Choledochoduodenal fistula paradoxically prevents biliary obstruction: a case report

Background: A 38-year-old woman with a 32-year history of recurrent biliary stones (cholecystectomy at age 6 and open choledochotomy with 6-month T-tube indwelling at age 16) presented with 2 h of postprandial right upper quadrant pain. Laboratory tests showed mild hyperbilirubinemia (total bilirubin 24.4 μmol/L, direct bilirubin 8.4 μmol/L) and elevated alanine transaminase of 287 U/L. Abdominal CT scan revealed common bile duct (CBD) stones without pneumobilia. Her symptoms resolved spontaneously before scheduled endoscopic retrograde cholangiopancreatography (ERCP). ERCP showed compensated dilatation of the CBD without residual stones, inadvertent contrast overflowing into the duodenum, a 5-mm choledochoduodenal fistula in the proximal descending duodenum, and a slender distal CBD segment confirmed by intraductal ultrasound. The fistula, further confirmed by enhanced CT, acted as a benign physiological drainage pathway. The slender distal CBD formed a specific pressure gradient, and spontaneous stone passage was achieved via this fistula, which was the core mechanism for the patient's long-term symptom-free survival. Conclusion: Choledochoduodenal fistula can, in rare circumstances, exert a protective rather than deleterious effect in patients with cholelithiasis. This case with a benign clinical course complements the clinical scenario beyond the conventional clinical paradigm that choledochoduodenal fistulas commonly require active intervention.