid-filled mass on the medial duodenal wall, as well as its specific intramural localization will suggest the diagnosis (Fig. 5). ERCP optimally displays the anatomical abnormality as well as the repercussion on the bile ducts (Fig. 6).