Choledochal cysts are rare; they are caused by a specific weakness in a part or the whole of the wall of the common bile duct, and usually presents in neonates or children.
Five varieties have been described. The most common type is a fusiform dilatation of the common bile duct from which the gallbladder arises. Another variant is dilatation of the intraduodenal portion of the common duct, so creating a choledochocoele. There may be associated dilatation of the intrahepatic ducts.
The cyst may contain up to 1 to 2 litres of bile. In adult life they often contain gallstones.