Gallbladder polyps, cholesterolosis, adenomyomatosis, and acute acalculous cholecystitis.

Acute acalculous cholecystitis is characterized by acute inflammation of the gallbladder in the absence of stones, usually occurring in elderly and critically ill patients with atherosclerosis, recent surgery or trauma, or hemodynamic instability. Patients may present with only unexplained fever, leukocytosis, and hyperamylasemia without right upper quadrant tenderness. If untreated, rapid progression to gangrene and perforation occurs.