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Outer Diameter of the Vermiform Appendix as a Sign of Acute Appendicitis: Evaluation at US¹

¹ From the Departments of Radiology and Nuclear Medicine (T.R., A.H., P.M., N.G.) and Surgery (F.T.), Hospital Barmherzige Brueder, Salzburg, Austria; the Department of Nuclear Medicine, State Hospital of Salzburg, Austria (L.R.); and the Institute of Medical Statistics, University of Vienna, Austria (B.S.). From the 1998 RSNA scientific assembly. Received March 22, 2000; revision requested May 12; revision received July 10; accepted August 15. Address correspondence to T.R., General Feursteinstr 19, A-6020, Innsbruck, Austria (e-mail: thomas.rettenbacher@uklibk.ac.at).

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
PURPOSE: To evaluate the usefulness and limitations of the outer diameter of the vermiform appendix at cross-sectional ultrasonography to confirm or rule out acute appendicitis.

MATERIALS AND METHODS: In a prospective study, outer appendiceal diameters in 240 control subjects and in 278 patients suspected of having acute appendicitis who did (n = 98) or did not (n = 180) have acute appendicitis were measured.

RESULTS: Outer appendiceal diameters in the control subjects ranged between 2 and 13 mm, and in 55 (23%) of 240 control subjects, diameters were 6 mm or more. Diameters in the symptomatic patients without acute appendicitis ranged between 2 and 11 mm, and 57 (32%) of 180 patients had diameters of 6 mm or more. Diameters of acutely inflamed appendices ranged between 6 and 30 mm. A diameter of 6 mm or more confirmed acute appendicitis with a sensitivity of 100%; a specificity of 68%; positive and negative predictive values of 63% and 100%, respectively; and an accuracy of 79%.

CONCLUSION: The outer appendiceal diameter of 6 mm or more as a sign of acute appendicitis provides high sensitivity but limited specificity. This diagnostic criterion is more useful in excluding acute appendicitis than in confirming it.

Index terms: Appendicitis, 751.291 • Appendix, US, 751.1298 • Receiver operating characteristic (ROC) curve

	INTRODUCTION

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The outer diameter of the vermiform appendix is one of the most important established cross-sectional imaging criteria in the preoperative evaluation of the appendix. This criterion was used in the majority of ultrasonographic (US) and computed tomographic (CT) studies to rule out or to confirm acute appendicitis (1–19). However, according to the literature, the optimum cutoff point is still controversial. The cutoff point most commonly used is 6 mm. A value greater than 6 mm is considered to be a sign of acute appendicitis, and a value less than 6 mm is regarded as typical for a normal appendix (1–14).

A study in which the diameters of normal and acutely inflamed appendices in a substantial patient population are compared would be fundamental to assess the optimum cutoff point and the limitations of this criterion. However, to our knowledge, no such study performed with any type of cross-sectional imaging method exists. Therefore, we initiated this research to compare the outer appendiceal diameters in control subjects and in patients suspected of having acute appendicitis to evaluate the usefulness and limitations of this criterion.

	MATERIALS AND METHODS

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Patient Population
In this prospective study, a total of 710 subjects underwent US of the vermiform appendix. The sonographers (T.R., A.H., P.M., N.G.) were able to visualize the appendix and measure the outer diameter in 518 subjects. These subjects made up the sample population, which consisted of two groups, 240 control subjects and 278 patients who were clinically suspected of having acute appendicitis.

Control subjects.—For approximately 15 weeks, we attempted to visualize the appendix with US in 380 consecutive adult subjects (age range, 25–82 years; mean age, 55 years). The subjects were part of our abdominal US screening program, which was offered to almost all of our admitted patients as common practice, even those without clinical abdominal symptoms. Informed consent was obtained from each subject, and the study was approved by the ethical committee at our hospital. The only selection criteria were that the subjects did not exhibit any signs of abdominal disease and that they had not previously undergone an appendectomy.

Subjects fasted overnight in preparation for the general screening investigation. At the end of the screening investigation, we attempted to detect the appendix at US within an examination time of 5–15 minutes. In 240 subjects, the appendix was clearly detected; this group served as the control sample for the study. During US, the outer diameters of the depicted appendices were prospectively measured and noted.

Patients clinically suspected of having acute appendicitis.—For approximately 3 years, 330 consecutive patients (age range, 7–97 years; mean age, 35 years) who were clinically suspected of having acute appendicitis were referred for US in our department. In 278 patients, we were able to detect the appendix with US. These appendices depicted in 278 patients with lower right quadrant pain who had either acutely inflamed or normal appendices made up the symptomatic sample population. Outer diameters of all depicted appendices were prospectively measured during US and noted.

Imaging Technique
All US examinations included in this study were performed by four radiologists (T.R., A.H., P.M., N.G.) experienced in gastrointestinal US. To detect vermiform appendices at US, we used the graded compression technique described by Puylaert (20). For this purpose, commercially available US equipment, including a US unit with 2–4-MHz curved-array, 4–7-MHz linear-array, and 5–10-MHz linear-array transducers (HDI 3000; Advanced Technology Laboratories, Bothell, Wash) and a US unit with 3.5–5.0-MHz curved-array and 7.5–10.0-MHz linear-array transducers (AU4; Esaote, Florence, Italy), were used.

Image Evaluation
For this study, the outer appendiceal diameters were measured as the distance between the outer borders of the hypoechoic tunica muscularis (outer muscle coat). This measurement was performed by the investigating radiologist during US by setting the electronic calipers. Only numbers rounded off to the nearest whole number (in millimeters) were used.

All outer diameters were measured in the transverse plane of the appendix and noted immediately after imaging. If a US image of the appendix showed several outer diameters within its entire length, we regarded the largest one as relevant for this study. Transverse appendiceal sections can appear either round or ovoid (3,21). In the case of a round transverse section, all circumferentially measured diameters can be considered relevant because they are equal. However, in the case of an ovoid transverse section, the relevant diameter must be determined. An aid in decision making is that ovoid appendiceal transverse sections seem to occur only in normal appendices or in the normal proximal portion of a distally inflamed appendix (21). This observation and discussion with other researchers led us to use the smallest (anteroposterior) outer diameter as the relevant diameter in cases of an ovoid transverse appendiceal section.

Among the 278 patients suspected of having acute appendicitis in whom the appendix was depicted and measured at US, 98 had acute appendicitis, and 180 did not. The diagnosis in all 98 patients with acute appendicitis was confirmed at surgery and histologic examination. In the 180 patients without acute appendicitis, diagnosis was confirmed at surgery and histologic examination in 56 patients and at clinical follow-up in 124 patients. Therefore, histologic examination of 154 appendices (98 acutely inflamed and 56 normal) and clinical follow-up of 124 normal appendices was considered the standard. Clinical follow-up consisted of symptom resolution during the hospital stay and confirmation of symptom resolution by telephone at least 8 weeks after hospitalization.

Statistical Analysis
For the description of outer appendiceal diameters and differences in diameter within an appendix, baseline characteristics are presented as range, mean, median, and SD. The outer appendiceal diameters of our three study groups were compared by using an analysis of variance, including the Tukey studentized range test.

To analyze the diameter as a discriminating variable to classify patients into two groups (patients with and patients without acute appendicitis), receiver operating characteristic (ROC) analysis was applied. We also calculated the area under the ROC curve, which can be interpreted as the probability that a randomly selected individual of the population with acute appendicitis has an outer appendiceal diameter that is greater than that of a randomly selected individual of the population without appendicitis.

We had two special cutoff points, 6 and 7 mm, for which we performed separate analyses. For these two cutoff points, 2 x 2 contingency tables were used. Using the resulting estimates for sensitivity and specificity, we computed the positive and negative predictive values by applying the Bayes theorem. The prevalence of acute appendicitis was derived from the population at our hospital during the past few years.

Data analyses were performed by using the SAS statistical software package (version 6.12; SAS Institute, Cary, NC) for Windows (Microsoft, Redmond, Wash).

	RESULTS

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The ranges, mean values, median values, and SDs of the outer appendiceal diameters of the three study groups are shown in Table 1. The ranges, mean values, median values, and SDs of the variability in the diameter of one appendix from the three study groups are shown in Table 2. The distribution of the outer diameters of all 518 appendices imaged for the study is shown in Figure 1. The ROC curve for the variable outer appendiceal diameter and the area under the curve are shown in Figure 2. Sensitivity, specificity, positive and negative predictive values, and accuracy for the outer appendiceal diameter as a positive finding for acute appendicitis with 6 and 7 mm as cutoff points are shown in Table 3.

View larger version (38K): [in this window] [in a new window] [Download PPT slide]   Figure 1. Bar graph shows the frequency and distribution of appendiceal diameters in 240 control subjects (white bars), in 180 patients with lower right quadrant pain and without acute appendicitis (gray bars), and in 98 patients with acute appendicitis (black bars).

View larger version (26K): [in this window] [in a new window] [Download PPT slide]   Figure 2. Graph shows the ROC curve with the outer appendiceal diameter as a variable. Appendiceal diameters of patients with lower right quadrant pain without acute appendicitis and those with acute appendicitis were analyzed. The area under the ROC curve, which can be interpreted as the probability that an individual with acute appendicitis has an outer appendiceal diameter that is greater than that of an individual without appendicitis, is 0.97. The ROC curve and the area under the curve demonstrate that the outer appendiceal diameter can be used as an important imaging criterion.

There was a statistically significant (P < .05) difference in the outer appendiceal diameters between the control subjects and patients with acute appendicitis and, among patients suspected of having appendicitis, between those with and those without acute appendicitis. There was no statistically significant difference between control subjects and patients in whom acute appendicitis was clinically suspected but not found.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Our study population consisted of a healthy control group and patients clinically suspected of having acute appendicitis. Our control group consisted of healthy individuals whose results were compared with those of the patients in whom acute appendicitis was suspected but not found, especially those who underwent clinical follow-up. This group of patients may have included some patients with spontaneously resolving appendicitis (15). In addition, we observed that inflammatory processes near the appendix can cause concomitant appendiceal thickening. The results of our study showed that outer appendiceal diameters in patients with lower right quadrant pain without evidence of acute appendicitis tended to be somewhat larger than those of the healthy control group (Fig 1). However, this difference was not statistically significant. To establish clinically relevant statements, only patients suspected of having acute appendicitis were considered in this study.

With regard to the US measurement of appendiceal diameters, we considered the following points. Outer diameters of all appendices imaged for this study were measured as the distance between the outer borders of the tunica muscularis (Figs 3–7), according to the methods used in previous studies (2,22). This measurement is practical because of the large difference in echogenicity between the hypoechoic tunica muscularis and the hyperechoic surrounding fatty tissue (Figs 3–7). Our results indicate that the same appendix can show a considerable difference in diameter over its entire length. Differences of up to 7 and 18 mm were found in normal and acutely inflamed appendices, respectively (Table 2). Since acute inflammation, and therefore thickening of the appendix, can be pronounced or limited to a portion of the appendix (3,9,23), the largest diameter over its entire length must be used as the relevant diameter. In cases of focal appendicitis, use of the smaller diameter of the least affected or nonaffected portion of the appendix would lead to a false-negative result, as illustrated in Figure 3.

View larger version (147K): [in this window] [in a new window] [Download PPT slide]   Figure 3a. (a) Transverse US image shows the proximal normal portion of a distally inflamed appendix. Image demonstrates an ovoid transverse section with an anteroposterior outer diameter of 5 mm (long arrows) and a transverse outer diameter of 11 mm (short arrows). (b) Transverse US image of the distal portion of the same appendix as in a shows an acutely inflamed, round appendix with an 11-mm outer diameter (long arrows) and shows the surrounding hyperechoic inflamed fatty tissue (short arrows).

View larger version (158K): [in this window] [in a new window] [Download PPT slide]   Figure 3b. (a) Transverse US image shows the proximal normal portion of a distally inflamed appendix. Image demonstrates an ovoid transverse section with an anteroposterior outer diameter of 5 mm (long arrows) and a transverse outer diameter of 11 mm (short arrows). (b) Transverse US image of the distal portion of the same appendix as in a shows an acutely inflamed, round appendix with an 11-mm outer diameter (long arrows) and shows the surrounding hyperechoic inflamed fatty tissue (short arrows).

View larger version (163K): [in this window] [in a new window] [Download PPT slide]   Figure 4. US image shows the transverse section of a normal appendix with a 4-mm outer diameter (arrows).

View larger version (176K): [in this window] [in a new window] [Download PPT slide]   Figure 5. US image shows two transverse sections of a normal appendix with an 8-mm outer diameter (arrows). Image shows two sections of the appendix because of the curved course of the appendix.

View larger version (155K): [in this window] [in a new window] [Download PPT slide]   Figure 6. US image shows the transverse section of an acutely inflamed appendix with a 6-mm outer diameter (narrow straight arrows) and a normal small-bowel loop (wide straight arrows) Note the adjacent hyperechoic inflamed fatty tissue (curved arrows).

View larger version (154K): [in this window] [in a new window] [Download PPT slide]   Figure 7. US image shows the transverse section of an acutely inflamed appendix with a 17-mm outer diameter (long arrows). Note the surrounding hyperechoic inflamed fatty tissue (short arrows).

Cross-sectional imaging detection rates of normal appendices has greatly increased in recent years. Therefore, studies dealing with the appearance of normal appendices are less numerous than those dealing with acutely inflamed appendices. Several study (2,3,17) findings showed that the diameters of normal appendices do not exceed 6 mm, while others (12,13,18,24) reported diameters greater than 6 mm. Reported (14) diameters of normal appendices were up to 10 mm. Normal appendices investigated in our study measured up to 13 mm, and 23% of our control subjects and 32% of our patients with lower right quadrant pain and without acute appendicitis had appendiceal diameters of 6 mm or more (Fig 1). Our study results resemble the observations made by Friedland and Siegel (18), who found appendiceal diameters of up to 8 mm, with a mean value of 6 mm in 25 control subjects at CT.

In 1997, Hahn et al (6) reported a marked overlap of diameters of normal and acutely inflamed appendices measured at US in children. They found diameters of 6 mm or more in 79 (82%) of 96 cases of histologically proved normal appendices with lymphatic hyperplasia. Hahn et al (6) therefore concluded that high diagnostic accuracy in children can be achieved only by regarding several US criteria simultaneously. Our study data suggest that this consideration should also be applied to an adult population, without exclusive use of the single criterion of the outer appendiceal diameter. US signs that we routinely and simultaneously apply and that are described in the literature include the outer appendiceal diameter, appendiceal compressibility, location of the point of tenderness, presence of hyperechoic periappendiceal inflamed fatty tissue, appendiceal shape, appendicoliths, gas in the appendiceal lumen, and blood flow in the appendiceal wall detected at color Doppler US (1,10, 11,21,23,30).

The area under the ROC curve of 0.97 unequivocally shows that the outer appendiceal diameter can be used as an important criterion (Fig 2). There is, however, a considerable overlap in the diameters of normal and acutely inflamed appendices, as shown in Figure 1; this overlap results in certain limitations. In our study, if a patient with lower right quadrant pain had an appendiceal diameter of 6 mm or more, acute appendicitis was present with a probability of 63% (positive predictive value). If this patient had an appendiceal diameter of less than 6 mm, it was a normal appendix with a probability of 100% (negative predictive value). The outer appendiceal diameter was, therefore, a more reliable sign in excluding acute appendicitis than in confirming its presence. If the cutoff point was defined at 7 mm, the positive predictive value was 81%, the negative predictive value 96%, and the accuracy increased from 79% to 90% (Table 3). A cutoff point of 7 mm or more, therefore, also seems reasonable. In our opinion, however, the cutoff point of 6 mm is preferable as a sensitive criterion for acute appendicitis, a disease that bears a considerable risk of developing serious complications in overlooked cases. On the other hand, by using a cutoff point of 6 mm, a marked limited specificity of 68% and an accuracy of 79% have to be considered.

Since this study focused on the usefulness and limitations of the outer appendiceal diameter as US criterion, the results may not be entirely applicable to CT. For example, at US, the graded compression applied to the abdominal wall by the transducers can influence the relevant anteroposterior diameter, compared with CT in which no artificial pressure to the abdominal wall is usually applied. Another reason for differences in diameter may be the orally and/or rectally administered contrast medium at CT, which can influence appendiceal content, especially with normal appendices (4). This increased appendiceal content may lead to greater appendiceal diameters at CT compared with US because patients usually do not receive contrast medium at US.

In summary, there is a marked overlap of outer appendiceal diameters in normal and acutely inflamed appendices. The cutoff point of 6 mm or more for the outer appendiceal diameter as a US criterion to confirm acute appendicitis provides high sensitivity but limited specificity. The cutoff point of 7 mm or more provides a higher accuracy than that of the cutoff point of 6 mm, but it may cause cases of acute appendicitis to be overlooked. As a diagnostic criterion, the outer appendiceal diameter is more useful in excluding acute appendicitis than in confirming its presence.

	FOOTNOTES

 
Abbreviation: ROC = receiver operating characteristic

Author contributions: Guarantors of integrity of entire study, T.R., A.H., L.R., N.G.; study concepts and design, T.R., A.H., L.R.; definition of intellectual content, T.R., A.H., L.R.; literature research, T.R., A.H.; clinical studies, T.R., A.H., P.M., F.T., N.G.; data acquisition, T.R., A.H., P.M., F.T.; data analysis, T.R., A.H., L.R., B.S., statistical analysis, T.R., B.S.; manuscript preparation and editing, T.R.; manuscript review, A.H., L.R., F.T., B.S.; manuscript final version approval, L.R., P.M.

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TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

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