IS case 535: Acute appendicitis
Imaging Sciences URMC 2010
Publication Date: 2010-09-11
The diagnostic criteria for appendicitis includes complete visualization of the appendix, measuring its outer diameter, compressibility, and echogenic inflammatory periappendiceal fat changes. Data for all age groups suggests that when the outer diameter of the appendix measures greater than 6 mm the sensitivity is 100%, but the specificity (about 64%) for appendicitis is low. So this criterion alone is not enough for the diagnosis of appendicitis. Compression of the appendix as a single criterion is specificity poor. Echogenic inflammatory periappendiceal fat change has proven to be a reliable factor (sensitivity 95%), but has a poor specificity (50%). Color Doppler is a weak diagnostic tool especially when the appendix is deep within the abdomen or is gangrenous.
It is particularly difficult to demonstrate a normal appendix (to exclude appendicitis) when the patient is obese. Obesity can be a factor that prevents a satisfactory examination because compression of the appendix is often inadequate. A CT scan can identify appendicitis in obese persons better than ultrasound exam. Other difficulties like an associated ileus in which a gas-filled bowel may overly the appendix producing shadowing. Sometimes the external iliac artery and vein can provide a good landmark for finding the appendix. The accuracy of ultrasound exam can be diminished by normal anatomical conditions like being retrocecal (60% of population), hidden from the transducer. About 28% of pediatric patients present with a retrocecal appendix that is not hidden. Other times underlying pathology can lead to a false-positive results like Meckel’s or cecal diverticulitis, inflammatory bowel disease, pelvic inflammatory disease, and endometriosis.
- Stocksley M. Abdominal Ultrasound. Greenwich Medical Media, 2001.