What is biliary dilation?

What is Biliary Dilation?

Biliary dilation refers to abnormal enlargement of the bile ducts beyond their normal diameter, defined as a common bile duct (CBD) diameter exceeding 8 mm in most patients, though this threshold varies with age and surgical history. 1, 2

Normal Bile Duct Measurements

• The normal CBD diameter should be less than 8 mm in the general population 1
• After cholecystectomy, the CBD may physiologically dilate up to 10 mm without indicating pathology 3, 2
• In elderly patients, some degree of duct dilation may occur as a normal aging phenomenon 4
• Any CBD diameter exceeding 10 mm warrants investigation, regardless of symptoms, as it carries a 39% incidence of CBD stones 3

Anatomic Patterns of Dilation

Biliary dilation can involve different segments of the biliary tree, and the pattern helps narrow the differential diagnosis:

Isolated Intrahepatic Bile Duct Dilation

• Intrahepatic ducts are considered dilated when they exceed 5 mm for peripheral ducts, 1 cm for right/left hepatic ducts 5
• Causes include Caroli disease (congenital cystic dilation), benign mass lesions in segment 4 compressing the common hepatic duct, intrahepatic cholangiocarcinoma, and sepsis-induced cholestasis 6
• The "central dot sign" (portal vein radicle within dilated duct) is pathognomonic for Caroli disease 5, 6

Combined Intrahepatic and Extrahepatic Dilation

• This pattern suggests obstruction at or below the level of the common hepatic duct 5
• Common causes include CBD stones (most frequent), cholangiocarcinoma, primary sclerosing cholangitis with its characteristic "beaded" appearance, and IgG4-related cholangitis 5

Isolated Extrahepatic Dilation

• The extrahepatic bile duct can be dilated while intrahepatic ducts remain normal, particularly in early or partial obstruction 7
• The common bile duct provides a sensitive indicator of biliary obstruction even when intrahepatic ducts appear normal 7

Clinical Significance

Symptomatic Patients

• Biliary dilation in symptomatic patients (abdominal pain, jaundice, fever) indicates likely obstruction requiring urgent investigation 1
• Charcot's triad (fever, chills, jaundice) suggests ascending cholangitis, a surgical emergency 5
• Cholestatic liver function tests (elevated alkaline phosphatase and GGT) support obstructive pathology 3

Asymptomatic Patients

• Incidental biliary dilation in asymptomatic patients with normal liver function tests has a 60.3% yield for identifying causative pathology on MRCP 8
• Even with normal liver function, 27.7% of asymptomatic patients have significant causes requiring intervention 8
• Asymptomatic patients with CBD >7 mm without obvious cause on ultrasound have a 59.2% rate of identifiable pathology on further investigation 9

Diagnostic Approach

Initial Imaging

• Ultrasound should be the first diagnostic modality due to wide availability and high accuracy for detecting biliary dilation, though it has low sensitivity (25-63%) for detecting CBD stones 1, 5
• Ultrasound reliably identifies the presence and pattern of dilation but is less accurate for determining the cause 1

Advanced Imaging

• MRCP is the gold standard non-invasive test with 96-100% sensitivity for CBD stones and superior visualization of biliary anatomy 6, 3
• MRCP has 85-93% sensitivity and 93-96% specificity for CBD stone detection, outperforming CT 1
• CT with IV contrast is indicated when malignancy is suspected, for unstable patients, or to evaluate for hepatic abscesses 1

Invasive Procedures

• Endoscopic ultrasound (EUS) has 93% sensitivity and 96% specificity for CBD stones, making it highly accurate for small stones <4 mm 1
• ERCP should be reserved for therapeutic intervention (stone removal, stent placement) rather than diagnosis alone 1, 4
• ERCP successfully clears CBD stones in 80-95% of cases via balloon sweep 1

Common Pitfalls

• Do not assume physiologic dilation for CBD >10 mm, even post-cholecystectomy, as this requires investigation 3
• Do not rely on negative ultrasound to exclude CBD stones given its poor sensitivity 3
• Do not skip MRCP and proceed directly to ERCP unless therapeutic intervention is clearly indicated 3
• Normal liver function tests do not exclude CBD stones or obstruction, as bilirubin may be normal with bile leaks 3
• Biliary dilation is unlikely to indicate obstruction in the absence of symptoms or elevated liver function tests, but further investigation may still be warranted based on the degree of dilation 2, 8