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Abdominal Imaging |
1 Department of Radiology, Northwestern University Feinberg School of Medicine,
676 N St. Clair St., Ste. 800, Chicago, IL 60611.
2 Department of Radiology, Hospital of the University of Pennsylvania,
Philadelphia, PA.
Received July 23, 2003;
accepted after revision March 22, 2004.
Address correspondence to P. Nikolaidis
(p-nikolaidis{at}northwestern.edu).
Abstract
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MATERIALS AND METHODS. After we received institutional review board approval, CT scans of 366 consecutive patients obtained for lower abdominal or right lower quadrant pain and to rule out appendicitis were retrospectively reviewed. Images were reviewed by an experienced abdominal radiologist and compared with the formal interpretation. The amount of right lower quadrant and pericecal fat was quantified on a scale of 0 (none) to 2 (abundant). Patients with a nonvisualized appendix and other findings compatible with acute appendicitis—such as abscess formation, localized perforation, periappendiceal fat stranding, or appendicolith—were excluded.
RESULTS. The appendix could not be visualized by both reviewers in 46 (13%) of 366 cases. CT findings indicated another cause for the patient's symptoms in 12 cases (26%), including gastrointestinal and genitourinary processes. An alternate diagnosis was subsequently reached in 11 additional patients (24%) with follow-up imaging or clinical evaluation. Only one patient (2%) with a nonvisualized appendix had acute appendicitis, proven by surgical pathology. In this patient, there was paucity of fat in the right lower quadrant.
CONCLUSION. In the absence of a distinctly visualized appendix and secondary inflammatory changes, the incidence of acute appendicitis is low. Nonvisualization of the appendix even when a small amount of fat is present in the right lower quadrant may safely exclude acute appendicitis if no secondary CT findings are present.
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Acute appendicitis has traditionally been a clinical diagnosis. With recent reports on the accuracy of CT examination in diagnosing appendicitis [3–7], CT has become part of the standard of care in managing patients with suspected acute appendicitis. CT signs include a distended appendix measuring greater than 6 mm in diameter, an appendicolith, appendiceal wall enhancement or thickening, adjacent or periappendiceal fat stranding, fluid collections, phlegmon or abscess formation, extraluminal air, adenopathy, adjacent bowel wall thickening, focal cecal apical thickening, the arrowhead sign, or the "cecal bar" sign. Sometimes, however, the appendix is not visualized on CT examination despite the use of optimal imaging parameters. This article will discuss the frequency of nonvisualization of the appendix and its significance in patients with a high clinical suspicion for acute appendicitis. To the best of our knowledge, this report is the first one about the relative importance of the nonvisualized appendix.
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Images obtained from all MDCT examinations were reevaluated by an experienced abdominal radiologist and compared with the formal interpretations by the staff radiologist at the time of examination. All cases in which the appendix was visualized were excluded. Patients who had a nonvisualized appendix but had other findings compatible with acute appendicitis (e.g., abscess formation, localized perforation, periappendiceal fat stranding, or appendicolith) were also excluded. For the remaining patients, the amount of fat at the level of the cecal tip, below the ileocecal valve, was quantified on a scale of 0 (scant) to 2 (abundant) for each examination. When fat was outlining only less than half the cecum and the rim of fat surrounding the cecum was less than 1.0 cm, a score of 0 was assigned (Fig. 1A). When fat outlined more than half the entire circumference of the cecum and the rim of fat measured less than 1.0 cm, a score of 1 was assigned (Fig. 1B). All remaining cases were assigned a score of 2 (Fig. 1C). The number of cases in which oral contrast material reached the cecum was also recorded. The medical records for these patients were reviewed to determine the outcome of the patient's abdominal complaint. Any other abdominal or pelvic abnormality to explain the patient's symptoms was also noted at the time of the review.
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Of the 46 cases for which both reviewers were in agreement that the appendix was not visualized, CT indicated another cause for the patient's symptoms in 12 (26%). Diagnoses included diverticulitis (n = 3), Crohn's disease and inflammatory bowel disease (n = 1), small-bowel obstruction (n = 1), nonspecific diffuse colitis (n = 1), mesenteric adenitis (n = 1), peptic ulcer disease and duodenitis (n = 1), ureterolithiasis (n = 2), pyelonephritis and urinary tract infection (n = 1), and postoperative abscess formation after gynecologic surgery (n = 1).
In the remaining 34 patients (34/46 [74%]), the CT examination was not able to detect an anatomic cause for the patient's abdominal complaints. Of those 34 patients, 20 patients had either follow-up imaging or further clinical evaluation. A diagnosis was subsequently made for 11 of those patients, which might have accounted for the presenting symptomatology. Diagnoses included Crohn's disease or inflammatory bowel disease (n = 4), pelvic inflammatory disease (n = 2), pyelonephritis (n = 1), endometriosis (n = 1), possible adhesions or small-bowel obstruction (n = 1), hepatitis (n = 1), and acute appendicitis (n = 1). The patient with appendicitis (1/46; 2%) underwent surgery on the basis of the strong clinical suspicion, despite the lack of supportive CT evidence. The diagnosis was confirmed by surgical pathology. Fourteen patients were lost to follow-up, 13 of whom had presented to the emergency department. Due to constrictions placed by the institutional review board, these patients could not be contacted directly by the investigators.
The amount of pericecal fat on the CT examinations of those patients with a nonvisualized appendix was quantified on a scale of 0 (scant) to 2 (abundant). Of the 10 cases for which the original and second reviewers disagreed about whether the appendix was visualized, most of the patients (8/10 [80%]) had ample pericecal fat (score, 2). Of the other two patients, one was scored as 1 and the other as 0 (paucity of pericecal fat). In the 46 cases for which both radiologists agreed on the nonvisualization of the appendix, eight cases were scored as 0, 20 were scored as 1, and the remaining ones were scored as 2. In the patient with appendicitis, a paucity of fat in the right lower quadrant resulted in a score of 0 (Fig. 2A, 2B).
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In 32 (70%) of 46 patients, oral contrast material had reached the cecum at the time the scan was obtained.
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The high incidence of misdiagnosis increases the risk of appendiceal perforation, thus resulting in higher rates of postoperative complications (39% for a perforated appendix vs 8% for a simple appendicitis) [13, 14]. On the basis of these findings, surgeons have struggled with finding the balance between decreasing false-negative appendectomy rates and making a diagnosis before perforation occurs. Therefore, surgeons generally consider 20% as an acceptable negative appendectomy rate to minimize the risk of perforation and its associated morbidity and mortality [14, 15].
The appendix arises from the posteromedial wall of the cecum approximately 2–3 cm inferior in relation to the ileocecal valve. The location of the appendix is variable because it is free-floating. Its two most common positions are retrocecal, in about two thirds of patients, and inferomedial in relation to the cecum, in about a third of patients [16]. However, the appendix can also be found in the retroileal, preileal, or in a variety of locations in the pelvis, thus making clinical diagnosis more difficult [16, 17]. With the use of CT for patients with suspected acute appendicitis, the diagnostic accuracy has improved to 93–98%, which has resulted in lower negative appendectomy rates without a proportionate increase in perforation risk [3–7]. Additional benefits of using helical CT technology include its widespread availability, its ease of use, its operator independence, and its cost effectiveness (in terms of eliminating unnecessary operations and the need to hospitalize the patient for observation) [18].
Although numerous studies have reported on the diagnostic accuracy and imaging findings of acute appendicitis on helical CT examination, none, to our knowledge, has reported the significance of not being able to confidently visualize the appendix. In our study population, the frequency of nonvisualization of the appendix was 13%. This finding is comparable to those of a recent study by Benjaminov et al. [19] who reported a 14% nonvisualization rate by any of three reviewers retrospectively reviewing CT scans of patients with renal colic on unenhanced helical CT. Although it occurs infrequently, it is difficult to definitively exclude appendicitis when a normal appendix is not seen. In our report, we found that in the absence of a distinctly visualized appendix and any secondary inflammatory changes, the incidence of acute appendicitis is low. The amount of fat surrounding the cecum influences our ability to visualize the appendix and therefore more confidently exclude the possibility of acute appendicitis. There was paucity of surrounding fat noted in the patient with appendicitis that was missed. Therefore, nonvisualization of the appendix even when only a small amount of fat is present in the right lower quadrant may safely exclude acute appendicitis as long as no secondary CT findings are present.
Limitations of our study include its retrospective nature. Because all cases were reviewed by a second reviewer (an experienced abdominal radiologist concentrating on the region of the cecum and appendix), there were some cases for which the original reviewer failed to recognize the appendix when in fact it was present (10/56 cases). In most of these patients, ample pericecal and periappendiceal fat was present. In eight (80%) of the 10 patients, a score of 2 was assigned and only one was assigned a score of 0. The amount of fat influenced the ability of the second reviewer to identify the appendix after careful scrutiny of the pericecal region.
Adequate opacification of the cecum may also influence a radiologist's ability to visualize the appendix. However, in our study, oral contrast material was identified in the cecum of most patients (70%) whose appendix was not visualized. For the single case of acute appendicitis in which the diagnosis was missed because of a nonvisualized appendix, oral contrast material had not reached the cecum.
Another limitation of the study is the lack of reliable follow-up information for a relatively large number of patients. However, given the transitory nature of the emergency department and our relatively young study population, we expected to lose some patients to follow-up. In fact, 13 of the 14 patients lost to follow-up had presented to the emergency department. Because of limitations and restrictions placed on investigators by the hospital institutional review board, we were not allowed to directly contact these patients for follow-up information. We are confident that patients were discharged from the hospital only after acute appendicitis was determined to be unlikely because appendicitis is primarily a clinical diagnosis. Furthermore, we know that most of these patients reside in the vicinity of our hospital and were given clear instructions to follow up with our hospital if symptoms continued or worsened. In this situation, we should have been aware of a subsequent diagnosis of appendicitis. Therefore, the probability that one of these patients who were lost to follow-up actually had acute appendicitis is low.
In conclusion, in the absence of a distinctly visualized appendix and secondary inflammatory changes, the incidence of acute appendicitis is low (2%). The missed case was associated with a paucity of surrounding fat. Nonvisualization of the appendix even when a small amount of fat is present in the right lower quadrant may safely exclude acute appendicitis if no secondary CT findings are present.
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