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Choledochal cysts represent congenital cystic dilatations of the biliary tree. Diagnosis relies on excluding other conditions as a cause of biliary duct dilatation, e.g. tumor, gallstone, inflammation.
Choledochal cysts are rare, with an incidence of 1:100,000-150,000. Although they may be discovered at any age, 60% are diagnosed before age ten years 1. Females are more frequently affected, with a male-to-female ratio of 1:4. There is a greater prevalence in East Asia.
A number of associations are recognized, including 1:
The classical presentation includes the triad of 1:
This triad is, however, only present in ~40% (range 19-60%) of cases, with palpable mass being the least common manifestation.
Their etiology is uncertain, but a close association with the anomalous formation of the pancreaticobiliary ductal junction is reported in some subtypes 1. Due to this anomaly, there is a large common channel draining the pancreatic and bile ducts. Thus the pancreatic juices cause cholangitis and bile duct wall destruction, which together with distal stenosis due to scarring result in the formation of a choledochal cyst.
Commonly accepted classification currently is one devised by Todani et al. There are five main types, with several subtypes some of which can be pathologically unrelated:
type I: most common, accounting for 80-90% 1 (this type can present in utero)
Ia: dilatation of extrahepatic bile duct (entire)
Ib: dilatation of extrahepatic bile duct (focal segment)
Ic: dilatation of the common bile duct portion of extrahepatic bile duct
type II: true diverticulum from extrahepatic bile duct
type III: dilatation of extrahepatic bile duct within the duodenal wall (choledochocele)
type IV: next most common
IVa: cysts involving both intra and extrahepatic ducts
IVb: multiple dilatations/cysts of extrahepatic ducts only
type V: multiple dilatations/cysts of intrahepatic ducts only (Caroli disease)
type VI: dilatation of cystic duct
The Todani classification scheme has been called into question in surgical literature, with claims that it may unfairly link multiple distinct processes into a spuriously coherent grading scheme 7,8.
The Komi classification classifies choledochal cyst into 3 types based on the anomalous union of the pancreatic-bile duct (AUPBD) 2.
Imaging of the biliary tree can be achieved with ultrasound, CT, direct contrast studies (ERCP, PTC), nuclear medicine examinations or MRI.
The key to the diagnosis is a dilated cystic lesion that communicates with the bile duct and is separate from the gallbladder. A careful search for other causes needs to also be undertaken (see differential below), remaining cognizant that both stone formation and malignancy are associated with choledochal cysts.
Findings are similar to ultrasound, with a greater ability to demonstrate intrahepatic disease and complications.
Radiolabelled HIDA is taken up by hepatocytes and excreted into the biliary system, so tracer accumulation will mimic the progression of bile through the biliary system and the cyst. Tracer will be slow initially to accumulate in the cyst, showing up as an area of reduced uptake compared to the remainder of the biliary tree, reflecting the biliary stasis that is typical of choledochal cysts, then subsequently tracer signal will increase and remain increased for longer than would be expected, due to the slow bile flow 9.
Treatment and prognosis
Patients with type I, II, or IV cysts usually undergo surgical resection of the cyst due to the risk of malignancy. A number of possible approaches exist, depending on cyst location and other factors. Typically a Roux-en-Y hepaticojejunostomy is performed 1.
The two most frequent complications of choledochal cysts are stone formation and malignancy. Complications include:
stone formation: most common
lifetime incidence 10-15% 5
the cyst may rupture, leading to bile peritonitis
most frequently seen in neonates 1
General imaging differential considerations include:
other causes of biliary tree dilatation