Choledochal Cyst Classification
Choledochal cysts are classified into five distinct types according to the Todani classification system, which is based on the anatomical location and pattern of biliary tree dilation. 1, 2, 3
The Five Types of Choledochal Cysts
Type I: Extrahepatic Bile Duct Dilation (Most Common)
- Accounts for 80-90% of all choledochal cysts 3
- Presents as either cystic (saccular) or fusiform (spindle-shaped) dilation of the common bile duct 4, 3
- The cystic lesion shows continuity with the draining bile ducts on contrast-enhanced imaging 4
- Carries the highest risk of malignant transformation to cholangiocarcinoma (5-30% lifetime risk) along with Type IV 4, 1
- Requires complete surgical excision with Roux-en-Y hepaticojejunostomy 4
Type II: Diverticulum of the Common Bile Duct (Rarest)
- Represents the rarest subtype of choledochal cysts 5
- Manifests as a diverticulum (outpouching) protruding from the common bile duct wall 5
- Most reported cases measure several centimeters, though cases as small as 2mm diameter have been documented 5
- The diverticulum wall is histologically identical to the common bile duct 6
Type III: Choledochocele
- Involves the intraduodenal portion of the common bile duct 1
- Presents as a cystic dilation within the duodenal wall at the ampulla 1
Type IV: Multiple Cysts
- Characterized by multiple cystic dilations affecting both intrahepatic and extrahepatic bile ducts 1, 2
- Second most common type after Type I 2
- Shares the highest malignancy risk with Type I cysts 4, 2
- Requires complete excision of extrahepatic components 1
Type V: Caroli Disease
- Exclusively involves intrahepatic bile ducts with multiple cystic dilations 4, 1
- Shows the characteristic "central dot sign" on imaging (portal vein radicle within the dilated duct) 4
- Must be distinguished from Type I cysts, which affect the extrahepatic biliary system 4
- Associated with congenital hepatic fibrosis 1
Mixed Subtypes
A rare mixed variant combining Type I and Type II features has been reported, occurring in approximately 1.1% of cases 6. These present with fusiform common bile duct dilation plus a diverticulum originating from the mid-CBD, with the cystic duct draining into the diverticulum 6. Despite the mixed morphology, surgical management follows the same principles as Type I cysts with complete excision 6.
Clinical Significance of Classification
- Accurate classification is critical because Types I and IV require aggressive surgical intervention due to their 5-30% malignancy risk 4, 1
- The risk of cholangiocarcinoma increases with age, with average diagnosis at 32 years in untreated patients 4
- Complete cyst excision is necessary for types with high malignant potential, while observation may be considered for certain Type V presentations 1
- Even after complete resection, 5.6% of patients develop metachronous malignant lesions, necessitating lifelong surveillance 7, 4