Imaging Studies of Appendicitis

The potential imaging modalities for diagnosis of acute appendicitis include plain radiographs, ultrasound, and computed tomography. Prior to the widespread use of modern imaging techniques, plain abdominal films were often obtained in patients with abdominal pain, and a right lower quadrant fecalith (or appendicolith) was considered pathognomonic for acute appendicitis. A number of studies question this teaching, however. Teicher and colleagues reviewed the abdominal radiographs of 200 appendectomy patients, 100 with pathologically proven appendicitis and 100 with a normal appendix. Of those with appendicitis, 10.5% had an appendicolith on x-ray, compared to 3.3% of those without appendicitis. An extensive review of appendectomy specimens at the Mayo Clinic showed that fecaliths or appendiceal calculi were present in 9% of patients with nonperforated appendicitis and 21% of those with perforated appendicitis. Interestingly, fecaliths were also present in 7% of patients with suspected appendicitis who had a pathologically normal appendix, and 2% of patients who had an appendectomy for other reasons.

These studies show that fecaliths are not pathognomonic for appendicitis, as some patients with abdominal pain and a fecalith have a normal appendix. In addition, fecaliths are not common enough in patients with appendicitis to be used as a reliable sign. As a result, plain abdominal radiographs are neither helpful nor cost effective and are not recommended for the diagnosis of acute appendicitis. Plain radiographs are indicated in elderly patients with severe abdominal pain, in whom a perforated viscus is included in the differential diagnosis. In this patient population, an upright chest x-ray can assess for the presence of free air.

Abdominal ultrasonography is a popular imaging modality for acute appendicitis. Findings that suggest appendicitis include thickening of the appendiceal wall, loss of wall compressibility, increased echogenicity of the surrounding fat signifying inflammation, and loculated pericecal fluid . The advantages of ultrasound include its widespread availability, as well as the avoidance of ionizing radiation and the side effects of intravenous contrast such as renal toxicity and allergic reactions. In addition, ultrasound (both abdominal and transvaginal) is particularly useful in assessing obstetric and gynecological causes of abdominal pain in women of childbearing age. Ultrasound is highly operator-dependent, however, and it is frequently unable to visualize the normal appendix. A recent meta-analysis of 14 prospective studies showed ultrasound to have a sensitivity of 0.86 and a specificity of 0.81.

Computed tomography (CT) is yet another imaging modality for acute appendicitis. CT benefits from a high diagnostic accuracy for appendicitis and visualization and diagnosis of many of the other causes of abdominal pain that can be confused with appendicitis. The radiographic findings of appendicitis on CT include a dilated (>6 mm), thick-walled appendix that does not fill with enteric contrast or air, as well as surrounding fat stranding to suggest inflammation.In a meta-analysis of 12 prospective studies, CT demonstrated a sensitivity of 0.94 and a specificity of 0.95.CT thus has a high negative predictive value, making it particularly useful in excluding appendicitis in patients for whom the diagnosis is in doubt. Appendicitis is highly unlikely if enteric contrast fills the lumen of the appendix and no surrounding inflammation is present. The clinician must remember, however, that a CT performed early in the course of appendicitis might not show the typical radiographic findings. In confusing cases, it is reasonable to repeat the CT after 24 hours of observation.

A number of recent prospective studies have compared the accuracy of CT and ultrasound in imaging the appendix .Balthazar and associates35 performed CT and ultrasound on 100 consecutive patients with suspected appendicitis. The sensitivity of CT was considerably higher (96% for CT, 76% for ultrasound), while the specificity was comparable (89% for CT, 91% for ultrasound), yielding a higher accuracy for CT (94% versus 83%). CT was also able to provide an alternative diagnosis in more patients and was better able to visualize abscesses or phlegmons . Horton and colleagues36 randomized patients with suspected appendicitis to either CT or ultrasound. Their findings echo those of Balthazar, with both CT and ultrasound having high specificity (100% for CT, 90% for US) but CT having significantly higher sensitivity (97% versus 76%). Yet another prospective study showed similar results, with CT having higher sensitivity (96% versus 62%) and specificity (92% versus 71%) than ultrasound.Again, CT was also better able to visualize other intra-abdominal pathology in the absence of appendicitis.

In a study of 100 patients evaluated by CT with rectal and intravenous contrast, Rao and coworkers showed that CT can reduce the use of hospital resources and costs. CT changed the management of 59 patients, avoiding 13 unnecessary appendectomies and eliminating a total of 50 inpatient hospital days for observation of unexplained abdominal pain. Even factoring in the cost of the CT scans, the authors calculated a net savings of $447 (U.S. dollars) per patient.

Taken together, these studies suggest an algorithm for evaluation of patients with suspected acute appendicitis. Patients with a history, physical examination, and laboratory studies classic for appendicitis should undergo urgent appendectomy. In those with an evaluation suggestive but not convincing for appendicitis, further imaging is indicated. In women of childbearing age, this should begin with a pelvic ultrasound to evaluate for ovarian pathology. Following this, the study of choice is an abdominopelvic CT because of its accuracy in diagnosing both appendiceal and other intra-abdominal pathology. This can be supplemented with rectal contrast CT, if needed, to better visualize the appendix.atients with a CT showing appendicitis are taken for appendectomy. In many instances, patients with a normal CT do not require hospital admission. If symptoms persist, admission to the hospital for observation and perhaps a repeat CT scan is warranted.