Gallstone Size and Passage from the Gallbladder
Gallstones smaller than 4-5 mm in diameter can pass through the cystic duct from the gallbladder into the common bile duct, with stones 1-4 mm being the most likely to migrate and cause complications such as pancreatitis, cholangitis, or biliary obstruction. 1
Critical Size Thresholds
Stones That Can Migrate (High Risk)
- Stones 1-4 mm in diameter are the primary culprits for migration through the cystic duct into the common bile duct 1
- Multiple small stones (<5 mm) in the gallbladder create a 4-fold increased risk for migration into the CBD 2
- Debris and particles ≤1 mm can pass through the cystic duct regardless of duct diameter 1
Anatomic Factors Enabling Passage
- Normal cystic duct diameter is ≤4 mm, and only 3% of patients with normal-caliber ducts experience stone migration 1
- Dilated cystic ducts (>4 mm) allow migration in 32.5% of patients, representing a 10-fold increase in risk 1
- Once small stones (1-4 mm) pass through, they can enlarge within the common bile duct and increase biliary pressure, which retrogradely dilates the duct system and allows progressively larger stones to follow 1
Clinical Consequences of Stone Migration
Immediate Complications
- Acute biliary pancreatitis: Gallstones cause up to 50% of all acute pancreatitis cases, with migrating stones being the primary mechanism 3, 4
- Acute cholangitis: Infection of the bile duct from obstruction by migrated stones 3
- Biliary obstruction with jaundice: Partial or complete blockage of bile flow 3
Prevalence of CBD Stones
- 10-20% of patients with symptomatic gallstones have concurrent common bile duct stones 3, 5
- In acute cholecystitis, the incidence drops to 5-15% 5
- In acute biliary pancreatitis, the prevalence is substantially higher due to the pathophysiologic mechanism of stone migration 4
Stones That Cannot Pass
Size Limitations
- Stones >20 mm in diameter rarely dissolve with medical therapy and are too large to pass through the cystic duct 2, 6
- Common bile duct stones are often larger than the cystic duct diameter, and when experimentally reintroduced into the gallbladder, they will not migrate back through 1
Composition Barriers
- Calcified gallstones cannot be dissolved medically and are unlikely to pass 2
- Pigment stones >1.5 cm with perihepatic localization are predicting factors for severe complications if spilled during surgery 2
Medical Dissolution Considerations
Treatable Stone Sizes
- Stones <6 mm: Best treated with oral bile acids (chenodeoxycholic acid 15 mg/kg/day or ursodeoxycholic acid 10 mg/kg/day), achieving up to 75% complete dissolution annually 7
- Stones up to 5 mm: Complete dissolution observed in 81% of patients with ursodiol therapy 6
- Stones <15 mm (radiolucent, cholesterol-rich): Nonsurgical therapy is generally limited to this size range 2
- Single stones <30 mm or multiple stones (n<3): Best treated with lithotripsy combined with oral bile acids, achieving 80% and 40% dissolution rates respectively 7
Stones Unsuitable for Medical Therapy
- Stones >20 mm rarely dissolve and are contraindications to ursodiol therapy 6
- Calcified stones prior to or during treatment predict failure of dissolution 6
Clinical Pitfalls
Detection Challenges
- Ultrasound sensitivity for CBD stones is only 22.5-75%, far lower than for gallbladder stones, because the subhepatic common duct may be obscured by bowel gas 2
- Normal CBD caliber on ultrasound has a 95-96% negative predictive value for choledocholithiasis 2
- Elevated liver function tests alone are insufficient for diagnosis, as 15-50% of patients with acute cholecystitis without CBD stones have elevated LFTs due to inflammation 5
Risk Stratification
- Ultrasound can detect dilated cystic ducts (>4 mm), identifying patients at 10-fold higher risk of stone migration 1
- Bilirubin elevation, age >55 years, and CBD dilatation 6-10 mm combined with tissue harmonic imaging increase sensitivity for CBD stone detection to 70-86% 2