Special Feature

Diagnostic Imaging for Appendicitis

By Theodore C. Chan, MD, FACEP

With more than 250,000 cases annually in the United States, appendicitis remains the most common abdominal surgical emergency confronting physicians. The disease carries a lifetime risk of 7% and occurs throughout all age groups in both genders, although it occurs more commonly in young men.1 Despite its prevalence, the diagnostic evaluation for appendicitis remains challenging. Clinical presentation can vary. Often, the classic findings of periumbilical pain migrating to the right lower quadrant (RLQ) associated with fever, anorexia, nausea, and vomiting are not present. Atypical presentations are common, particularly in young children and elderly patients. As a result, diagnosis is often delayed, leading to complications such as perforation, as well as significant morbidity and mortality.2

Historically, laboratory and other studies have had limited utility. Although commonly obtained, both the white blood cell (WBC) count and plain abdominal radiology have limited use in either diagnosing or excluding appendicitis. As a result, hospitalization and high negative appendectomy rates (as high as 50%) in order to prevent delayed diagnosis and complications have traditionally been acceptable, leading to significant costs and use of health care resources.3 In the last 15 years, however, advances in diagnostic imaging, particularly for ultrasonography and computed tomography, have significantly changed the approach to diagnosing appendicitis.

Plain Abdominal Radiography. Plain abdominal radiography remains the first and most common diagnostic imaging test obtained in patients with suspected appendicitis. Findings consistent with the disease include an appendolith, RLQ soft tissue mass, distal small bowel obstruction, focal RLQ ileus (sentinel loop), appendiceal luminal air or double lucency sign (extraluminal air bubbles), indistinct right psoas margin with obliteration of adjacent properitoneal fat, and pneumoperitoneum in the case of perforation. Of these, only the appendolith is specific for appendicitis, yet only 10%-15% of patients with appendicitis will have this finding on radiograph. Other findings are neither sensitive nor specific and often can be misleading. Moreover, a normal plain radiograph does not exclude the diagnosis. Overall, plain abdominal radiography is of limited utility.4

Barium Enema. Barium enema studies have been reported to have high accuracy (as high as 91.5%) in the setting of appendicitis. Findings consistent with the diagnosis include nonvisualization of the appendiceal lumen and an apical cecal extrinsic mass impression. However, to exclude appendicitis, barium must fill the entire appendiceal lumen up to its bulbous tip. Partial filling, such as with distal appendicitis, may be mistaken for complete filling. Moreover, up to 30% of normal appendices will not fill with barium. In many cases, the lumen cannot be visualized, resulting in an indeterminate study. The general consensus remains that overall, barium studies have low sensitivity, low specificity, and limited utility in this setting.1

Nuclear Medicine Scan. Radioisotope-labeled WBC scans have long been touted as potentially important diagnostic adjuncts for appendicitis, with a few select centers reporting impressive sensitivities (as high as 98%).5 However, as the key finding for appendicitis is increased RLQ isotope uptake, these scans remain highly nonspecific, and overall accuracy can be quite low. Moreover, scanning takes a minimum of 2-3 hours, and may require repeat scanning at 24 hours. Such delay can lead to significant complications. While newer labels and scanning techniques continue to be developed, the overall utility of these scans remains questionable for most institutions.

Magnetic Resonance (MR). While commonly used for nervous and musculoskeletal system imaging, MR of the abdomen has been limited. In fact, MR has poor ability to image the normal appendix. However, with gadolinium enhancement, MR has been reported to have high sensitivity (up to 97%) and accuracy (up to 95%) for appendicitis.6 In such enhanced studies, the inflamed appendix appears as a blind-ending, fluid-filled tubular structure with a thickened wall. The emergent use of MR, however, remains limited by its high cost, lack of easy availability, and long scanning delays. Moreover, MR is contraindicated for many individuals, further restricting its use on a widespread basis.

Ultrasonography (US). While long used for gynecologic imaging, US use for appendicitis has markedly increased in the last 15 years. Because of its easy availability, low cost, and lack of ionizing radiation, ultrasound appears to have a number of advantages. The key technique of "graded compression" was first described in 1986, in which the probe is used to gradually compress the RLQ until the appendix is visualized anterior to the iliac vessels and psoas muscle. The normal appendix is compressible like other bowel, nontender, and less than 6-7 mm in diameter. The inflamed appendix is noncompressible, fluid-filled, tender, and usually greater than 7 mm in diameter. A hyperechoic appendolith with acoustic shadowing is very specific for the disease. Other findings include periappendiceal inflammatory mass, RLQ fluid collection, and regional adenopathy. The use of Doppler to visualize increased flow in the appendix wall and periappendiceal region has recently been described.7 Unfortunately, RLQ inflammation and adenopathy are nonspecific and may be seen in other diseases, such as diverticulitis. In such instances, visualization of a normal appendix on ultrasound can be extremely helpful. Yet, in up to 50% of nonappendicitis cases, the organ cannot be visualized. Other pitfalls include the fact that ultrasound is operator-dependent and can be more difficult in patients who are obese, have variant RLQ anatomy, or are already perforated.

Reports have varied regarding the sensitivity (ranging from 68% to 96%), specificity (73% to 100%), and overall accuracy (71% to 95%) of ultrasound. Greater success has been reported in Europe, where ultrasound is performed by physicians.8,9 Reports from the United States are less conclusive. Wade reported that nearly one-quarter of patients with normal ultrasound findings ultimately had appendicitis.10 Other reports suggest a decreased diagnostic accuracy with ultrasound.11 Even in children, where US is more commonly used for appendicitis, reported sensitivities have varied from as low as 44% to as high as 92%.12,13

Additionally, some have questioned whether ultrasound has a beneficial effect on patient outcome. In a review of 231 pediatric cases, Roosevelt reported the use of ultrasound did not result in earlier diagnosis, did not reduce overall perforation and complication rates, and delayed surgery in those with the disease.14 In a meta-analysis involving more than 3000 cases, Orr reported an overall sensitivity of 85% and specificity of 92% with ultrasound. Interestingly, Orr found ultrasound to be the least helpful in cases in which the pre-test probability and clinical suspicion for appendicitis were either high or low. The investigators suggest that ultrasound only be utilized in cases in which clinical suspicion for the disease is intermediate.15 Overall, the role of ultrasound for appendicitis continues to be debated. Its use will continue to be focused on children (given concerns regarding radiation exposure) and women of childbearing age (in whom gynecologic disorders must be considered).

Computed Tomography (CT). In large measure, CT remains the imaging modality of choice for the abdomen.

Early studies on the use of conventional abdominal CT with oral and intravenous (IV) contrast suggested an excellent sensitivity (96%-98%) and high accuracy 93%- 94%) for appendicitis. Moreover, CT had higher rates of visualizing the normal appendix when it was present and enabling determination of an alternative diagnosis for non-appendicitis cases.3 On CT, the normal appendix appears as a tubular structure with a collapsed or patent lumen (filled with air or contrast), having a maximum diameter of 7 mm. The specific finding of an appendolith is seen in up to 46% of true appendicitis cases. Other findings include an enlarged appendix that fails to completely fill with oral contrast, wall thickening and enhancement, extraluminal air, evidence of periappendiceal inflammation (fat stranding, mistiness, or haziness), RLQ inflammatory mass, cecal apical thickening, and evidence of pneumoperitoneum from perforation.

Conventional CT with oral and IV contrast, however, has significant drawbacks. First, the study requires time for contrast administration and scanning. Second, many patients cannot tolerate oral contrast. Third, IV contrast itself presents significant risk for adverse events. In this respect, the advent of helical CT in which scanning is performed continuously during a single inspired breath represents a major advance. Helical CT has the advantages of reduced scanning time, decreased image noise and artifact from respiration, and reduced need for contrast.16 Recent reports suggest that helical abdominal CT without any contrast has excellent sensitivity (as high as 96%), specificity (99%), and accuracy (97%) for the disease.17 In addition, Rao has reported that a focused lower abdominal helical CT with only rectal/colon contrast has sensitivity and specificity for appendicitis equal to those of conventional abdominal CT with IV/oral contrast. This appendiceal CT results in one-third the radiation and significantly shorter scanning times. Rao reported that CT use changed the management of 59% of patients with suspected appendicitis, avoiding unnecessary surgeries and hospitalizations, and resulting in significant cost savings.18 Because of the success of CT, many centers have developed algorithms for scanning patients suspected of appendicitis. Our particular institution calls for an initial abdominal helical CT scan without contrast in those with intermediate clinical suspicion. If evidence of appendicitis is seen or the normal appendix is visualized, the scan is complete. For indeterminate scans, contrast is administered.

While CT use for appendicitis is increasing, there are potential pitfalls, including failure to achieve optimal cecal opacification and discrimination, particularly in thin patients who have little intraperitoneal fat to help visualize the appendix. The focused scan also may miss upper abdominal processes that could provide alternative diagnoses for the patient’s presentation.

Choice of Imaging Modality. The decision to obtain an imaging study for suspected appendicitis will still depend on clinical impression. Likely, patients in whom there is very high clinical suspicion should not have an expensive imaging study performed as it is unlikely to change their care. Similarly, patients in whom there is very low clinical suspicion will also obtain little benefit from imaging. The choice of modality will likely depend on a number of factors, not the least of which include institutional radiology resources, availability, and expertise. Recent work suggests helical CT scan, with limited or no contrast, will increasingly become the imaging modality of choice for most patients.

References

1. Rao PM, Boland GW. Imaging of acute right lower abdominal quadrant pain. Clin Radiol 1998;53:639-649.

2. Wagner J, et al. Does this patient have appendicitis? JAMA 1996;276:1589-94.

3. Rao PM, et al. Helical CT of appendicitis and diverticulitis. Radiol Clin North Am 1999; 37:895-910.

4. Rao PM, et al. Plain abdominal radiography in clinically suspected appendicitis: Diagnostic yield, resource use, and comparison with CT. Am J Emerg Med 1999; 17:325-328.

5. Rypins EB, et al. Tc-99m-HMPAO white blood cell scan for diagnosis of acute appendicitis in patients with equivocal clinical presentation. Ann Surg 1997; 226:58-65.

6. Incesu L, et al. Acute appendicitis: MR imaging and sonographic correlation. Am J Roentgenol 1997;168: 669-674.

7. Gutierrez CJ, et al. Doppler ultrasound accurately screens patients with appendicitis. Am Surg 1999;65: 1015-1017.

8. Zielke A, et al. Influence of ultrasound on clinical decision making in acute appendicitis: A prospective study. Europ J Surg 1998;164:201-209.

9. Allemann F, et al. Ultrasound scans done by surgeons for patients with acute abdominal pain: A prospective study. Europ J Surg 1999;165:966-970.

10. Wade DS, et al. Accuracy of ultrasound in the diagnosis of acute appendicitis compared with the surgeon’s clinical clinical impression. Arch Surg 1993;128:1039-1044.

11. Ford RD, et al. Diagnostic ultrasound for suspected appendicitis: Does the added cost produce a better outcome? Am Surg 1994;60:895-898.

12. Garcia Pena BM, et al. Ultrasonography and limited computed tomography in the diagnosis and management of appendicitis in children. JAMA 1999;282: 1041-1046.

13. Ramachandran P, et al. Ultrasonography as an adjunct in the diagnosis of acute appendicitis: A 4-year experience. J Pediatr Surg 1996;31:164-167.

14. Roosevelt GE, Reynolds SL. Does the use of ultrasonography improve the outcome of children with appendicitis? Acad Emerg Med 1998;5:1071-1075.

15. Orr RK, et al. Ultrasonography to evaluate adults for appendicitis: Decision making based on meta-analysis and probabilistic reasoning. Acad Emerg Med 1995;2: 644-650.

16. Gupta H, Dupuy DE. Advances in imaging of the acute abdomen. Surg Clin North Am 1997;77:1245-1263.

17. Lane MJ, et al. Suspected acute appendicitis: Nonenhanced helical CT in 300 consecutive patients. Radiology 1999;213:341-346.

18. Rao PM, et al. Effect of computed tomography of the appendix on treatment of patients and use of hospital resources. N Engl J Med 1998;338:141-146.