What is the differential diagnosis (ddx) and management approach for a patient presenting with abdominal pain, jaundice, and elevated liver enzymes, considering gallstone pancreatitis (inflammation of the pancreas caused by gallstones) vs Mirizzi syndrome (obstruction of the bile duct caused by a gallstone) vs choledocholithiasis (presence of gallstones in the bile duct)?

Differential Diagnosis: Gallstone Pancreatitis vs Mirizzi Syndrome vs Choledocholithiasis

Initial Diagnostic Approach

Start with transabdominal ultrasound and liver function tests as the first-line investigation for any patient presenting with abdominal pain, jaundice, and elevated liver enzymes. 1 Ultrasound has 65-95% sensitivity for detecting biliary obstruction and can identify dilated bile ducts, though it has limited sensitivity (22.5-75%) for detecting common bile duct stones due to overlying bowel gas. 1

Key Clinical and Laboratory Distinctions

Gallstone Pancreatitis:

• Presents with severe epigastric pain radiating to the back, elevated amylase/lipase (typically >3x upper limit of normal) 1
• Gallstones are the causative factor in up to 50% of acute pancreatitis cases 1
• Multiple small gallstones (<5 mm) create a 4-fold increased risk for migration into the common bile duct 1
• Transaminases may be markedly elevated initially but typically normalize within days 1

Choledocholithiasis:

• Common bile duct stones occur in 10-20% of gallstone cases, with 5-15% incidence in acute cholecystitis 1
• Gamma-glutamyl transpeptidase (GGT) is the most reliable liver function test with 80.6% sensitivity and 75.3% specificity at cutoff of 224 IU/L 1
• Elevated liver function tests alone have only 15% positive predictive value for common bile duct stones and should never be used as the sole diagnostic criterion 1
• Normal common bile duct caliber on ultrasound has 95-96% negative predictive value 1

Mirizzi Syndrome:

• Rare complication occurring in <1% of gallstone patients 1, 2
• Caused by impacted stone in cystic duct or gallbladder infundibulum causing extrinsic compression of common hepatic duct 2, 3
• Ultrasound findings: shrunken gallbladder, impacted stone in cystic duct, dilated intrahepatic ducts with normal-sized distal common bile duct 2
• Majority of cases (up to 99%) are diagnosed intraoperatively rather than preoperatively 1, 4

Advanced Imaging Algorithm

When Ultrasound Shows Dilated Bile Ducts

Proceed directly to MRCP (magnetic resonance cholangiopancreatography) with contrast-enhanced MRI as the next diagnostic step. 1 MRCP has 86% sensitivity and 94% specificity for diagnosing biliary obstruction and can differentiate between the three conditions. 1

MRCP advantages:

• Superior to CT for assessing biliary sources of right upper quadrant pain with 85-100% sensitivity 1
• Can visualize cystic duct and distinguish Mirizzi syndrome from choledocholithiasis 1, 2
• Identifies extent of inflammation around gallbladder to differentiate Mirizzi syndrome from malignancy 2
• Non-invasive alternative to ERCP for diagnosis 1

When to Use ERCP

Reserve ERCP for therapeutic intervention rather than diagnosis. 1 ERCP should be performed when:

• MRCP confirms common bile duct stones requiring extraction (80-95% clearance rate) 1
• Tissue acquisition needed for suspected malignancy 1
• Mirizzi syndrome confirmed and requires stone extraction or stent placement 2, 5

Critical pitfall: ERCP carries risk of post-procedural pancreatitis (up to 6.3% complication rate with endoscopic ultrasound) 1, so diagnostic ERCP has been replaced by MRCP in modern practice. 1

Distinguishing Features on Imaging

Gallstone Pancreatitis:

• Peripancreatic inflammation and fluid collections on CT/MRI
• May show small stones passed into duodenum or impacted at ampulla
• Pancreatic duct dilation possible

Choledocholithiasis:

• Filling defects within dilated common bile duct on MRCP 1
• Uniform dilation of intra- and extrahepatic bile ducts 1
• Stones appear as hyperechoic foci with acoustic shadowing on ultrasound 1

Mirizzi Syndrome:

• Stone impacted in cystic duct or gallbladder neck 2, 3
• Dilated intrahepatic and proximal common hepatic duct with normal distal common bile duct caliber 2
• Dense adhesions and distorted anatomy at Calot's triangle 3
• ERCP shows extrinsic compression rather than intraluminal filling defect 5, 4

Management Implications

All three conditions require stone extraction in fit patients to prevent unfavorable outcomes. 1 Conservative management of common bile duct stones results in 25.3% unfavorable outcome rate (pancreatitis, cholangitis, obstruction) versus 12.7% with active treatment. 1

Treatment pathways:

• Gallstone pancreatitis: ERCP within 24-48 hours if cholangitis present; otherwise after inflammation subsides, followed by cholecystectomy 1
• Choledocholithiasis: ERCP with stone extraction, then cholecystectomy 1
• Mirizzi syndrome: Open cholecystectomy is standard (laparoscopic contraindicated in severe cases due to 11/14 conversion rate); may require T-tube placement, choledochoplasty, or bilioenteric anastomosis 3, 4

Common pitfall: Mirizzi syndrome misdiagnosed as simple choledocholithiasis leads to increased bile duct injury risk during surgery due to distorted anatomy. 6, 3 Preoperative MRCP recognition allows appropriate surgical planning and prevents complications. 2, 4