Acute cholecystitis may be complicated by necrosis of the gallbladder wall (gangrenous cholecystitis), empyema, perforation and pericholecystic abscess. The clinical presentation and imaging findings of complicated cholecystitis differ from simple acute cholecystitis, and it is important to recognize the difference on imaging studies. Sonography and CT are useful in distinguishing the two situations. Gangrenous cholecystitis is a fulminant form of acute cholecystitis characterized by mural necrosis and microabscess formation. The term does not always imply infection with a gas-forming organism but does imply severe inflammation which may be associated with gallbladder perforation and/or empyema. When inflammation is the result of ischemia, Clostridia superinfection and even emphysematous cholecystitis may result. On sonography, gangrenous cholecystitis is suggested by the presence of linear echogenic intraluminal membranes representing desquamated mucosa, coarse intraluminal echoes indicative of debris and focal wall irregularities.

Perforation usually develops in the relatively avascular gallbladder fundus and is most commonly associated with acute gangrenous cholecystitis or empyema. Perforation results in a localized, pericholecystic abscess or fistula to an adjacent organ more often than it does to free rupture. Free rupture can occur, however, and is characterized by resolution of an enlarged gallbladder with temporary relief of symptoms followed by signs of peritonitis. Emergent operation is required.

CT is helpful in demonstrating pericholecystic abscess or perforation seen with complicated cholecystitis. Extension of the inflammatory reaction into the liver, adjacent fat and surrounding structures can be detected. A thick, shaggy gallbladder wall, high density bile and air in the gallbladder, or adjacent abscess may also be seen. The main differential diagnosis on CT and US is carcinoma of the gallbladder. The presence of pericholecystic fluid and inflammatory changes in the adjacent fat as well as the clinical history are helpful in making the distinction.