Occurs in 20% if untreated (esp. diabetics, elderly, delay in seeking care)
Consider if patient presents with sepsis in addition to cholecystitis
Occurs in 2% after development of gangrene
Usually localized, leading to pericholecystic abscess
Due to cholecystoenteric fistula
Bowel obstruction due to impaction of gallstone at terminal ileum
Gallstone enters small bowel through biliary-duodenal fistula
Diagnosis suggested by pneumobilia, bowel obstruction, ectopic gallstone
Gallstone enters small bowel through biliary-duodenal fistula
Due to secondary infection of GB by gas-forming organisms (C. perfringens)
Presents like cholecystitis but often progresses to sepsis and gangrene
IV antibiotic and cholecystectomy are essential
Ultrasound report may mistake GB wall gas for bowel gas
Mortality as high as 15% due to gangrene or perforation
Partial obstruction of common hepatic duct due to stone impaction / chronic inflammation
Symptoms of acute cholecystitis + dilated intrahepatic ducts + jaundice
Inflammation can cause erosive fistula from Hartmann pouch into common hepatic duct
US and CT can usually delineate the fistula
Treatment = open cholecystectomy
US and CT can usually delineate the fistula
Acute calculous cholecystitis
Gallbladder disease
Acalculous cholecystitis
Symptomatic cholelithiasis 
Acute pancreatitis
Choledocholithiasis
Ascending cholangitis
Acute cholecystitis