What type of choledochal cyst is characterized by a cystic dilation of the extrahepatic (outside the liver) biliary tree?

Type I Choledochal Cyst: Cystic Dilation of the Extrahepatic Biliary Tree

Type I choledochal cyst is characterized by a fusiform or saccular dilation of the extrahepatic bile duct, representing the most common type of choledochal cyst according to the Todani classification system. 1

Classification and Characteristics

• Type I choledochal cysts are solitary, extrahepatic bile duct dilations that have the greatest predisposition to malignant transformation along with Type IV cysts 1
• These cysts appear as fusiform or saccular dilations of the common bile duct, with continuity between the cystic lesion and the draining bile ducts visible on contrast-enhanced imaging 2
• Type I cysts must be distinguished from other biliary pathologies such as Caroli disease (Type V), which primarily affects the intrahepatic bile ducts 2

Diagnostic Imaging

• Magnetic resonance cholangiopancreatography (MRCP) offers the highest diagnostic accuracy for choledochal cysts, allowing optimal visualization of the biliary tree 2, 1
• On imaging, Type I cysts appear as well-defined dilations of the extrahepatic bile duct, which can be fusiform (gradually tapering) or saccular (balloon-like) in shape 1
• Contrast-enhanced MRI can demonstrate the continuity between the cystic dilation and the biliary system, a key diagnostic feature 2
• Ultrasound may serve as an initial screening tool but has limitations in fully characterizing the extent and classification of the cyst 1

Clinical Significance and Complications

• Type I choledochal cysts carry a significant lifetime risk of malignant transformation to cholangiocarcinoma, ranging from 5% to 30% if left untreated 2
• The risk of malignancy increases with age, with the average age at cholangiocarcinoma diagnosis being 32 years in patients with choledochal cysts 2
• These cysts can cause symptoms including jaundice (84-90%), abdominal pain (30%), and weight loss (35%), though many patients may be asymptomatic and diagnosed incidentally 1

Management Approach

• Complete surgical excision of the choledochal cyst with Roux-en-Y hepaticojejunostomy is the treatment of choice for Type I cysts 3, 1
• Simple drainage procedures or partial excision are inadequate as they leave behind abnormal epithelium with malignant potential 3
• Early intervention is recommended, particularly in pediatric patients, to prevent complications including recurrent cholangitis, pancreatitis, and malignant transformation 3

Long-term Follow-up

• Even after complete resection, patients require long-term surveillance due to a 5.6% median incidence of metachronous malignant lesions 3
• Recommended follow-up includes annual liver function tests and CA19-9 for 20 years (then biannually), along with biannual ultrasound for 20 years (then every 3 years) 1
• The risk of malignancy appears primarily limited to the first 20 years after resection 1

Differential Diagnosis

• Type I choledochal cysts must be differentiated from other conditions such as primary sclerosing cholangitis, which presents with multifocal strictures and a "beaded" appearance of bile ducts 2
• Caroli disease (Type V choledochal cyst) affects the intrahepatic bile ducts rather than the extrahepatic system and typically shows the characteristic "central dot sign" on imaging 2
• Adult forms of ABCB4/MDR3 deficiency can mimic choledochal cysts with large spindle-shaped bile duct dilations but can be distinguished by family history and genetic testing 2