CT Evaluation of Appendicitis and Its Complications: Imaging Techniques and Key Diagnostic Findings

Transcription

1 Pinto Leite et al. CT of Appendicitis Abdominal Imaging Review Nuno Pinto Leite 1 José M. Pereira 1 Rui Cunha 1 Pedro Pinto 1,2 Claude Sirlin 1,3 Pinto Leite N, Pereira JM, Cunha R, Pinto P, Sirlin C Received September 22, 2004; accepted after revision December 6, Presented at the 2004 annual meeting of the American Roentgen Ray Society, Miami Beach, FL. Dedicated to our friend and colleague, Pedro Pinto ( ). 1 Department of Radiology, Hospital São João, Oporto Medical School, Oporto, Portugal. 2 Deceased. 3 Present address: Department of Radiology, University of California, San Diego, 200 W Arbor Dr., San Diego, CA Address correspondence to C. Sirlin. AJR 2005; 185: X/05/ American Roentgen Ray Society CT Evaluation of Appendicitis and Its Complications: Imaging Techniques and Key Diagnostic Findings OBJECTIVE. This article reviews various CT protocols for appendicitis, identifies key CT findings for diagnosing appendicitis, discusses unusual manifestations such as chronic and recurrent appendicitis, and profiles imaging features that differentiate appendicitis from other inflammatory and neoplastic ileocecal conditions. Patients were studied with helical CT. CONCLUSION. CT is a highly accurate, noninvasive test for appendicitis, but the optimal CT technique is controversial. Major complications of appendicitis (perforation, abscess formation, peritonitis, bowel obstruction, septic seeding of mesenteric vessels, gangrenous appendicitis) and their management are discussed. Abdominal CT is a well-established technique in the study of acute abdominal pain and has shown high sensitivity and specificity for diagnosing and differentiating appendicitis, providing an accurate diagnosis in the early stages of disease. cute appendicitis is one of the A most common causes of acute abdominal pain, the most common condition that requires abdominal surgery in childhood, and the most common condition associated with lawsuits against emergency physicians. Acute appendicitis occurs when the appendiceal lumen is obstructed, leading to fluid accumulation, luminal distention, inflammation, and, finally, perforation [1 4]. Classic symptoms of appendicitis are well described [5]. However, up to one third of patients with acute appendicitis have atypical presentations [6]. Moreover, patients with alternative abdominal conditions may present with clinical findings indistinguishable from acute appendicitis [7]. Thus, although appendicitis traditionally has been a clinical diagnosis, many patients are found to have normal appendixes at surgery. The misdiagnosis of this acute condition has led to the inappropriate removal of a normal appendix in 8 30% of patients [8]. A rate of unnecessary removal as high as 20% has been considered acceptable in the surgery literature [9, 10]. However, negative laparotomy can be avoided in many patients if modern diagnostic methods are used to confirm or exclude acute appendicitis. In the mid 1980s, graded-compression sonography emerged as a promising imaging technique for the evaluation of suspected appendicitis, especially in children [11 18]. Sonography is a noninvasive, rapid, widely available, and relatively inexpensive technique. Most important, sonography does not involve the use of ionizing radiation, a key consideration when imaging otherwise healthy pediatric and young adult patients, who are up to 10 times more sensitive to the effects of radiation than are middle-aged and elderly adults [19 21]. On the other hand, sonography is highly operator-dependent, requires a high level of skill and expertise, and may be difficult in some situations (severe pain, overlying gas). Sonography is particularly challenging in large and overweight adults, which is a major limitation to its use in North America and parts of Europe. Moreover, sonography frequently does not allow the detection of normal or perforated appendixes [12, 17, 22 29]; thus, sonography may be of limited benefit in evaluating patients at the extremes of the disease spectrum. The reported diagnostic accuracy of graded compression sonography varies widely; reported sensitivity of sonography in children ranges from 44% to 94%, and specificity, from 47% to 95% [11 13, 16 18, 25, 30]. In 1995, Orr et al. [31] performed a meta-analysis of pediatric and adult studies published between 1986 and 1994, showing an overall sonography sensitivity of 85% and specificity of 92%. Anecdotally, our personal experience with 406 AJR:185, August 2005

2 CT of Appendicitis Fig. 1 CT scan after oral contrast administration in 32-year-old woman with normal appendix. Note normal appendix with intraluminal enteric contrast material and gas (arrows). Appendix wall is nearly imperceptibly thin. A Fig. 2 6-year-old girl with acute appendicitis. A and B, CT scans obtained before (A) and after (B) IV contrast administration illustrate benefit of IV contrast material in difficult cases. Unenhanced scan is indeterminate because appendix is not confidently visualized. Enhanced scan shows dilated appendix with thickened, hyperenhancing wall (arrows, B). Notice mural stratification of appendix wall. B sonography in the diagnosis of appendicitis has been disappointing. We reserve sonography as the initial examination in children, adolescents, thin adults, and women of reproductive age with possible gynecologic presentations, but if the sonographic results are negative or inconclusive, we generally proceed with CT. CT has high accuracy for the noninvasive assessment of patients with suspected appendicitis, with reported sensitivities of %, specificities of 91 99%, positive predictive values of 92 98%, negative predictive values of %, and accuracies of 94 98% [8, 32 34], and has emerged as the technique of AJR:185, August

3 Pinto Leite et al. Fig year-old man with early, acute appendicitis. Unenhanced CT scan shows inflamed appendix measuring 10 mm in transverse diameter (arrows). Note lowattenuation edema in submucosal layer of appendix. No appendicoliths, free air, adjacent fluid collection, or fat stranding is seen. Surgery confirmed early, nonperforated appendicitis. choice in many centers for imaging evaluation of these patients [35 37]. More recently, several authors have also reported the accuracy of helical CT for the diagnosis of acute appendicitis in children [11, 36, 38 40]. Important advantages of CT are that it depicts the appendix, the periappendiceal tissues, and other intraabdominal structures. Thus, CT allows the radiologist to confidently exclude appendicitis if a normal appendix is visualized and to diagnose appendicitis if the appendix is abnormal. Importantly, by depicting the severity and extension of the inflammatory process, CT can also help guide appropriate management. CT has several important disadvantages, however. The most serious is that it uses ionizing radiation. Radiation dose depends on CT technique. This article reviews CT technique, key CT findings, complications, unusual manifestations, and differential diagnosis. CT Technique Because visualization of both the normal (Fig. 1) and the inflamed appendix can be challenging, especially in asthenic patients with a paucity of visceral fat [41], meticulous technique is important. Nevertheless, the optimal CT technique for appendicitis remains controversial, and a variety of methods have been advocated. It is generally accepted that appendiceal CT should incorporate thin-section scanning (5 mm) through the right lower quadrant (RLQ) to improve identification of the appendix, but debate exists regarding the need for IV contrast material, the use and route of enteric contrast agents, and the necessity for scanning the entire abdomen and pelvis versus performing a focused data acquisition through the RLQ. The most commonly used CT technique for studying the appendix is a scan of the entire abdomen and pelvis after both oral and IV administration of contrast material [42], but several other approaches are possible. We describe the most commonly used CT techniques in the evaluation of appendicitis. Fig year-old man with acute appendicitis. Oral and IV contrast-enhanced CT scan shows thickened, fluid-filled appendix (arrows). Unenhanced CT Some centers advocate examination without oral or IV contrast material [8, 32]. Unenhanced scanning eliminates patient preparation time to receive enteric contrast material thus expediting the examination and diminishing the risk of appendiceal perforation before scanning and also eliminates the risks associated with IV contrast injection. Ege et al. [41] reported a sensitivity of 96%, specificity of 98%, positive predictive value of 97%, and negative predictive value of 98%. On the basis of these results, the authors recommended that if no definite inflammatory changes are detected with unenhanced CT, patient clinical monitoring could be done [43]. However, other authors found less promising results for unenhanced CT. Heaston et al. [3] showed a sensitivity of 84% and a specificity of 92%. Our anecdotal experience is that unenhanced CT accuracy probably depends on the patient s body habitus (particularly visceral fat content), although to our knowledge this hypothesis has not yet been tested. Focused CT Some authors advocate a focused CT examination from the right renal lower pole through the entire pelvis with various combinations of oral, rectal, and IV contrast media. Focused CT has the advantage of decreasing patient radiation dose, which is especially desirable in pe- 408 AJR:185, August 2005

4 CT of Appendicitis diatric patients [34, 42, 44, 45]. In one study, Fefferman et al. [44] reported high sensitivity (97%), specificity (93%), positive predictive value (90%), and negative predictive value (98%). However, the focused CT technique has some limitations. In one study of 100 patients presenting to the emergency department with RLQ pain, Kamel et al. [45] showed that if only focused CT had been performed, 7% of patients with abnormalities outside the pelvis (4% of whom required surgery) would be undiagnosed. They concluded that both abdominal and pelvic CT examinations are necessary because there are many possible upper abdominal causes of RLQ pain in patients with clinically suspected appendicitis. Use of Enteric Contrast Material Most investigators recommend the use of enteric contrast material, either oral or rectal [33, 46 49], claiming that positive enteric contrast material decreases the number of false-negative cases and improves characterization of appendicitis and detection of its complications. Rectal administration Cecal opacification and distention may be achieved by rectal administration of 800 1,500 ml of contrast material [33, 43, 49]. The contrast agent is given with the patient on the CT gurney as a bolus under gravity control without fluoroscopic visualization. Several studies have shown high accuracy of appendicitis CT in both adults and children after rectal contrast material administration [34, 43, 50]. In one study, helical CT with rectal contrast material was as accurate (98%) as helical CT with both oral and rectal contrast material [33]. Rectal contrast material distends the cecum, delineates the thickness of its wall, and opacifies an unobstructed appendix. By distending the cecal lumen, this technique depicts several cecal signs of appendicitis, including the arrowhead sign, the cecal bar sign, and focal cecal apical thickening [46 49]. These signs are discussed further in the subsection Cecal Changes under Key CT Findings. An important advantage of rectal contrast administration is that it is relatively fast to perform and the patient does not need to wait the 1 2 hr usually required with the oral route for terminal ileal and cecal visualization. Disadvantages of routine rectal contrast administration include patient discomfort, inconsistent opacification of the terminal ileum, and logistical or procedural difficulties. Rectal contrast material is contraindicated in neutropenic patients and those with peritoneal signs or other evidence of gross perforation. Oral administration Distal small-bowel and cecal opacification may be achieved by oral administration of 800 1,000 ml of contrast material in small increments over hr. Opacification of these structures is often helpful because otherwise they may mimic or obscure an abnormal appendix. Unfortunately, oral contrast administration delays the examination. Moreover, optimal opacification of the ileocecal region is often not achieved because of variability in gastrointestinal transit time and patient compliance; patients with abdominal pain are often nauseated and may not tolerate oral contrast material [51]. In our experience, oral contrast material is not usually beneficial except in cases of perforation, when oral contrast material can help identify extraluminal fluid collections. Use of IV Contrast Material Although some authors believe that the use of enteric contrast material alone is adequate to diagnose appendicitis, other authors believe that IV contrast material is necessary. IV contrast material can be especially helpful in subtle cases and in patients with minimal intraabdominal fat by showing enhancement of the appendiceal wall [49, 52 54] (Fig. 2). Complications such as appendiceal perforation, extraappendiceal fluid collections, abscess formation, and septic seeding of the mesenteric portal venous system are also better characterized after IV contrast administration [52, 55]. Furthermore, IV contrast material is useful to diagnose and assess other causes of abdominal pain, including pancreatitis, inflammatory bowel disease, and pyelonephritis [49, 52 55]. Disadvantages include possible adverse reactions and costs. Unenhanced CT with Selective Use of Contrast Material An alternative, theoretically more elegant approach, is unenhanced CT with the selective use of contrast material. In this approach, patients with suspected appendicitis are initially evaluated with unenhanced CT. If unenhanced images are conclusive (i.e., positive or negative for appendicitis), no further imaging is necessary. However, if findings are inconclusive, a repeat scan is performed with contrast material. The type of contrast material (IV, oral, rectal) and the imaging volume (e.g., focused RLQ scan or scan of the entire abdomen and pelvis) are chosen by the interpreting radiologist. This approach permits immediate imaging assessment of patients with suspected appendicitis and rational choice of contrast material tailored to a particular patient if the preliminary unenhanced scan is inconclusive. Disadvantages of this approach are that it requires monitoring by the radiologist to determine whether contrast administration is needed and that it results in additional scanning in patients in whom unenhanced images are inconclusive, thereby increasing radiation exposure and potentially delaying diagnosis. Tamburrini et al. [56] found that in 25% of patients, the preliminary images were inconclusive and additional scanning with contrast material was necessary. The frequency with which additional scanning is necessary may be influenced by patient demographic factors (age, sex) and visceral fat content; this is under active investigation. Our Approach The large number of proposed CT techniques presents a challenge to radiologists who wish to start using CT for diagnosis of appendicitis at their institution. The simplest and most widely used technique is CT with both oral and IV contrast material. However, as discussed previously, we find that positive oral and IV contrast material are not helpful in most patients. On the basis of our personal experience and to satisfy the needs of our emergency department colleagues, we tailor our protocol according to the patient s clinical presentation and other factors. If the patient is a child, adolescent, thin young adult, or reproductive-age woman with a possible gynecologic source of pain, try sonography first; if that is inconclusive, perform CT with IV contrast material. If the patient is a large adult, try unenhanced CT with selective use of contrast material. This method expedites the CT examination, which is critical for our emergency department physicians. If the symptoms have persisted for more than 72 hr, try CT with oral and IV contrast material because of the high probability of perforation. If the patient has a history of cancer, inflammatory bowel disease, immune deficiency, or lower abdominal or pelvic surgery, try CT with oral and IV contrast material because there is a high pretest probability of disorders other than appendicitis and possibly distorted anatomy. The individualized approach advocated here may be impractical in nonacademic institutions or in institutions that rely on remote AJR:185, August

5 Pinto Leite et al. coverage of after-hours cases. A uniform protocol may be preferred. The chosen protocol must satisfy the needs of referring clinicians and be appropriate for the patient population. Our emergency department colleagues, for example, place a premium on expediting the examination, and most of our patients are overweight adults. For this reason, we perform unenhanced CT with the selective use of contrast material in most patients. Key CT Findings CT findings of acute appendicitis are divided into appendiceal, cecal, and periappendiceal changes [4, 34, 46, 57]. Appendiceal Changes One of the CT hallmarks of acute appendicitis is appendiceal thickening (Figs. 3 and 4). Most authors have extrapolated from the sonography literature on appendicitis [27] and define appendiceal thickening on CT as outer-wall-to-outer-wall transverse diameter greater than 6 mm. Unfortunately, sonographic data of appendiceal diameter were based on images obtained with graded compression of the RLQ, which may alter appendiceal diameter, whereas CT images are obtained without compression. Thus, extrapolation from sonography to CT may not be valid. For this and other reasons, some authors define appendiceal thickening on CT as transverse diameter greater than 7 mm [11]. However, even a diameter threshold of 7 mm may be inappropriate. Brown et al. [58] showed that the normal appendix measures greater than 6 mm in 42% of asymptomatic outpatient adults; they defined the upper limit of normal as 10 mm. On the basis of these results, an appendiceal diameter of 6 10 mm is indeterminate. Because of considerable overlap between the normal and abnormal appendix, we believe that any single-diameter threshold is too simplistic. Instead, the appendiceal diameter probably should be interpreted in the context of clinical and other CT findings. Table 1 summarizes a proposed algorithm for the interpretation of CT findings. This algorithm is based on personal experience and the authors interpretation of presented and published data. It has not been tested. Testing and refinement of the proposed algorithm will require further study. Additional appendiceal signs include appendiceal wall thickening (wall 3mm) (Figs. 5 and 6), appendiceal wall hyperenhancement (Figs. 2 and 4 6), mural stratification of the appendiceal wall (Fig. 2), appendicolith(s) (Figs. 5 and 7), and intramural gas. Appendicoliths are present in one third of patients with appendicitis. Although associated with appendicitis, appendicoliths are not diagnostic and have low specificity as isolated findings because they are commonly present in asymptomatic subjects. Appendicoliths may have prognostic importance, however, because their presence increases the likelihood of appendiceal perforation (Fig. 5; also see the following text). Cecal Changes Key CT findings involving the cecum involve the cecal apex and include cecal apical thickening (Fig. 8), the arrowhead sign (Fig. 9), and the cecal bar sign. Diffuse as opposed to apical cecal thickening is also possible, but this is less specific for appendicitis. The arrowhead sign refers to focal cecal wall thickening centered on the appendiceal orifice: enteric contrast material in the cecal lumen points to the abnormal appendix and assumes a triangular configuration, mimicking an arrowhead. The cecal bar sign refers to inflammatory soft tissue at the base of the appendix that separates the appendix from the contrast-filled cecum. The cecal arrowhead and bar signs are applicable only in patients in whom enteric contrast material distends the cecum; these signs are best visualized after rectal contrast administration. TABLE 1: Proposed Algorithm for Interpretation of Appendicitis CT in Symptomatic Patients Inflammatory Changes in RLQ Periappendiceal inflammation includes periappendiceal fat stranding (Figs. 5 7), thickening of the lateral conal fascia (Fig. 7) and mesoappendix, extraluminal fluid (Fig. 7), phlegmon, abscess (Fig. 10), ileocecal mild lymph node enlargement, and inflammatory thickening of contiguous structures. Structures that may be secondarily inflamed depend on the anatomic location of the appendix and include the ascending colon, terminal ileum, sigmoid, and urinary bladder (Fig. 6). Periappendiceal signs are not specific for appendicitis [4], however, because they are present in a wide spectrum of other RLQ disorders. Moreover, the sensitivity of these signs may be decreasing because CT is being performed earlier in the course of acute appendicitis. This gradual change in practice has reduced the prevalence and severity of periappendiceal fat stranding and other inflammatory changes on CT [57]. Comparison of Individual CT Signs In a recent study, Choi et al. [57] performed a statistical analysis of the individual CT findings and concluded that appendiceal enlargement, appendiceal wall thickening, periappendiceal fat stranding, and appendiceal wall enhancement were significantly more associated with acute appendicitis than with other findings. Major Complications Perforation If appendicitis is allowed to progress, portions of the appendiceal wall eventually become ischemic or necrotic [1, 59] and the appendix perforates. On CT, perforation is suggested by the presence of localized periappendiceal inflammation, although this is a nonspecific finding. Interestingly, visualization of appendicoliths on CT increases the probability of appendiceal perforation [1, 27, 60], possibly because appendicoliths accelerate the rate at which perforation occurs. Thus, the presence of one or more appendicoliths in association with periappendiceal inflammation is virtually diagnostic of perforation [60] (Figs. 5 and 7). Even in the ab- Interpretation CT Findings Recommendation Excludes appendicitis < 6-mm appendix or > 6-mm appendix completely gas-filled Work up other causes of RLQ pain Possible appendicitis 6- to 10-mm appendix without any other CT signs Observation if symptomatic Probable appendicitis 6- to 10-mm appendix + WT + WHE (no FS) Surgery if symptomatic Definite appendicitis > 10-mm appendix or 6- to 10-mm appendix + WT + WHE + FS Surgery if symptomatic Note This algorithm is based on authors personal experience and interpretation of published data from their and other institutions. Accuracy of the proposed algorithm has not been tested. RLQ = right lower quadrant, WT = wall thickening (appendiceal wall 3 mm), WHE = wall hyperenhancement, FS = fat stranding. 410 AJR:185, August 2005

6 CT of Appendicitis Fig year-old man with acute appendicitis. Axial oblique reformatted image of CT study after administration of oral and IV contrast material shows distended appendix with wall enhancement (arrow) and appendicolith (arrowhead). Note periappendiceal fat stranding. Surgery confirmed perforated appendix. Fig year-old man with acute appendicitis. Unenhanced CT shows appendicolith (arrowhead), periappendiceal fat stranding (black arrows), lateral conal fascia thickening (white arrow), and periappendiceal fluid. Fig year-old woman with acute appendicitis. Axial CT image after IV and oral contrast administration shows thickened appendiceal wall, with wall enhancement (arrow) and fat stranding. Note thickening of adjacent bladder wall (arrowheads) caused by inflammatory process. Fig year-old boy with acute appendicitis. CT with oral contrast material shows cecal apical wall thickening (arrowheads). sence of periappendiceal changes, a CT finding of a thickened appendix and one or more appendicoliths is suspicious for perforation or impending perforation. In a retrospective study, Horrow et al. [61] showed 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 (95%) and specificity (95%) for perforation in patients with known appendicitis who underwent preoperative CT. AJR:185, August

7 Pinto Leite et al. Fig year-old girl with acute appendicitis. Axial CT after oral and IV contrast material shows cecal wall thickening around appendiceal orifice. Enteric contrast material in cecal lumen points to enlarged appendix (arrow) and assumes triangular configuration (arrowhead sign [arrowhead]). Fig year-old man with periappendiceal abscess. Helical CT after IV contrast injection shows periappendiceal abscess extending into psoas muscle (arrowheads). Fig year-old man with mesenteric adenitis. A and B, Unenhanced axial CT shows enlarged mesenteric lymph nodes (A, arrows) and normal appendix (B, arrowhead). A B 412 AJR:185, August 2005

8 CT of Appendicitis Fig year-old man with cecal diverticulitis. Helical CT after oral and IV contrast administration shows diverticulum in cecum (arrow) and mild surrounding fat stranding. Note normal appendix (arrowhead) and engorged vasa recta. In that study, the individual finding with highest sensitivity was a mural enhancement defect (64%). Periappendiceal Abscess Abscess is the most frequent complication of perforation. The abscess remains localized if periappendiceal fibrinous adhesions develop before rupture. CT shows a loculated, rim-enhancing fluid collection that may have mass effect on adjacent bowel loops [59] (Fig. 10). If the abscess is large (> 4 cm), percutaneous drainage followed by delayed appendectomy is the preferred treatment [43]. Peritonitis Bacterial peritonitis, a dangerous complication, is due to early appendiceal rupture before formation of inflammatory adhesions. This complication is more common in young children because progression to perforation tends to be rapid [59, 62]. CT and sonography show interloop fluid and free-fluid tracking along the peritoneal reflections, sometimes far from the appendix. Common locations are the pelvis; the paracolic gutters; and the subhepatic, subphrenic, and hepatorenal spaces. Fluid in the lesser sac suggests other diagnoses, such as pancreatitis or perforated peptic ulcer. Contrast-enhanced CT helps differentiate bacterial peritonitis from ascites by showing enhancement and thickening of peritoneal reflections, inflammatory changes in the mesentery and omentum, engorgement of regional mesenteric vessels, and hyperemic changes in contiguous bowel segments. Bowel Obstruction Uncommonly, patients with acute appendicitis present with mechanical obstruction, likely secondary to entrapment of the distal ileum in a periappendiceal inflammatory mass. More commonly, small-bowel obstruction is a late complication of appendectomy and is caused by postoperative fibrous adhesions in the peritoneal cavity. Fig year-old woman with epiploic appendagitis. Axial IV contrast-enhanced CT shows small fat-attenuation lesion (arrowhead) adjacent to right colon with round hyperdense focus in center (arrow). Note also asymmetric thickening of adjacent colon and infiltration of mesenteric fat. Septic Seeding of Mesenteric Vessels Appendicitis can be complicated by pylephlebitis, pylethrombosis, or hepatic abscess caused by ascending infection along the draining mesenteric portal venous system. Occasionally, patients with cryptogenic portal hypertension due to pylethrombosis have a recent or remote history of appendicitis. Gangrenous Appendicitis Gangrenous appendicitis is the result of intramural and arterial thromboses. CT findings include pneumatosis, shaggy appendiceal wall, and patchy areas of mural nonperfusion. Unusual Manifestations In addition to acute appendicitis, appendicular obstruction may occasionally produce milder, more chronic inflammation. Two similar entities have been described, recurrent appendicitis and chronic appendicitis. Recurrent appendicitis refers to repeated episodes of RLQ pain that, after appendectomy, are proven to be the result of an inflamed appendix. Chronic appendicitis refers to RLQ pain of at least 3 weeks duration that completely disappears after appendectomy. Pathologic examination shows chronic active inflammation of the appendiceal wall or fibrosis of the appendix. In both entities, clinical presentation is usually more insidious than with acute appendicitis. CT findings are generally indistinguishable from those of early acute appen- AJR:185, August

9 Pinto Leite et al. Fig year-old man with omental infarction. Abdominal CT shows well-circumscribed region of inflamed omental fat, with streaklike areas of inflammatory stranding (arrowheads). dicitis: mild wall thickening and mural hyperenhancement with minimal to no fat stranding. Surgery is curative but does not necessarily need to be performed on an emergent basis. With increasing frequency, these patients are being treated with antibiotics and elective surgery weeks to months later, rather than with emergent surgery. Differential Diagnosis Several alternative conditions may mimic appendicitis clinically and on CT. Correct diagnosis is important because many of these differential entities are self-limited and respond to conservative management. Mesenteric Adenitis Mesenteric adenitis is the most common alternative condition identified at negative appendectomy. It is a benign inflammation of the ileocolic lymph nodes that is usually caused by Yersinia enterocolitica, Y. pseudotuberculosis, or Campylobacter jejuni. CT findings include enlargement (> 5 mm) of mesenteric lymph nodes, thickening of the adjacent cecum and ileum, and a normal appendix [63] (Fig. 11). Cecal Diverticulitis Cecal diverticulitis is relatively uncommon in North American and European populations, accounting for only 5% of all diverticulitis cases. For reasons that are not completely clear, it is distinctly more common in Asians. CT findings of cecal diverticulitis include focal pericecal inflammatory changes, slight mural thickening, and visualization of one or more diverticula. Identifying the inflamed cecal diverticulum allows accurate diagnosis of cecal diverticulitis [64]. The inflamed diverticulum contains gas, fluid, contrast material, or calcified material. Visualization of a normal appendix helps confirm the diagnosis. If the normal appendix is not visualized, the differential diagnosis is difficult [63, 65] (Fig. 12). Epiploic Appendagitis Epiploic appendagitis is an uncommon condition caused by inflammation, torsion, or ischemia of an epiploic appendage. On CT, there is a small fat-attenuation mass contiguous with the colon and having a hyperattenuating rim. A round or linear hyperdense focus in the center of the mass thought to represent a thrombosed central vein is characteristic but is not always present. Other possible findings are focal thickening of the adjacent bowel, infiltration of mesenteric fat, and focal thickening of the surrounding peritoneum [66] (Fig. 13). Omental Infarction Omental infarction is a rare condition in which there is segmental infarction of some portion of the omentum. CT features include a well-circumscribed region of inflamed omental fat with haziness and streaklike areas of inflammatory stranding [63] (Fig. 14). Depending on the location, omental infarction may mimic acute appendicitis, epiploic appendagitis, or diverticulitis. Crohn s Disease Crohn s disease is a chronic granulomatous inflammatory condition that can involve any segment of the gastrointestinal tract but most commonly involves the terminal ileum and right colon. CT helps exclude appendicitis and shows features characteristic of Crohn s disease. Affected bowel usually shows prominent circumferential wall thickening. In acute and subacute cases, IV contrast administration shows bowel wall mural stratification (target sign). Characteristically, skip lesions are present. Local proliferation of mesenteric fat around the affected bowel, prominent vessels in the hypertrophied fat, fistulas, sinus tracts, and abscesses are frequently found [43, 67, 68]. Importantly, Crohn s disease may involve the appendix and cause a chronic granulomatous appendicitis, which is usually managed conservatively. Infectious Terminal Ileitis Bacterial, mycobacterial, parasitic, and viral pathogens can cause terminal ileitis either in isolation or in association with mesenteric adenitis. CT typically shows mild terminal ileal wall thickening (< 5 mm) and, if present, findings of mesenteric adenitis. 414 AJR:185, August 2005

10 CT of Appendicitis A B Fig year-old woman with appendiceal mucocele. A and B, Axial (A) and coronal reformatted (B) CT scans obtained after oral and IV contrast administration show distended appendiceal lumen caused by abnormal mucus accumulation (arrows). Perforated Cecal and Appendiceal Carcinoma Small cecal and appendicular carcinomas can occlude the appendicular lumen, causing secondary appendicitis. Also, cecal carcinoma can perforate and mimic acute appendicitis. Although benign and malignant causes of appendicitis can be similar in appearance, they usually occur in different age groups. Malignant causes are extremely rare in young patients and usually are encountered only in elderly patients. CT findings include nodular and asymmetric thickening of the cecum or appendiceal base. An intraluminal lesion may be visible and show enhancement after IV contrast administration. Correctly suggesting the possibility of malignant appendicitis is important because it may alter the surgical approach. Appendiceal Mucocele Appendiceal mucocele is a well-capsulated cystic mass in the pericecal region representing the distended appendiceal lumen caused by abnormal mucus accumulation (Fig. 15). Usually the cystic mass displaces the adjacent bowel loops, typically without periappendiceal inflammation or abscess. Unenhanced CT may show curvilinear or punctate calcifications in the mucocele wall. The wall enhances after IV contrast administration. Focal nodular thickening in the wall of the mucocele suggests the presence of a mucinous cystadenocarcinoma [69]. Conclusion Helical CT is an accurate, effective technique for diagnosing acute appendicitis. Although the optimal CT technique for evaluation of patients with suspected acute appendicitis is controversial, results from many studies show appendicitis CT to be highly accurate independently of the chosen protocol. Familiarity with CT findings is important for the correct diagnosis of acute appendicitis, differentiation of appendicitis from other entities, and identification of complications. CT does have important disadvantages, however. These include the use of ionizing radiation, possible adverse reactions to IV contrast material, discomfort caused by enteric agents, limited assessment of acute gynecologic disorders, and potentially inadequate RLQ visualization in thin individuals. For these reasons, sonography will continue to play an important role. In our opinion, sonography probably should be performed first in children, adolescents, thin young adults, and women of reproductive age with possible gynecologic causes of pain. References 1. Birnbaum BA, Wilson SR. Appendicitis at the millennium. Radiology 2001; 215: Berry J Jr, Malt RA. Appendicitis near its centenary. Ann Surg 1984; 200: Heaston DR, McClellan JS, Heaston DK. Community hospital experience in 600+ consecutive patients who underwent unenhanced helical CT for suspected appendicitis. AJR 2000; 174[American Roentgen Ray Society 98th Annual Meeting Abstract Book suppl]:53 4. Rao PM, Rhea JT, Novelline RA. Helical CT of appendicitis and diverticulitis. Radiol Clin North Am 1999; 37: Silen W. Cope s early diagnosis of the acute abdomen, 19th ed. New York. NY: Oxford University Press, 1996:5 6. Lewis FR, Holcroft JW, Boey J, et al. Appendicitis: a critical review of diagnosis and treatment in 1000 cases. Arch Surg 1975; 110: Birnbaum BA, Jeffrey RB. CT and sonographic evaluation of acute right lower quadrant abdominal pain. AJR 1998; 170: Lane MJ, Liu DM, Huynh MD, Jeffrey RB Jr, Mindelzun RE, Katz DS. Suspected acute appendicitis: nonenhanced helical CT in 300 consecutive patients. Radiology 1999; 213: Bongard F, Landers DV, Lewis D. Differential diagnosis of appendicitis and pelvic inflammatory AJR:185, August

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