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ACUTE CALCULOUS CHOLECYSTITIS

Sonographic Examination of Gallbladder:

The gallbladder is generally imaged with patients in both supine and posterior oblique (LPO) positions. Upright and, on occasion, prone scans may be helpful to visualize very small gallstones. When stones are not immediately apparent, the region of the cystic duct and neck of the gallbladder should carefully be scrutinized for stones in both transverse and sagittal views. When a gallstone is identified in the neck of the gallbladder, it is important to scan the patient in multiple positions to determine if the stone is impacted. All patients with acute right upper quadrant pain should be evaluated for sonographic signs of acute cholecystitis.

Sonography of the gallbladder is best performed with a 5-MHz curved array transducer. A 5-MHz linear array transducer may be helpful in patients with very superficial gallbladders. In obese patients or in patients with difficult sonographic access, a 3,5-MHz sector or curved linear transducer is necessary.

Role of Sonography in Diagnosis of Acute Calculous Cholecystitis:

Sonography has diagnostic accuracies exceeding 90% for suspected acute calculous  cholecystitis. It is preferred as a initial imaging study due to several clear advantages in the evaluation of patients with possible acute cholecystitis:

• It is less expensive than other techniques.

• It often can be performed and interpreted more quickly.

• It provides considerably more anatomic information.

• It can definitely diagnose gallstones as well as other important secondary findings (se below).

• It is independent of hepatic function and biliary obstruction (unlike scintigraphy).

• It may be performed at the bedside in critically ill patients.

• In patients with gangrenous cholecystitis, sonography may guide percutaneous cholecystostomy when clinically appropriate.

• Sonography can survey the entire abdomen if the gallbladder is normal (an important capability as only a minority of patients referred for imaging with right upper quadrant pain prove to have cholecystitis).

Despite these advantages, sonography has a number of significant limitations in the diagnosis of acute cholecystitis:

• Cystic duct stones may be missed due to the small lumen of the cystic duct and lack of surrounding  bile.

• While cystic duct obstruction can generally be inferred when a stone is identified impacted in the neck of the gallbladder, sonography cannot directly diagnose cystic duct obstruction. 

Sonographic Diagnosis of Acute Calculous Cholecystitis:

Gallstones are the single most important finding. In the absence of stones, other observations, such as wall thickening or focal tenderness over the gallbladder (sonographic Murphy's sign) are diagnostic.

The sonographic Murphy's sign is an important finding in suspected acute cholecystitis. It differs significantly from the clinical sign and refers to focal tenderness directly over the gallbladder when pressure is applied by the ultrasound transducer. Gangrenous cholecystitis may cause necrosis and denervation of afferent nerve fibers from the gallbladder and result in a negative sonographic Murphy's sign. However, there are generally other significant sonographic abnormalities such as gallstones, wall thickening, or pericholecystic fluid to suggest the diagnosis of cholecystitis.

Thickening of the gallbladder wall is an important sonographic observation in acute cholecystitis. The normal gallbladder wall measures 3mm or less. Measurements are most accurate when obtained from the anterior subhepatic gallbladder wall using a long axis image. This avoids side-lobe artifacts from adjacent bowel gas and difficulties encountered by dependent intraluminal sludge. Measurements of the gallbladder wall are not reliable in postprandial patients with a contracted  gallbladder.

Symmetric thickening of the gallbladder wall (Picture1) is often not related to intrinsic biliary disease. In the absence of gallstones, this finding must be interpreted with caution in patients with possible cholecystitis. Generalized gallbladder wall thickening may occur in a broad spectrum of other disorders including hepatitis, acquired immunodeficiency syndrome (AIDS), congestive heart failure, hypoalbuminemia, ascites, hyperplastic cholecystosis, and chronic cholecystitis.

Picture 1.

Asymmetric thickening of the gallbladder wall may be due to carcinoma, metastases, adenomyomatosis, or gangrenous cholecystitis. 

Four distinct patterns of gallbladder wall thickening have been identified with sonography:

1. A striated pattern of multiple hypoechoic layers separated by echogenic zones.

2. Asymmetric thickening of the gallbladder wall with echogenic tissue projecting into the gallbladder lumen.

3. A single central hypoechoic zone separated by two echogenic layers.

4. A uniformly echogenic appearance.

Unfortunately, no one pattern of wall thickening as an isolated finding is specific for acute cholecystitis. Furthermore, there is considerable overlap with many other causes of gallbladder wall thickening.

In general, sonography is the most reliable technique for diagnosing acute cholecystitis, if multiple signs are present such as gallstones, a thickened gallbladder wall, and a positive sonographic Murphy's sign. In patients with a sonographically normal gallbladder, acute cholecystitis can be excluded with a high degree of confidence.

REFERENCES:
[1]Sonography of the Abdomen. R.B.Jeffrey, P.W.Rolls. 1995