Cystic dilatation of the common bile duct (CBD), also known as choledochal cyst, is a fairly uncommon anomaly of the biliary tract. Although it was first described by Vater and Ezler in 1723, Douglas published the first complete clinical description of the anomaly in a patient in 1853. He speculated about the congenital nature of this anomaly.
In 1959, Alonso-Lej et al published an extensive review of 94 cases in the literature and added two cases of their own.  They classified choledochal cysts into 3 types. In 1977, Todani et al further classified this anomaly into 5 types.  Subsequent subtypes based on cholangiographic findings have been described.
The management of choledochal cysts has evolved during the last 3 decades. Today, complete excision of the cyst and biliary bypass have become the treatment of choice. Other procedures, such as cystoduodenostomy or cystojejunostomy, have been associated with high morbidity rates and the potential for malignant transformation in the residual cyst. Even after complete excision, sporadic cases of cancer in the remaining biliary tree have been reported in the last decade.
Population prevalence estimates of choledochal cysts range from approximately 1 case in 13,000 people to 1 case in 2 million people. Cystic diseases of the bile duct are more common in Japan and Asia.  In an extensive review published in 1980, Yamaguchi et al reviewed 1433 cases from the literature; of these, 1204 (more than two thirds) were from Japan. 
In all reported series, the most frequent type of choledochal cyst is type I (see the image below; see Relevant Anatomy). In types I and IV choledochal cysts (fusiform dilation of the CBD without or with dilated intrahepatic ducts, respectively), the female-to-male ratio is roughly 4:1. However, in types II (diverticulum of the CBD), III (choledochocele), and V (dilation of intrahepatic ducts only), the cysts occur with equal frequency in both sexes.
Choledochal cysts can occur in persons of any age. Two thirds of the cysts are diagnosed before the patient is aged 10 years. [5, 6, 7] Approximately 20% of cysts are diagnosed in much older patients. [8, 9, 10] In rare cases, choledochal cysts have been detected at prenatal ultrasonography as early as 15 weeks' gestation.
The exact cause of choledochal cyst remains obscure. Many authors believe that they are congenital because most of the cysts are diagnosed in infants and children. However, because approximately 20% are diagnosed in adults, including elderly patients, several theories have been postulated, as follows: [8, 7]
In 1969, Babbitt and colleagues carefully analyzed the cholangiograms of patients with choledochal cysts and found many with an anomaly of the pancreaticobiliary junction.  In these patients, a small distal CBD entered the pancreatic duct at 2-3.5 cm from the ampulla of Vater, whereas the normal common channel is 5 mm or less. This may represent failure of normal separation of these two ducts during embryologic development. This proximal junction precludes the proper functioning of the sphincter of Oddi. The pressure in the pancreatic duct (30-50 cm H2 O) exceeds the pressure in the CBD (25-30 cm H2 O) favoring reflux of pancreatic secretions into the CBD. They also noted a high amylase content in the fluid from the cysts. The reflux of pancreatic juice could lead to weakness and dissolution of the wall of the CBD (see the image below).
Experimental support for this concept was reported in 1974 by Kato and associates who anastomosed the main pancreatic duct to the gallbladder in dogs.  Within 9 days after the anastomosis, all the tested animals had varying degrees of dilatation of the CBD, with edematous changes of the CBD wall. They concluded that proteolytic enzymes were responsible for the damage.
In 1977, Spitz supported the concept of distal CBD obstruction as a cause of choledochal cyst when he demonstrated bile duct dilatation in lambs by means of ligation of the duct near the duodenum.  However, the same experiment failed to reproduce dilatation of the bile duct in mature sheep.
Miyano et al (1981) established an experimental model of an anomalous choledochopancreatic ductal junction by creating a choledochopancreatic end-to-side ductal anastomosis in puppies.  They successfully reproduced the dilatation of the CBD in all experimental animals without exception.
In 1984, Todani et al conducted an analysis of endoscopic retrograde cholangiopancreatograms and other cholangiograms and confirmed this long common-channel anomaly.  The anomaly was found in most patients. Other authors have reported the same findings in their series. [22, 15, 16, 9, 23] Reflux of pancreatic enzymes into the CBD can happen early in life, even in fetal life, resulting in damage to the ductal wall. The distal portion of the CBD is most at risk, and with repeated irritation, it can become stenotic.
All of these theories are applicable to choledochal cyst type I, III, and IV anomalies, but they cannot be used to explain type II and V choledochal cysts in which the CBD is normal. Perhaps genetic factors play a role. [3, 24, 25] Despite this, the two most accepted theories are still reflux of pancreatic enzymes into the CBD secondary to an anomalous pancreaticobiliary junction and obstruction of the distal CBD.
Grossly, the size of a type I choledochal cyst widely varies. [3, 22, 5, 6, 8, 26, 10, 27] The cysts contain a few hundred milliliters of bilious fluid that is rich in pancreatic enzymes. The cyst wall thickness also varies, ranging from very thin to a few millimeters in thickness.
In type IV and V anomalies, intrahepatic cysts can be fusiform or saccular and are in continuity with the CBD. Dilatation of the proximal common duct is frequent, particularly just proximal to the long common channel. Sludge and stones are sometimes present within the cyst. [3, 24] The bile duct distal to the cyst is usually stenotic. The liver may have variable degrees of fibrosis or established cirrhosis with portal hypertension. Histologic studies of the wall of choledochal cysts show dense fibrous connective tissue with inflammation and ulceration of the mucosa and submucosal layers (see the image below).
The inflammation is significantly less in younger patients than in older children in whom chronic inflammatory changes abound. [24, 23, 28, 29, 27] With severe inflammation, only a relatively thin and fragmented lining, rather than the typical biliary mucosa, is observed lining the cyst. Intramucosal glandular structures are frequently observed; this suggests epithelial metaplasia secondary to repeated destruction and regrowth of the lining of the cyst. Inflammation has been noted to be more severe in intrahepatic cysts than in extrahepatic cysts.
Carcinoma arising in a choledochal cyst wall or remaining biliary tree after complete cyst excision is well recognized.  Malignancy is believed to be the result of chronic inflammation and metaplasia (see the image below). The typical malignancy is adenosquamous carcinoma or occasional cases of small cell carcinoma.
Choledochal cysts and malignancies
Malignancies in choledochal cyst can arise from the distal CBD, the wall of the cyst (even after successful drainage at cystoenterostomy), or the intrahepatic bile ducts. The risk of cancer appears to be related to the age of the patient; it is high (>20 times) compared with that of the healthy population. [12, 31, 32, 7, 2, 33, 34, 35] The risk of detecting a biliary tree malignancy in a resected cyst is 0.7% in patients who undergo surgery before age 10 years, 6.8% in patients who undergo surgery at age 11-20 years, and 14.3% in patients who undergo surgery after age 20 years.
More than half of the cancers arise from the cyst wall, even after successful internal drainage. Total cyst excision has not prevented the risk of malignancy in the remaining bile ducts. Malignancy can develop many years after excision of the cyst and can develop in areas of the biliary tree remote from the cyst such as the gallbladder and terminal common duct, which is left behind after excisional surgery.
Any type of cyst is susceptible to malignancy, but the greatest prevalence is observed with types I, IV, and V. Factors thought to contribute to the development of malignancy include prolonged bile stasis and chronic inflammation of the cyst wall. Inflammatory and metaplastic changes increase with patient age, and they are frequently observed in association with carcinoma of the bile duct. The increased risk of biliary tract malignancy, even after surgery, warrants close surveillance in any case of choledochal cyst.
With the use of prenatal ultrasonography, an increasing number of choledochal cysts have been reported in the fetus. [36, 37, 8, 38, 39, 40, 41] Incomplete gastric obstruction by a large cyst is one of the typical clinical manifestations in newborns and young infants.  The earliest reported choledochal cyst was detected in a fetus at 15 weeks' gestation, which may correspond to the timing of the formation of pancreatic enzymes.
The prenatal demonstration of a cystic structure inferior to the liver strongly suggests the diagnosis. Fetal development should be carefully monitored with serial ultrasonography after such a discovery. Most centers prefer to excise the cyst shortly after birth. A waiting period of a few weeks is necessary to stabilize the baby and allow for proper preoperative evaluation. Surgical excision in the neonatal period has been shown to be technically feasible and well tolerated by the patient.
Two distinct clinical groups of patients are recognized with regard to age at presentation. [6, 8, 7, 28, 29, 27] The first group is the infantile group consisting of babies younger than 1 year, with or without obvious hepatomegaly, with obstructive jaundice and acholic stools. This clinical picture is indistinguishable from that of biliary atresia in the absence of a palpable mass in the right side of the abdomen. However, the cystic mass can usually be detected either at clinical examination or on ultrasonography; this finding suggests a diagnosis of choledochal cyst.
In 1995, Todani et al found that 26 of 28 infants younger than 1 year had a mass, whereas only 3 of 8 infants aged 13-24 months had a mass.  Other symptoms, such as vomiting, fever, and abdominal pain with hyperamylasemia are extremely infrequent.  In infants with a prenatal diagnosis of choledochal cyst, jaundice often does not manifest until 1-3 weeks after birth. [43, 44]
In contrast, infants older than 1 year, with the so-called adult form of choledochal cyst, generally have one or more components of the classic triad: pain, jaundice, and a palpable mass. The entire triad is present in fewer than 30% of patients.  Jaundice is intermittent and often associated with vague abdominal pain. The pain has been described as being similar to that of cholangitis or recurrent mild pancreatitis. Undiagnosed choledochal cysts can lead to choledocholithiasis, cirrhosis with portal hypertension, cyst rupture, or biliary carcinomas.
Total excision of the cyst with adequate bile drainage is the standard treatment for choledochal cyst.  However, in the setting of extensive inflammation, a cyst mucosectomy and cystoenterostomy can be performed.
Regarding the anatomic classification of choledochal cyst, in 1977, Todani et al modified the classic Alonso-Lej classification by adding two new types (types IV and V).  Others have subsequently added further subtypes (see the image below). The types are as follows:
Type I - Cystic or fusiform dilatation of the common bile duct (CBD); most frequent type (90-95% of the cases).
Type II - Diverticulum of the CBD, with normal size CBD
Type III - Choledochocele, a cystic dilatation of the distal intramural portion of the CBD, typically protruding into the second portion of the duodenum
Type IV - Cystic or fusiform dilatation of the CBD associated with cystic, fusiform, or saccular dilatation of intrahepatic bile ducts, also termed form fruste
The treatment of choledochal cyst is typically complete surgical excision at the time of presentation, with the exception of type V (multiple intrahepatic cysts) which may benefit from conservative therapy including percutaneous drainage and medical management.