Acute cholecystitis is the most common cause of acute right upper quadrant pain. Most cases are caused by cystic duct obstruction from an impacted gallstone. Approximately one third of the patients with gallstones will develop cholecystitis. Acalculous cholecystitis accounts for 5-10% of acute cholecystitis cases and typically occurs in critically ill patients who have not been fed orally for a prolonged period of time. In patients with acquired immunodeficiency syndrome (AIDS), acute cholecystitis can be caused by infection with cytomegalovirus, cryptosporidium, or microsporidium.

Before widespread use of imaging, up to 22% of patients who were surgically explored for suspected cholecystitis did not have it. Imaging is also used to look for emergent complications of acute cholecystitis, such as gangrene (2-38% of cases) or perforation (3-15%).

Acute cholecystitis usually presents with sudden onset of right upper quadrant or epigastric pain and tenderness, frequently precipitated by a fatty meal. Most cases subside spontaneously due to antegrade or retrograde stone passage. Attacks lasting more than 1 day are unlikely to resolve spontaneously. Most patients experience nausea and vomiting. Fever and mild leukocytosis are common. Mild elevation of liver function tests is not unusual; however, persistent or significant elevation suggests choledocholithiasis.

Clinical presentation of acute cholecystitis can be mimicked by pancreatitis, appendicitis, peptic ulcer disease, gastroesophageal reflux, hepatitis, liver abscess, liver neoplasm, right lower lobe pneumonia, and cardiac disease.

Ultrasonography is the preferred modality for evaluation of suspected acute cholecystitis because of ready availability and high accuracy. It can also provide additional information about nonbiliary pathology in cases where the gallbladder is found to be normal.

If possible, the examination should be performed after 6-hour fast to ensure adequate distension of the gallbladder. The gallbladder should be examined with the patient in supine and left posterior oblique positions. If cholelithiasis is present, every effort should be made to evaluate for stone impaction in the gallbladder neck or cystic duct by turning or standing the patient to determine if the gallstone is mobile.