Choledochal cysts are rare congenital cystic dilations of the biliary tree and are classified by the portion of the biliary tree affected. The most common form consists of fusiform dilation of the common bile duct (Figure 67-1). Type I choledochal cyst accounts for 85% to 90% of all cases. All cases of choledochal cyst that have been diagnosed prenatally, to date, have been of the type I variety (Bancroft et al., 1994). The less common forms of choledochal cyst include diverticulum of the common bile duct (type II), which are intraduodenal or intrapancreatic; choledochocele (type III); multiple extrahepatic cysts with or without intrahepatic cysts (type IV); and Caroli’s disease, which consists of single or multiple intrahepatic cysts associated with hepatic fibrosis and a normal extrahepatic biliary tree (type V) (Caroli et al., 1958).

The wall of a choledochal cyst is usually thickened with dense connective tissue interlaced with strands of smooth-muscle fibers associated with some inflammatory reaction (O’Neill et al., 1987). Normal biliary mucosal lining is absent, although sparse islands of columnar epithelium and microscopic bile ducts within the wall may be seen. In older patients, stones may be seen within the choledochal cyst or within intrahepatic ducts (Landing, 1974; Bhagvat and Chaudhrey, 1989). Most newborns with choledochal cysts have complete biliary obstruction at the level of the duodenum, which has been likened to a form of biliary atresia (Landing, 1974; Vergnes et al., 1990). Approximately 60% of choledochal cysts detected prenatally were found postnatally to have complete obstruction of the distal bile duct (Bancroft et al., 1994). Liver histology in infants with choledochal cysts is usually normal, but may show mild bile duct proliferation consistent with chronic biliary obstruction (Hanai et al., 1967). Rare cases have been reported with coexistence of choledochal cysts and congenital hepatic fibrosis (Murray-Lyon et al., 1973; Mall et al., 1974; Yamaguchi, 1980; Orenstein and Whittington, 1982; Ramirez et al., 1989). Although malignancies, usually adenosquamous or small cell carcinomas, have been reported in association with choledochal cysts, these lesions occur only after decades of chronic inflammation and recurrent cholangitis, and have not been reported in an infant or a child (Joseph, 1980).

Theories regarding the cause of choledochal cyst disease have evolved from abnormalities in the early stages of embryonic development, to congenital weakness of the wall combined with distal obstruction, to the “common channel” hypothesis (O’Neill et al., 1987). Some authors have suggested that choledochal cysts result from embryologic abnormalities in the growth of hepatic diverticulum and support their assertion by the frequently observed associated anomalies of duplications, gallbladder agenesis, and duodenal atresia (Barlow et al., 1976; Martin and Rowe, 1979). Miyano et al. (1980) have emphasized that weakness of the choledochal wall results primarily from obstruction of the distal duct during fetal development. Spitz (1977) was able to produce cystic dilatation of the common bile duct in neonatal lambs by ligation of the distal common bile duct. In contrast, only dilatation of the gallbladder occurred in their adult controls. The cause of distal common bile duct obstruction in choledochal cyst has been variously suggested to be due to congenital stenosis, persistence of epithelial membrane, abnormal valves, or neuromuscular incoordination of the sphincter (Ito et al., 1984). Kusanoki et al., (1988) suggested that the obstruction may be due to aganglionosis of the distal bile duct, similar to Hirsh-sprung’s disease of the colon.

Todani et al., (1977) noted that the majority of patients with choledochal cyst have an anomalous arrangement of the pancreatic or biliary duct system in which the pancreatic duct enters at an abnormal angle proximal to the circular muscle fibers of the ampulla of Vater (Todani et al., 1977; Giordani et al., 1984). They suggested that this arrangement would permit reflux of pancreatic enzymes containing trypsin upward into the common bile duct, resulting in damage to the duct wall in utero (Oguchi et al., 1988).

Rustad and Lilly (1987) noted that the “long common channel” that results from an anomalous pancreaticobiliary junction is quite common and seen in up to 50% of patients with biliary disorders other than choledochal cyst. In addition, choledochal cyst has been diagnosed as early as 15 weeks of gestation, well before the development of pancreatic exocrine function (Schroeder et al., 1989). This raises doubts about the common channel theory as the cause for choledochal cyst disease.

Choledochal cyst is a rare congenital anomaly with an estimated occurrence ranging from 1 case in 100,000 to 150,000 livebirths (Lippett and Pitt, 2003) to 1 case in every 2 million livebirths (Dewbury et al., 1980). It is more common in females in all ethnic groups, with a ratio of 2.5:1, which some have suggested may indicate that it is a sex-linked defect (Alonzo-Lej et al., 1959; Kim, 1981). Asian populations, particularly Chinese and Japanese, have a much higher incidence of choledochal cyst than any other race.

The characteristic sonographic finding in fetal choledochal cyst is a simple anechoic cyst in the upper abdomen in close proximity to the porta hepatis (Figure 67-2). Rarely, the cyst can be imaged in continuity with the bifurcation of the right and left hepatic ducts or joined by the cystic duct from the gallbladder. Elrad et al. (1985) were the first to describe finding dilated ducts leading into a cystic mass that was later confirmed to be a choledochal cyst (Frank et al., 1981). It may be difficult to distinguish a choledochal cyst from gallbladder duplication, duodenal atresia, mesenteric cyst, or even an ovarian cyst that has migrated up into the upper abdomen (Crombleholme et al., 1997). Color flow Doppler studies may be helpful in demonstrating the cyst in relation to the portal vein, hepatic artery, and umbilical vein (Gallivan et al., 1996). Recently, fetal MRI has been used to diagnose choledochal cyst (MacKenzie et al., 2001; Chen et al., 2004; Want et al., 2005).

The differential diagnosis of a cystic mass in the right upper quadrant includes hepatic or omental cysts, adrenal cysts, renal cysts or hydronephrotic renal pelvis of the right kidney, dilated loop of bowel, mesenteric cysts, duodenal duplications, gallbladder duplications, simple hepatic cysts, duodenal atresia, and situs inversus. Because of the small fetal pelvis, ovarian cysts can move up into the right upper quadrant (Crombleholme et al., 1997). A diagnosis of choledochal cyst is more likely if the location is subhepatic and intraperitoneal and there is no change in appearance with observed peristalsis of the fetal bowel. Color Doppler studies that demonstrate an intimate association of the cyst with the hepatic artery and portal vein also suggest a biliary origin (Gallivan et al., 1996). The diagnosis is most convincing if tubular structures can be identified entering and leaving the cyst.

Dilation of intrahepatic bile ducts occurs in the fetal liver, most often in association with infantile polycystic kidney disease. Substantial ectasia with grossly identifiable cyst formation is rare, but may be seen in Meckel–Gruber syndrome, infantile polycystic kidney disease, and a variant of Ivemark syndrome (Strayer and Kissone, 1979; Blankenberry et al., 1987).

The earliest reported prenatal diagnosis of a choledochal cyst has been at 15 weeks’ gestation (Schroeder et al., 1989) (Table 67-1). This early prenatal diagnosis challenges one commonly held theory that choledochal cysts arise from the reflux of pancreatic enzymes into the common bile duct via the “long common channel.” At this gestational age, pancreatic acini are only just appearing and zymogen granules are immature, with no evidence of secretion on electron microscopy (Laitio et al., 1974).