How should patients with acute cholecystitis be prepared? What manipulations are used to shorten the study or increase its reliability?
- Traditionally, acute cholecystitis is diagnosed on functional cholescintigraphy by noting a lack of filling of the gallbladder on both the initial 60-minute study and subsequent 4-hour delayed images.
- Patient preparation is vital in ensuring that lack of gallbladder visualization is a true-positive finding.
- Instead of 4-hour delayed images, morphine can be used to shorten the time needed to complete this study.
- Because food is a potent and long-lasting stimulus for endogenous cholecystokinin (CCK) release, the patient should not eat for 4 hours prior to the study because endogenous CCK will prevent normal gallbladder relaxation and consequently impair normal filling, leading to a false-positive study.
- These patients should wait 4 hours to ensure an optimal study.
- On the other hand, patients who have had a prolonged fast (longer than 24 hours), are receiving intravenous hyperalimentation, or are severely ill can develop viscous bile formation, which may not be adequately emptied out of a normal gallbladder.
- This can impair radiopharmaceutical filling of the gallbladder, which in turn can also cause a false-positive study. In these patients at risk for viscous bile formation, the short-acting CCK analog sincalide can be administered (0.02 mcg/kg intravenously in 20-30 minutes), prior to cholescintigraphy.
- This ensures proper emptying of the gallbladder before the radiopharmaceutical is administered and will prevent a false-positive event from occurring.
- Despite these manipulations, the gallbladder may not be visualized during the initial 60 minutes of the study.
- Rather than reimage at 4 hours, the study can be expedited using morphine (0.04 mg/kg intravenously), provided small bowel activity is seen within the initial 60 minutes. After morphine administration, imaging is continued for another 30 minutes.
- Because morphine causes sphincter of Oddi contraction, the resultant increased biliary tree pressure will overcome a functional obstruction of the cystic duct.
- If the gallbladder is still not seen, delayed imaging is not necessary and acute cholecystitis is diagnosed.
- Overall, the sensitivity for acute calculous cholecystitis is 97% with a specificity of 85%. The sensitivity and specificity are slightly lower in acute acalculous cholecystitis with a sensitivity and specificity of 79% and 87%, respectively.
- If there is pericholecysitic hepatic activity with a subsequent rim sign, the potential for a complicated cholecystitis (i.e., gangrenous or perforated gallbladder) is significantly higher. In adults, absence of activity in the intrahepatic ducts or small bowel can represent a high grade obstruction. In the early stages, conventional imaging will be normal.
- Fortunately, scintigraphy will demonstrate abnormal excretion before anatomic abnormalities are detectable.
