The common bile duct (CBD) is the main transport pathway for bile, a fluid produced by the liver that aids in fat digestion. This tube-like structure is formed by the junction of the common hepatic duct from the liver and the cystic duct from the gallbladder. Its function is to carry bile from the liver and gallbladder down to the duodenum, the first section of the small intestine.
When a physician reports a “dilated” common bile duct, it signifies an abnormal widening of the duct’s diameter. While a normal CBD diameter is generally up to 6–8 millimeters, a measurement of 7 millimeters or greater on an ultrasound is often accepted as a sign of dilation that warrants further investigation. This widening results from increased pressure within the biliary system, most often caused by a downstream blockage.
Primary Causes of Common Bile Duct Dilation
The most frequent reason for the common bile duct to dilate is an obstruction that prevents the normal flow of bile into the small intestine. When bile cannot pass the obstruction, it backs up, increasing pressure and causing the duct walls to stretch. This mechanical blockage is often caused by gallstones (choledocholithiasis) that have migrated from the gallbladder and become lodged within the CBD.
Gallstones can lead to complications like ascending cholangitis (an infection of the bile duct) or pancreatitis if the blockage is near the pancreas. The backup of bile into the liver and bloodstream can also cause jaundice, characterized by yellowing of the skin and eyes. Tumors represent another cause of obstruction, frequently originating in the head of the pancreas, the bile duct itself (cholangiocarcinoma), or the ampulla of Vater.
These malignant obstructions create a progressive narrowing of the duct. Benign strictures, which are areas of scarring, can also cause dilation by creating a chronic partial blockage. Chronic inflammation, such as that seen with chronic pancreatitis or prior surgical trauma, may lead to these strictures.
Non-obstructive causes also exist; for example, the CBD often appears slightly wider in older adults or in patients who have had their gallbladder removed (cholecystectomy). Nonetheless, when dilation is newly discovered or accompanied by symptoms, a thorough investigation is required to rule out a significant underlying obstruction.
Investigating the Cause
The diagnostic process begins with laboratory tests that assess liver function and the degree of bile backup. Physicians order Liver Function Tests (LFTs), which look for elevated enzymes like alkaline phosphatase (ALP) and bilirubin. Significant elevations in these markers suggest biliary obstruction is present and guide the need for immediate imaging.
The initial imaging method is transabdominal ultrasound, which confirms the dilation and may immediately identify the presence of gallstones within the duct. If the ultrasound is inconclusive or suggests a more complex blockage, advanced imaging is used. Magnetic Resonance Cholangiopancreatography (MRCP) creates detailed images of the biliary and pancreatic ducts.
MRCP locates the precise site of an obstruction, whether it is a stone or a tumor. Endoscopic Retrograde Cholangiopancreatography (ERCP) is a more invasive procedure. ERCP can be used for both diagnosis and immediate treatment, making it a valuable tool when an obstruction needs to be relieved quickly.
Treatment Pathways
The management of a dilated common bile duct depends on the underlying cause. When dilation is caused by a gallstone, the primary goal is to remove the stone and restore bile flow. This is most often accomplished using ERCP.
For strictures or blockages caused by benign scarring or malignant tumors, the immediate concern is to relieve the pressure. This is achieved by placing a stent across the narrowed segment of the duct. The stent holds the duct open, allowing bile to drain.
Surgical intervention is necessary for complex or malignant causes of dilation. For instance, a complex benign stricture or a cancer in the head of the pancreas may require an operation, such as a Whipple procedure. If the dilation is found to be mild and non-obstructive, the approach is often watchful waiting and periodic monitoring.