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Differentiation of Perforated from Nonperforated Appendicitis at CT¹

¹ From the Department of Radiology, Albert Einstein Medical Center, 5501 Old York Rd, Philadelphia, PA 19141 (M.M.H., D.S.W.); and Department of Anesthesiology, Drexel University College of Medicine, Philadelphia, Pa (J.C.H.). From the 2001 RSNA scientific assembly. Received March 7, 2002; revision requested April 26; final revision received July 19; accepted August 17. Address correspondence to M.M.H. (e-mail: horrowm@einstein.edu).

	ABSTRACT

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PURPOSE: To evaluate the sensitivity and specificity of five computed tomographic (CT) criteria in the differentiation of perforated from nonperforated appendicitis.

MATERIALS AND METHODS: CT scans of 94 patients with surgically proven appendicitis were classified on review as showing perforation if one of five CT findings was present. The authors calculated the sensitivity and specificity for each finding by comparing the predicted outcome to the surgical and pathologic outcome.

RESULTS: The perforated group comprised 39 patients and the nonperforated group comprised 55 patients. Sensitivity for abscess, phlegmon, extraluminal air, extraluminal appendicolith, and focal defect in enhancing appendiceal wall individually were 36%, 46%, 36%, 21%, and 64%, respectively. Sensitivity for any one of the five findings was 94.9%. Specificities were 100% for all findings except for phlegmon (95%). Groups differed with respect to age: 47 years ± 19 (mean ± SD) for perforated appendicitis and 30 years ± 13 for nonperforated appendicitis (P < .001). Groups also differed with respect to appendiceal diameter: 15 mm ± 4.9 for perforated appendicitis and 12 mm ± 3.3 for nonperforated appendicitis (P = .049).

CONCLUSION: A dedicated search for five specific CT findings allowed an overall sensitivity of 94.9% for perforated appendicitis. Among findings with 100% specificity, a focal defect in the enhancing appendiceal wall achieved the highest sensitivity.

© RSNA, 2003

Index terms: Appendicitis, 751.291 • Appendix, CT, 751.1211

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
During the past decade numerous investigators have established computed tomography (CT) as the imaging study of choice for the diagnosis of appendicitis. Sensitivities and specificities range from 94% to 98% based on the findings of a thickened appendix with some degree of adjacent inflammation (1–7). The use of CT has led to a substantial decrease in the negative appendectomy rate without a concomitant increase in the perforation rate (8,9).

In this same period of time we have witnessed an evolution in the treatment of appendicitis. Though surgery remains the traditional approach to appendicitis, many surgeons now avoid a surgical approach once perforation has occurred because of perioperative complications (10). Instead they choose conservative medical treatment or percutaneous drainage with or without interval appendectomy (11,12). The choice among differing management options demands preoperative diagnostic imaging with excellent sensitivity and specificity.

Few authors have investigated the sensitivity of CT in the diagnosis of perforated appendicitis. The classic findings of bowel perforation, abscess, and extraluminal air are well known but are not always present in patients with perforated appendicitis. Furthermore, the appendix may not actually be distinguishable after perforation. Because the treatment can vary depending on the recognition of perforation and the severity of the findings, we undertook this study to evaluate the sensitivity and specificity of CT for the diagnosis of perforated appendicitis based on the presence of specific CT findings.

	MATERIALS AND METHODS

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Patients
We performed a computerized search of medical records at the Albert Einstein Medical Center from June 1995 through April 2001 for all patients who underwent an appendectomy and preoperative CT of the abdomen and pelvis. We excluded any patient who underwent appendectomy as part of a procedure unrelated to acute appendicitis. Patients with pathologically proven appendicitis, surgical reports, and available preoperative CT scans constituted the study population. Our institutional review board judged this study to be exempt from the need for informed consent.

CT Imaging
All CT examinations were performed helically with a pitch of 1.0. The section thickness varied from 5 to 10 mm. The use of oral and/or intravenous contrast medium varied depending on the clinical history provided. Examinations in patients with abdominal pain were performed with oral and intravenous contrast medium, unless there was a contraindication. No oral or intravenous contrast medium was used in patients suspected of having nephro-ureterolithiasis.

Image Interpretation
An experienced abdominal imager (17 years; M.M.H.) reviewed the CT scans without knowledge of the original CT reports or of the surgical and pathologic outcome regarding perforation. We recorded the presence of each of five specific findings indicative of perforated appendicitis: abscess (Fig 1), phlegmon (Fig 2), extraluminal air (Fig 1), extraluminal appendicolith (Fig 3), and a defect in the enhancing appendiceal wall (Figs 1–3). An abscess was characterized by a well-delineated, discrete collection with rim enhancement. A phlegmon was characterized by diffuse and substantial inflammation of the periappendiceal fat with ill-defined fluid collections. In addition we recorded the presence and type of intraluminal air, intraluminal appendicolith, maximal appendiceal thickness, appendiceal wall enhancement, and degree of adjacent inflammation (none, minimal, mild, moderate, and severe) in patients without abscess or phlegmon. Minimal inflammation was considered as just perceptible inflammatory changes. Mild, moderate, and severe inflammation represented progressively increased haziness and linear areas of high attenuation in the periappendiceal fat. Associated findings such as adjacent bowel wall thickening, small fluid collections, small-bowel obstruction, and abscesses involving other organs were also noted. Patient demographics, including age and sex, were compiled.

View larger version (151K): [in this window] [in a new window] [Download PPT slide]   Figure 1a. Transverse CT images obtained with oral and intravenous contrast medium in a 14-year-old girl with perforated appendicitis. (a) CT image demonstrates a 17-mm appendix (arrow) with an intraluminal air-fluid level and a surrounding abscess (A). The enhancing appendiceal wall is intact at this level. (b) CT image obtained more inferiorly demonstrates a break in the enhancing wall (arrows) and extraluminal air.

View larger version (161K): [in this window] [in a new window] [Download PPT slide]   Figure 1b. Transverse CT images obtained with oral and intravenous contrast medium in a 14-year-old girl with perforated appendicitis. (a) CT image demonstrates a 17-mm appendix (arrow) with an intraluminal air-fluid level and a surrounding abscess (A). The enhancing appendiceal wall is intact at this level. (b) CT image obtained more inferiorly demonstrates a break in the enhancing wall (arrows) and extraluminal air.

View larger version (165K): [in this window] [in a new window] [Download PPT slide]   Figure 2a. Transverse CT images obtained with oral and intravenous contrast medium in a 50-year-old woman with perforated appendicitis. (a) CT image demonstrates a 13-mm appendix with an intact, enhancing wall (arrow) and an adjacent phlegmon (P). (b) CT image obtained just inferiorly demonstrates a defect in the enhancing appendiceal wall (arrow) and the adjacent phlegmon (P).

View larger version (164K): [in this window] [in a new window] [Download PPT slide]   Figure 2b. Transverse CT images obtained with oral and intravenous contrast medium in a 50-year-old woman with perforated appendicitis. (a) CT image demonstrates a 13-mm appendix with an intact, enhancing wall (arrow) and an adjacent phlegmon (P). (b) CT image obtained just inferiorly demonstrates a defect in the enhancing appendiceal wall (arrow) and the adjacent phlegmon (P).

View larger version (156K): [in this window] [in a new window] [Download PPT slide]   Figure 3. Transverse CT image obtained with oral and intravenous contrast medium in a 46-year-old man with perforated appendicitis. Image demonstrates an appendicolith (arrow) almost extruded through the enhancing appendiceal wall.

Statistical Analysis
For each finding and for selected groupings of the five findings, we calculated accuracy statistics: sensitivity, specificity, positive predictive value, and negative predictive value. The unpaired, unequal variance Student t test compared continuous data between groups. The Fisher exact test compared discrete data between groups with P < .05 denoting significance for all comparisons.

	RESULTS

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Our study population consisted of 94 patients, ranging in age from 6 to 82 years (mean ± SD, 37 years ± 18). Eleven patients were 18 years of age or less and 83 were greater than 18 years of age. There were 43 male patients and 51 female patients. On the basis of the surgical and pathologic outcome, 39 patients had perforated appendicitis and 55 had nonperforated appendicitis. Ten patients did not receive intravenous contrast medium; of these, five were from the perforated group and five were from the nonperforated group. Of these 10 patients, six underwent CT for symptoms of nephro-ureterolithiasis and did not receive oral contrast medium. Allergy or renal insufficiency contraindicated the use of intravenous contrast medium in the remaining four.

CT Depiction of Five Specific Findings
By using the presence of any one of the five specific findings, the sensitivity of detecting perforation was 94.9% with a specificity of 94.5%. Our review detected a right lower quadrant abscess in 14 patients in the perforated group (sensitivity, 36%) and in no patients in the nonperforated group (specificity, 100%). A phlegmon was present in 18 patients from the perforated group and three patients from the nonperforated group, yielding a sensitivity of 46.2% and a specificity of 94.5% for perforation. A focal defect in the enhancing wall was noted in 18 of the 28 patients (sensitivity, 64.3%) with perforation who received intravenous contrast medium and in whom the appendix could be visualized and in none of the patients receiving intravenous contrast medium in the nonperforated group (specificity, 100%). Extraluminal air was present in 14 patients with perforation (sensitivity, 36%) and in none of the patients without perforation (specificity, 100%). An extraluminal appendicolith was present in eight patients in the perforated group (sensitivity, 20.5%) and in none of the patients in the nonperforated group (specificity, 100%) (Table 1).

Grouping of Findings
Because the sensitivities of individual specific CT findings were too small for clinical use, we then analyzed the findings in groups. Group 1 included all patients with one or more of the classic findings of appendiceal perforation: abscess, extraluminal air, and extraluminal appendicolith. Group 2 included all patients with any of these three findings or a phlegmon. Group 3 included all patients with any of the three findings or a defect in the enhancing wall. Thus group 1 is a subset of group 2 and a subset of group 3. The sensitivity, specificity, positive predictive value, and negative predictive value, respectively, are 69.2%, 100%, 100%, 82.1% for group 1; 94.9%, 94.5%, 92.5%, 96.3% for group 2; and 96.4%, 100%, 100%, 97.6% for group 3 (Table 2).

Appendiceal Diameter, Intraluminal Air, and Appendicolith
Comparisons were made between the two groups for appendiceal diameter, intraluminal air, and the presence of an appendicolith either within or outside of the appendix. Among patients with a visualized appendix, mean appendiceal diameter in the perforated group (32 patients) was 15 mm ± 4.9 (range, 8–30 mm). This differed significantly from the mean appendiceal diameter in the nonperforated group (55 patients), which was 12 mm ± 3.3 (range, 8–20 mm) (P = .015 by Student t test). An appendicolith occurred more often in perforated appendicitis (19 patients, 49%) than in nonperforated appendicitis (15 patients, 27%) (P = .049 by Fisher exact test). Appendicoliths occurred in 34 patients (36%) overall. Among patients with a visualized appendix, intraluminal air was present in six of 32 (19%) in the perforated group and in 11 of 55 (20%) in the nonperforated group (P > .99, Fisher exact test). This air appeared in several ways, including an air-fluid level, individual dots of air, and intramural air (Table 3).

Inflammation
We assessed the degree of inflammation in both the periappendiceal fat and the cecum. In the nonperforated group the degree of inflammatory stranding was variable: none (one patient), minimal (12 patients), mild (29 patients), moderate (10 patients), and severe (no patients); phlegmon was observed in three patients. In the perforated group, excluding the 14 patients with abscess, the degree of inflammation also varied but was significantly more severe: None (no patients), minimal (no patients), mild (two patients), moderate (four patients), severe (one patient), and phlegmon (18 patients) (P < .001, Fisher exact test) (Table 4).

	DISCUSSION

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These results demonstrate poor sensitivity (69.2%) for the diagnosis of perforated appendicitis when using only the three classic CT findings: abscess, extraluminal air, or extraluminal appendicolith. Although inclusion of phlegmon increases sensitivity to 94.9%, it decreases specificity. Alternatively, inclusion of a defect in the enhancing wall yielded 96.4% sensitivity while preserving 100% specificity. For CT examinations performed without intravenous contrast medium or with suboptimal wall enhancement, however, inclusion of phlegmon still improves sensitivity with only a slight decrease in specificity. Considering the five specific findings separately is not clinically useful since the sensitivities of the individual findings are small, ranging from 20.5% for an extraluminal appendicolith to 64.3% for a defect in the enhancing wall.

All of the CT findings of perforation that we studied had excellent specificities. Only the finding of a phlegmon was less than 100% specific. This is not surprising since the findings of a phlegmon are more subjective, and phlegmon is part of a spectrum of inflammatory changes.

Preoperative knowledge of whether the appendix is perforated has clinical relevance. Once perforation has occurred, the complications, which include reoperation and intraabdominal sepsis, increase (10). Surgeons consider initial percutaneous drainage an appropriate therapy for patients with perforated appendicitis and a palpable mass (13). As the use of CT for the diagnosis of appendicitis has increased, surgeons have begun to use conservative management in patients who do not have a clinical periappendiceal mass (11,14,15). In these studies, when CT demonstrated a localized abscess or phlegmon, nonsurgical management was successful in 92%–95% of patients, with 32%–48% requiring percutaneous aspiration or catheter drainage. Jeffrey et al (12) documented the safety and utility of medical therapy for phlegmons and abscesses smaller than 3 cm in diameter and CT-guided drainage of abscesses greater than 3 cm in diameter as alternatives to surgery. Most authors recommend interval appendectomy in at least 6 weeks (13,16), and some recommend this as a laparoscopic procedure (15).

If initial surgical management is chosen for patients with perforated appendicitis, they may undergo either a laparoscopic or an open procedure. On the basis of a retrospective study, Khalili et al (17) recommend the laparoscopic procedure, while Horwitz et al (18) suggest an open procedure on the basis of their review of 56 pediatric cases. To our knowledge, no large prospective trial comparing these methods in cases of perforation has yet been reported. Such a trial would certainly require an accurate preoperative imaging study.

Though a substantial body of literature documents the excellent accuracy of CT for the diagnosis of appendicitis, few authors have studied the ability of CT to distinguish perforated from nonperforated appendicitis. Maniatis et al (19) retrospectively studied a group of 76 patients with a variety of alimentary tract perforations. Their diagnoses were based on direct findings of extraluminal air or oral contrast medium, and indirect findings of abscess, phlegmon surrounding an appendicolith, or a bowel wall–related phlegmon or abscess involving fluid in the mesentery or surrounding a radiopaque foreign body. Most patients did not receive intravenous contrast medium. The sensitivity for perforation in general was 85.5%, which included six cases of appendicitis. Oliak et al (20) reviewed the CT scans of 84 patients with proven appendicitis, 59 of whom had gangrenous and/or perforated appendicitis proven pathologically. The presence of any one of three findings—abscess, phlegmon, or extraluminal air—had a sensitivity of 92%, a specificity of 88%, and a positive predictive value of 96% for perforated or gangrenous appendicitis. Most patients received oral and intravenous contrast medium.

Graded-compression ultrasonography (US) of the right lower quadrant is also a valuable imaging modality for appendicitis, with sensitivities ranging from 75 to 90%. US criteria for perforation include loculated pericecal fluid, prominent pericecal fat greater than 10 mm, and circumferential loss of the echogenic submucosal layer. For perforation, the reported sensitivities vary from a low of 29% (21) to a high of 84% (22). Not surprisingly, CT should be more sensitive than US for perforated appendicitis, since extraluminal air, extraluminal appendicoliths, and interloop abscesses are more easily detected with use of CT.

Though we did not include them in our group of five specific findings of perforated appendicitis, several other findings may help to distinguish perforated from nonperforated appendicitis. Patients with perforated appendicitis are older than patients without perforated appendicitis, with the oldest group of patients in this study (>60 years) all in the perforated group. The appendix was larger in the perforated group, although these data provide no specific cutoff to distinguish between the two groups. The appendix was visualized in all patients with nonperforated appendicitis but in only 82% of patients with perforated appendicits. Thus, if there is considerable pericecal inflammatory change or a right lower quadrant abscess without visualization of the appendix, one must consider perforated appendicitis in the differential diagnosis. In addition, our study showed that secondary findings, which include cecal thickening and inflammatory stranding, tend to be more severe in the perforated group.

The most important criticism of these results derives from the retrospective nature of the data. Although we did not know which patients had perforated appendicitis when we reviewed the CT scans, we did know that all patients had appendicitis. Particularly in the subgroup without an identifiable appendix, our sensitivities and specificities might conceivably have been lower if analyzed in a true prospective fashion.

We chose to evaluate five specific CT findings to make the diagnosis of perforated appendicitis. A variety of other CT findings for appendicitis have been studied, including adenopathy, the "arrowhead" sign, terminal ileal wall thickening, and focal cecal thickening (1). We did not consider these or other findings because, in our clinical experience, they did not seem useful. Nonetheless, it is possible that one or more other findings may also be predictive of perforation.

Factors such as CT protocol and patient habitus may affect the sensitivities and specificities of our five CT findings. All of our patients received oral contrast medium, except those originally suspected of having urinary tract calculi (patients had flank pain). Oral contrast medium can decrease one’s ability to distinguish an appendicolith and may have decreased our sensitivity for this finding. By using a protocol of no oral or intravenous contrast medium, Lane et al (5) found an appendicolith in 46% of their patients with appendicitis, a percentage arguably higher than in the current study (36%). In addition, in patients with very little adipose tissue it is more difficult to appreciate and quantitate the extent of inflammation.

Thinner collimation can improve visualization of both the appendix and an appendicolith (4). Presumably it would also help one detect a focal defect in the enhancing wall. The section thickness varied in our study between 5 and 10 mm, possibly affecting our sensitivities and specificities.

Many authors have shown excellent sensitivities and specificities of CT for appendicitis without using intravenous contrast medium. We justify the use of intravenous contrast medium because the resulting wall enhancement makes it easier to identify the appendix, particularly in patients with minimal fat. In addition, the generalized enhancement of bowel allows one to appreciate more easily a loculated, extraluminal fluid collection among bowel loops. Furthermore, our experience with intravenous contrast medium has led to the new observation of a defect in the enhancing wall as a sign of appendiceal perforation.

In conclusion, we found that a dedicated search for five specific CT findings—extraluminal air, extraluminal appendicolith, abscess, phlegmon, and a defect in the enhancing appendiceal wall—allows excellent sensitivity (94.9%) and specificity (94.5%) for the diagnosis of perforated appendicitis when evaluated in a group of patients with known appendicitis. A defect in the enhancing appendiceal wall had the highest sensitivity (64.3%) of any individual finding.

	FOOTNOTES

 
Author contributions: Guarantor of integrity of entire study, M.M.H.; study concepts, M.M.H.; study design, M.M.H., D.S.W.; literature research, M.M.H., D.S.W.; clinical studies, M.M.H., D.S.W.; data acquisition, M.M.H., D.S.W.; data analysis/interpretation, M.M.H., J.C.H., D.S.W.; statistical analysis, M.M.H., J.C.H.; manuscript preparation and definition of intellectual content, M.M.H., D.S.W., J.C.H.; manuscript editing, revision/review, and final version approval, M.M.H., D.S.W., J.C.H.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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This Article Abstract Figures Only Full Text (PDF) All Versions of this Article: 2272020223v1 227/1/46    most recent Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Horrow, M. M. Articles by Horrow, J. C. Search for Related Content PubMed PubMed Citation Articles by Horrow, M. M. Articles by Horrow, J. C.