Nonenhanced Helical CT Accurate in Suspected Acute Appendicitis
Nonenhanced Helical CT Accurate in Suspected Acute Appendicitis
Abstract & Commentary
Synopsis: Nonenhanced abdominopelvic CT, performed without oral, rectal, or intravenous contrast material, is highly accurate in the evaluation of patients for suspected acute appendicitis when the primary criterion for radiologic diagnosis is an enlarged appendix with periappendiceal inflammation. Alternative diagnoses are commonly identified in patients without appendicitis. Ultrasound remains important in evaluating pediatric, pregnant, or very thin patients.
Source: Lane MJ, et al. Suspected acute appendicitis: Nonenhanced helical CT in 300 consecutive patients. Radiology 1999;213:341-346.
As many as approximately one-quarter of patients with acute abdominal pain have acute appendicitis while approximately one-quarter of patients with suspected acute appendicitis present with atypical clinical and laboratory findings. These statistics indicate the need for a good (sensitive and specific) diagnostic test for acute appendicitis. In the absence of such a test, many normal appendices must be removed (typically about 15% of surgical specimens negative for acute appendicitis) in order to avoid missing a case of a not-yet-perforated appendicitis progressing to perforation and peritonitis.
Nonenhanced CT—without oral, rectal, or intravenous contrast—appears to be just what the doctor ordered. CT can be performed rapidly and at low risk. As confirmed in the present study of Lane and associates, CT is accurate in cases of suspected acute appendicitis in all but asthenic patients and has the benefit of frequently demonstrating a different cause for the symptoms when appendicitis is not present.
The study of Lane et al involved 300 consecutive patients (ages, 8-86) referred for CT for the diagnosis of appendicitis suspected by surgeons and emergency medicine physicians. Patients underwent nonenhanced helical CT from the top of the T12 vertebral body to the pubic symphysis at 5 mm collimation and 8 mm/sec table speed (i.e., pitch of 1.6), reconstructed every 5 mm. An appendix of diameter greater than 6 mm was considered potentially abnormal but not sufficient for diagnosis. The diagnosis of acute appendicitis also required that periappendiceal inflammatory changes be present. The presence of an appendicolith was not part of the primary criteria for diagnosing acute appendicitis (but contributed to the mistaken diagnosis of appendicitis in one of the false-positive cases). CT findings were correlated with surgical findings or clinical follow-up in the nonoperative cases.
Prospective interpretations yielded 110 (37%) true- positives, 181 (60%) true-negatives, four (1%) false-positives, and five (2%) false-negatives. In the 181 true-negative interpretations, a normal appendix was identified in 139 (77%). In 63 (35%) of the 181 true negative CTs for appendicitis, a correct alternative diagnosis was made, including renal or ureteral calculi, gynecologic disease, diverticulitis, small-bowel disease, mesenteric adenitis (in 3 patients), neoplasms, cholelithiasis, hernia, and abscess. In retrospective review of the four false-positive cases, two showed dilated appendices (including 1 with an appendicolith), but neither demonstrated periappendiceal inflammation and, thus, were interpretative errors. One false-positive case demonstrating the CT diagnostic findings, albeit subtly, was histologically normal at surgery. The fourth false-positive case proved to be perforated cecal diverticulitis with secondary involvement of the appendix. Retrospective review of the five false-negative cases demonstrated three interpretative errors (occurring early in the study), one dilated appendix without periappendiceal inflammation, and one case in which the appendix was not visualized in a thin patient.
Lane et al note that the appendix frequently drapes over the right external iliac artery and vein and that tracing the course of these vessels can help in identifying the appendix. The entire appendix must be evaluated because early appendicitis can present with findings only at the tip. Because the key finding of periappendiceal inflammation requires periappendiceal fat in which to see the stranding, false-negative findings may occur in thin patients; an appendix deep in the pelvis also appears to be a risk factor for a false-negative interpretation. Lane et al conclude that nonenhanced helical CT should be the initial study for evaluation of suspected acute appendicitis in adults with normal or obese body habitus. Ultrasound may be the better choice for pediatric, pregnant, or very thin patients.
Comment by James H. Ellis, MD
The applications for helical CT just keep on coming. In their article on the use of helical CT in the evaluation of suspected acute appendicitis, Lane et al describe a large series of 300 patients that confirms prior work showing that CT is accurate in this condition in most patients. Of particular importance, Lane et al demonstrate that the CT can be performed without oral, rectal, or intravenous contrast. This has several benefits. It costs less than scanning with intravenous contrast. It takes less time for a complete study than if the patient must ingest oral contrast. It is more pleasant for the patient than rectal (or other) contrast, and it avoids the small but not zero risk of intravenous contrast administration. Finally, it does not require the administration of any contrast material that may interfere with a subsequent examination in the event that the CT does not answer the question. But that isn’t all. CT is not only accurate in determining whether the patient has acute appendicitis, it also frequently shows, in those patients who do not have appendicitis, what they do have.
The present work, while dramatic in its positive and negative predictive values of 97% each, should not be swallowed whole. Of 300 cases, there were 110 true-positive CTs and five false-negative CTs, meaning that 115 (38%) of the patients scanned had acute appendicitis, more than the 28% of acute abdominal pain patients who had appendicitis in a study cited Lane et al. They state that referrals came from surgeons and emergency medicine physicians, but they don’t explicitly state whether all patients with suspected appendicitis were referred for CT or whether some were taken to the operating room without CT because the symptoms were classic. (And if so, we don’t know what the accuracy of the clinical diagnosis was in the non-CT group.) Even though it isn’t entirely clear what the group being studied really was, it does appear that the group studied was a selected population. The predictive values might differ when a more unselected population of patients with abdominal pain (e.g., including patients from general outpatient clinics) is evaluated.
This minor point is not meant to slow the accelerating movement toward nonenhanced helical CT as a virtual extension of the physical exam. It is wonderful that nonenhanced CT can be useful for so many acute conditions, including appendicitis, urinary tract calculi, and a host of other diseases that have been reported in various studies of patients scanned for evaluation of the first two disorders mentioned. In real life (unlike television), it is often difficult to get an ER physician to commit to a suspected diagnosis, the differential set is frequently appropriately broad, time pressures lead to a desire to perform a complete diagnostic workup before the first results are back, and physicians may be harried and haven’t the time for a careful history and physical examination. Indeed, CT may be most helpful in this situation. However, it does mean that it may be more difficult to achieve the high accuracy levels of the present study in other situations.
An important consideration (emphasized by the non-appendiceal diagnoses made) is that the CT technique should be one that will not only identify appendicitis, but other conditions (renal stones, abscess, pelvic inflammatory disease, diverticulitis, etc.) as well. Lane et al used the same protocol for appendicitis as they do for renal colic studies. They argue, and I agree, against a focused pelvic CT for suspected appendicitis because the kidneys are not imaged. Another reason to abandon pelvic CT for appendicitis is that some appendices lie above the pelvic brim and won’t be included in the scan field, requiring the patient to be brought back if he or she has left the CT suite prior to interpretation. Furthermore, such causes of acute abdominal pain as pancreatitis or contained perforated duodenal ulcer may not commonly present in the same way as acute appendicitis, but are we sure when the requisition says "appendicitis" that that is the only condition the referring physician is considering? In our practice, we image the pelvis at 3 mm collimation to better see ureteral calculi and distinguish them from vascular calcifications, though I wouldn’t argue with scanning the pelvis at 5 mm. One may need two breath hold spirals: an abdomen CT at 5 mm collimation, and a pelvis at 3 mm or 5 mm.
Lane et al correctly remind us that ultrasound is also available to look for appendicitis. Though technically demanding, ultrasound may be preferred as the initial examination for thin patients as well as for pediatric1 or pregnant patients. However, it may be tough to get clinicians to factor the amount of patient fat into their algorithms for diagnostic evaluation. CT may also be demanding on the interpretation side, because some appendicitis cases have subtle findings and a dilated appendix can be mistakenly identified as normal small bowel.
One could argue that CT should not delay treatment in patients with a classic appendicitis presentation. However, as the current technique of Lane et al requires no delay or patient discomfort for contrast material administration and literally only minutes of imaging time (though with a variable amount of time for interpretation), there will probably be increasing use of CT to avoid clinical diagnostic errors. It will be of interest to see if more widespread CT imaging of appendicitis and appendiceal post-rupture abscess may lead to increasing use of minimally invasive treatment of these conditions, using antibiotics alone or laparoscopic resection for acute appendicitis and catheter drainage of periappendiceal abscess.2
The interested reader may profit from the immediately preceding article in the same issue of Radiology,3 a review of the use of helical CT in a wide variety of emergency situations including appendicitis. (This article is co-authored by a supporter of the use of rectal contrast and scanning initially limited to the abdominopelvic junction, thus providing a contrasting point of view.)
References
1. Garcia Pena BM, et al. Ultrasonography and limited computed tomography in the diagnosis and management of appendicitis in children. JAMA 1999;282: 1041-1046.
2. Jamieson DH, et al. Interventional drainage of appendiceal abscesses in children. AJR Am J Roentgenol 1997;169:1619-1622.
3. Novelline RA, et al. Helical CT in emergency radiology. Radiology 1999;213:321-339.
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