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. tumour, gallstone, inflammation.
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Epidemiology
Choledochal cysts are rare, with an overall incidence of 1:100,000-150,000. There is a greater prevalence in East Asia, particularly Japan, where there is an incidence as high as 1:1000, and two-thirds of reported cases occur 11,12. Although they may be discovered at any age, 40-60% are diagnosed before the age of ten years old 1,10. The female-to-male ratio is reported to be between 3:1 and 4:1 10,12.
Associations
Associations include 1:
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associated with type V (Caroli disease)
Clinical presentation
The classical presentation includes the triad of 1:
abdominal mass
This triad is only present in ~40% (range 19-60%) of cases, with palpable mass being the least common manifestation.
Pathology
Their aetiology is uncertain, but some subtypes are closely associated with the anomalous formation of the pancreaticobiliary ductal junction which results in a large common channel draining the pancreatic and bile ducts 1. This allows pancreatic juices to enter the bile ducts and cause cholangitis and wall destruction. Distal scarring results in a choledocal cyst more proximally.
Classification
Commonly accepted classification is one devised by Todani et al. There are five main types, with several subtypes some of which can be pathologically unrelated:
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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)
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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.
Radiographic features
Imaging of the biliary tree can be achieved with ultrasound, CT, direct contrast studies (ERCP, PTC), nuclear medicine examinations or MRI.
Ultrasound
A dilated cystic lesion that communicates with the bile duct and is separate from the gallbladder may be demonstrated. Other causes for this appearance may be identified (see the differential below), and associated pathology including calculi and malignancy.
CT/MRI
Findings are similar to ultrasound, with a greater ability to demonstrate intrahepatic disease and complications.
Nuclear medicine
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
Approximately 20% of untreated patients will develop malignancies of the gallbladder and biliary tree, thought secondary to chronic inflammation 7,12,13. 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.
Complications
The two most frequent complications of choledochal cysts are stone formation and malignancy. Complications include:
stone formation: most common
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malignancy
lifetime incidence 10-21% 5,11-13
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the cyst may rupture, leading to bile peritonitis
most frequently seen in neonates 1
Differential diagnosis
General imaging differential considerations include:
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cystic tumours
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other causes of biliary tree dilatation
impacted gallstone
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