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REVIEW OF LITERATURE

The term ‘Acute Abdomen’ refers to any clinical condition characterised by severe abdominal pain that develops over a period of hoursi.It is in many cases a medical emergency, requiring urgent and specific diagnosis. Computed tomography has proved its value as the imaging modality of choice in evaluation of acute abdominal trauma with contribution towards triage of patients and decrease of negative exploratory laprotomies. CT is also indicated when there is deterioration of patient condition or non-response to initial clinical therapy. Computed tomography provides an excellent means to detect a number of acute abdominal disorders and allows rapid assessment with high diagnostic accuracy. With modern scanners, CT has broadened its wings and its accuracy has increased over the years. The advent of multi detector computed tomography (MDCT), CT angiography and perfusion studies is an example of this. For diagnosing the cause of ‘acute abdomen’, CT is superior to clinical evaluation basic laboratory investigations and inconclusive imaging examination regardless of duration of signs and symptoms in both traumatic and nontraumatic patients Acute abdominal pain may still present a diagnostic dilemma for clinicians. The accuracy of clinical assess- ment is variable (47–76%), and depends upon clinical experience and whether laboratory data are available [2, 3]. For this reason, there has justifiably been an increasing reliance on CT imaging to guide management; the role of intravenous (i.v.) contrast-enhanced CT is well established, with evidence demonstrating improved speed and accuracy of diagnosis, with resultant reduc- tion in hospital admission rates and length of stay, as well as reduced morbidity and mortality [3–9]. In a minority of cases, the utility of CT is more limited, especially in the diagnosis of early inflammatory changes such as in inflammatory bowel disease, mesenteric ischaemia and mild acute pancreatitis, and in patients who have a history of chronic abdominal disease, with disagreement between the radiological and clinical diagnoses more likely [5–8, 10, 11].

CT Examination Use of CT in the evaluation of acute ab- dominal pain has increased to a large ex- tent. For example, in the United States, the number of CT examinations per- formed for this indication increased 141% between 1996 and 2005 (18). This increase was related to the high accuracy of CT in the diagnosis of specific diseases (eg, appendicitis and diverticulitis [12,13]) and the rapid patient throughput that can be achieved with use of multide- tector CT scanners. The CT technique used to examine patients with acute abdominal pain gen- erally involves scanning of the entire ab- domen after intravenous administration of an iodinated contrast medium. Although abdominal CT can be performed without contrast medium, the intrave- nous administration of contrast material facilitates good accuracy—with a positive predictive value of 95% reported for the diagnosis of appendicitis (19)—and a high level of diagnostic confidence, especially in rendering

diagnoses in thin patients, in whom fat interfaces may be almost ab- sent (Fig 1). Although rectal or oral con- trast material may be helpful in differen- tiating fluid-filled bowel loops from ab- scesses in some cases, the use of oral contrast material can markedly increase the time these patients spend in the ED (23). The lack of enteral contrast medium does not seem to hamper the accurate reading of CT images obtained in patients with acute abdominal pain as it does in postoperative patients. For example, in a series of 1021 consecutive patients with acute abdominal pain in whom only intravenous contrast medium was adminis- tered, there were no inconclusive CT scans due to the lack of enteral contrast medium (21). Multiplanar reformation is beneficial, especially in cases of equivocal CT scans, and it increases the radiolo- gist’s level of confidence in the diagnosis.

Fig 1 Axial CT images in 26-year-old slen- der woman clinically suspected of having acute appendicitis. Differentiation between pelvic in- flammatory disease and appendicitis on US im- ages was difficult; therefore, CT was performed. (a) Nonenhanced CT scan findings were also in- conclusive because of absence of delineating fat. (b) CT scan obtained after intravenous and rectal contrast material administration shows appendici- tis: a distended appendix with thickened wall (ar- row) and surrounding infiltration. B bladder, C cecum. Appendicitis was confirmed at sur- gery and histopathologic analysis. (Images cour- tesy of Saffire S.K.S. Phoa, MD, Academisch Medisch Centrum, Universiteit van Amsterdam, Amsterdam, the Netherlands.)

Studies to evaluate the accuracy of abdominal CT performed in patients with acute abdominal pain generally are scarce. In the cohort study of 1021 con- secutive patients with acute abdominal pain, US and CT were compared for the determination of urgent diagnoses (21). CT was significantly more sensitive than US (89% vs 70%, P .001). The highest sensitivity (only 6% missed urgent cases) was obtained with a diagnostic strategy involving the use of initial US, followed by CT, only in negative or inconclusive US cases. Use of this approach also led to a reduction in radiation exposure because CT was needed for only 49% of the pa- tients. Alternative strategies based on the body mass index or age of the patient or on the location of the pain resulted in a loss of sensitivity. In the literature, there are two randomized controlled trials in which standard practice was compared with early CT— in one study, early CT was performed within 1 hour of presen- tation, and in the other study, it was per- formed within 24 hours—in patients who presented with acute abdominal pain (22,23). In these two studies, standard practice involved conventional abdominal and chest radiography and, if necessary, additional CT. CT was requested in half the patients in the standard practice group. In the

first trial (22), patients in the early CT group had shorter hospital stays, but this finding was not reproduced in the second randomized trial, which was powered to evaluate reductions in hospital stay (23). The percentages of correct diagnoses 24 hours after admis- sion did not differ significantly between the two patient groups (76% for early CT, 75% for standard practice). However, a significantly greater percentage of serious diagnoses were missed in the standard practice group (21% vs 4%, P .001). Prospective studies involving the ex- amination of patients for whom the clini- cian ordered CT scanning have shown that CT findings have a significant effect on diagnoses. In one study, the accuracy of the clinical diagnosis made before CT was performed improved from 71% to 93% after CT was performed (24). The accompanying change in treatment man- agement was 46%. Another study re- vealed a significant increase in the level of confidence of the diagnosis made with CT: The treatment management for 60% of patients was changed (25). Abdominal CT reportedly yields good overall interob- server agreement and very good interob- server agreement for the determination of specific urgent diagnoses, with re- ported values of 0.84, 0.90, and 0.81 for agreement regarding the diagnoses of ap- pendicitis, diverticulitis, and bowel ob- struction, respectively (26). Exposure to ionizing radiation is a dis- advantage of CT. The effective radiation dose for abdominal CT is approximately 10 mSv. In comparison, the annual back- ground radiation dose in the United States is approximately 3 mSv. A 10-mSv CT examination performed in a 25-year- old person is associated with an esti- mated risk of induced cancer of one in 900 individuals and a risk of induced fatal cancer of about one in 1800 individuals (27). For older individuals, these risks are considerably lower. These risks should be weighed against the direct diagnostic ben- efit and related to the lifetime cancer risk: One in three people will develop cancer within their lifetime. It is important to note that the effective radiation dose at abdominal CT may be reduced to some extent. In studies of appendicitis and di- verticulitis, standard-radiation-dose (100- and 120-mAs) nonenhanced CT was compared with 30-mAs nonen- hanced CT (28,29). There were no signif- icant differences in accuracy between the low- and standard-dose CT examinations. In a series of 58 patients suspected of having appendicitis, low-dose CT with oral contrast medium had accuracy com- parable to that of standarddose CT with intravenous contrast medium (30). In general, the effective CT dose is influ- enced by the current dose modulation methods, which balance image quality and dose. The use of intravenous contrast medium is a drawback in patients with imminent renal insufficiency.

Acute Appendicitis

The prevalence of appendicitis in patients who present with abdominal pain to the ED is approximately 14% (Table E1 [on-line]). The starting symptom is generally nondescriptive visceral pain in the peri-umbilical region, followed by nausea and vomiting. When the disease progresses, the pain typically migrates to the right lower quadrant because of more localized peritoneal inflammation. Owing to this frequent cause of acute abdominal pain, approximately 250 000 appendectomies are performed annually in the United States (31). After mortality and morbid-ity, the important quality indicators of care in patients suspected of having ap-pendicitis are negative appendectomy rate and percentage of perforated appen-dicitis.

Making an accurate and timely diag-nosis of appendicitis is challenging for cli-nicians. A false-positive diagnosis may lead to unnecessary surgical exploration, which is associated with increased mor-tality risk, prolonged hospital stay, and increased infection-related complication risk (32). A false-negative (missed) diag-nosis can lead to prolonged time to treat-ment and increased risk of perforation. Several nonmedical factors (ie, prehospi-tal time, availability of operating room for emergency surgery, time of presentation) have been shown to be significantly asso-ciated with perforated appendicitis (40). Compared with uncomplicated appendi-citis, perforated appendicitis is associated with a two- to tenfold increase in mortal-ity (Fig 2) (34).

Fig 2 Coronal contrast-enhanced reformat- ted CT image in 28-year-old man clinically suspected of having appendicitis shows inflamed appendix (straight arrow). Inflammation is more pronounced at the appendiceal tip, and discontinuity of the appen- diceal wall is suggested. Small amounts of fluid (curved arrow) adjacent to the appendix are present. Adjacent fat inflammation (arrowheads) is more pronounced at the appendiceal tip. Surgery and his- topathologic analysis results proved the presence of perforated retrocecal appendicitis.

Traditionally, acute appendicitis has been diagnosed on the basis of clinical findings. There has been a low threshold to perform appendectomy on the basis of the assumption that missed appendicitis—and thus the chance of perforation— has more hazardous consequences than does appendectomy that reveals negative findings. As a result, negative-finding ap-pendectomy rates of 12%– 40% have been reported (35). Despite having high sensitivity (up to 100%), clinical evalua-tion has relatively low specificity (73%) (36). This means that surgeons are likely to overestimate the presence of appendi-citis in patients who present to the ED. Some clinicians hold the view that imaging should be performed only in patients who have equivocal clinical findings at presentation. Direct appendectomy can be performed in patients with classic signs and symptoms at presentation, particularly young men (37), whereas mak-ing a clinical diagnosis is more difficult in women. A combination of clinical fea-tures, including pain migration, abdomi-nal rigidity, and elevated inflammatory parameters (38), has a high predictive value for appendicitis but is present in only a small proportion of patients sus-pected of having appendicitis. This makes clinically determining the

diagnosis diffi-cult in the majority of patients and em-phasizes the added value of imaging in patients suspected of having appendicitis.

The exact role of imaging in the set-ting of suspected appendicitis is still a matter of debate. In general, radiography does not play a role in the work-up; US and CT have important roles, al-though CT has better accuracy (Figs 3, 4). CT is the preferred imaging technique for the diagnosis and assessment of appendicitis in the United States (39) and has been shown to reduce the negativefinding appendectomy rate from 24% to 3%, with a simultaneous increase in CT use, from 20% to 85%, over a period of 10 years (40). Two randomized trials re-vealed negative-finding appendectomy rates of 5% and 2% in patients who un-derwent routine CT, compared with rates of 14% and 19% in patients in whom CT was performed selectively on the basis of clinical judgment (41,42). Furthermore, it has been shown that the routine use of CT is cost-effective because it facilitates a reduction in in-hospital costs of $447 per patient by preventing unnecessary hospi-talizations and surgical explorations (43).

There are several individual CT find-ings that suggest a diagnosis of appendi-citis; an enlarged ( 6 mm) appendix has a high positive predictive value (44,45). Likewise, the sensitivity of adjacent fat infiltration is high for the diagnosis of ap-pendicitis (45,46). However, the visual-ization of appendicoliths has been shown to have a low positive predictive value for the diagnosis of appendicitis because these may also be present in individuals who do not have appendicitis (Fig 5) (44). CT has limitations in the detection of ap-pendiceal perforation. For the detection of perforated appendicitis, extraluminal gas, abscess, focal appendiceal wall de-fect, and small-bowel obstruction (SBO) have high specificity at CT; however, these findings are not very sensitive (47,48). If appendicitis can be ruled out, the most common alternative imaging-based diagnoses are gynecologic diseases (Fig 6), diverticulitis, and colitis (48). Other alternative conditions, which require conservative treatment, include right-sided diverticulitis and epiploic ap-pendagitis (Fig 7).

Despite the high diagnostic perfor-mance of CT, this modality has the afore-mentioned drawbacks. Therefore, alter-native strategies for the diagnosis of acute appendicitis that involve less use of CT have been proposed—for example, US performed as the initial diagnostic test, with CT performed only secondarily, af-ter US has yielded nondiagnostic findings. However, US can be limited by gasfilled bowel, which may obscure the underlying

Fig 3 US image in 43-year-old man clini- cally suspected of having acute appendicitis shows noncompressible thickened (10-mm) ap- pendix surrounded by inflamed mesenteric fat (arrows) and some fluid (arrowhead). Calipers mark the appendix. A iliac artery, V iliac vein

Fig 4 Axial CT image obtained after intra- venous contrast medium administration in 47-year-old man with 2-day history of right lower quadrant pain and clinically suspected of having acute appendicitis shows thickened appendix (arrow) with maximal diameter of 14 mm and adja- cent fat infiltration (arrowheads). C cecum.

Fig 5 Axial abdominal CT image in 62- year-old woman with known factor V Leiden trom- bophilia and a fever without an apparent cause for more than 2 weeks, obtained after the administra- tion of oral and intravenous contrast media to exclude lymphoma shows an appendicolith (ar- row) in a noninflamed appendix as an incidental finding.

Fig 6 Axial CT scan obtained after intrave- nous contrast medium administration in 24-year- old woman with right lower quadrant pain, a clinical differential diagnosis of gynecologic disorder (tuboovarian abscess, pelvic inflammatory dis- ease, ovarian torsion) or appendicitis, and incon- clusive US findings shows a normal appendix (straight arrow) and an enlarged right ovary (ar- rowheads), which most likely is due to tubo-ovar- ian abscess or ovarian torsion. Free fluid and some thickening of the peritoneum (curved ar- rows) are also visible. Laparoscopy revealed ovar- ian torsion. U uterus.

Fig 7 Axial CT images in 25-year-old woman suspected of having appendicitis. At US, the appendix was not well visualized; therefore, CT was performed after intravenous contrast medium administration. (a) Image shows right-sided diver- ticulitis, indicated by right-sided colon diverticula (arrow) and fecalith with thickened wall, wall en- hancement, and adjacent fat infiltration (arrow- heads). (b) Image shows some secondary wall thickening of the adjacent appendix (arrow), with air in the lumen. Only some secondary changes— and no appendicitis—are seen. C cecum.

Acute Diverticulitis Acute colonic diverticulitis is the second most common cause of acute abdominal pain and leads to 130 000 hospitalizations in the United States annually (49). The prevalence—and thus the incidence—of diverticulosis increases with age. Ten per- cent of the general population younger than 40 years and more than 60% of peo- ple older than 80 years are affected by diverticulosis (50). Ten percent to 20% of the affected people will develop divertic- ulitis, which is localized on the left side of the colon in 90% of cases (51). Seventy- two percent of patients admitted to the hospital for diverticulitis have uncompli- cated diverticulitis. A sensitivity of 64% for the clinical diagnosis of acute divertic- ulitis in the ED has been reported—that is, one-third of the cases are missed clinically (52). These patients are most often suspected of having acute appendicitis. Because the treatments for acute appendi- citis (appendectomy) and acute diverticuli- tis (mainly conservative treatment) are dif- ferent, the differentiation of these two diag- noses is important. The reported positive predictive value of 53% for the presence of diverticulitis after clinical evaluation indi- cates that approximately 40% of primary clinical diagnoses are false-positive (52). The disease stage in patients with di- verticulitis is often determined by using the modified Hinchey classification sys- tem (53), in which imaging and/or surgi- cal findings are incorporated (Table). Most patients with uncomplicated di- verticulitis can follow a conservative treatment regimen of antibiotics and diet modification. In mildly ill patients with a presentation clearly suggestive of uncomplicated diverticulitis (Hinchey stage 0 or 1a), the treatment decision is not based on the imaging results but rather on the patient’s clinical status. In patients who have Hinchey stage 1b diverticulitis with a small ( 2 cm) abscess, treatment can be conservative as well. Patients with larger abscesses are treated with percutaneous drainage. Diverticulitis-associated abscesses are found at CT in approximately 15% of patients (54) (Fig 8). The majority of these collections, approximately 36%–59%, are me- socolic abscesses, which can be treated with percutaneous drainage. The divertic- ulitis recurrence rate is the highest (40%) in this group (55,56). If patients do not respond to or dete- riorate while undergoing conservative treatment, they will undergo surgery (57). Approximately 13% of patients who are treated conservatively will have a recurrence of diverticulitis, and only 4% of patients will have a third episode (58). On the basis of the low recurrence rates, Broderick-Villa et al (58) proposed that elective surgery is not indicated in these patients. In another study, 10% of the patients underwent surgery after initial conservative treatment for diverticulitis. CT plays a role in confirming the diagnosis and staging suspected complicated disease. CT assists in therapeutic decisions and in the detection of alterna- tive diseases, according to guidelines of the American Society of Colorectal Sur- geons (51,57). In a recent metaanalysis, the accuracies of US and CT in the assess- ment and diagnosis of diverticulitis were not significantly different. Overall sensitivities were 92% for US and 94% for CT (P .65), and overall specificities were90%forUSand99%forCT(P .07). The sensitivity of CT for the diagno- sis of alternative diseases was higher and ranged between 50% and 100%. Two frequently present CT findings that have high sensitivity for the diagnosis of diverticulitis are wall thickening (95% sensitivity) and fat stranding (91% sensi- tivity). Although fascial thickening and

in- flamed diverticulum are less frequent findings, they have reported specificities of 97% and 91%, respectively (59). CT is used not only to make diagnoses but also to stage disease in patients with divertic- ulitis. CT can also be used to differentiate colorectal cancer from diverticulitis. Fea- tures associated with the diagnosis of co- lon carcinoma are pericolonic lymph nodes and luminal mass, whereas pericolonic in- flammation and segment involvement larger than 10 cm are more commonly as- sociated with diverticulitis (Figs 8, 9). However, these signs are not very accurate, and cancer can be missed (Fig 8). There- fore, endoscopy and biopsy are often re- quired to make this differentiation after the clinical symptoms have resolved— often after 6 weeks.

Fig 8 Axial CT image obtained after intra- venous contrast medium administration in 54- year-old man with a history of colitis that was diag- nosed at age 15 and a several-month-long history of abdominal pain and weight loss, who presented to the ED with progressive abdominal pain of 1 week duration. Image shows a thickened sigmoid colon with some surrounding infiltration (ar- row), a contained perforation (arrowhead), and multiple abscesses (A). Histopathologic analy- sis revealed extensive perforated diverticulitis and adenocarcinoma. (Image courtesy of C. Yung Nio, MD, Academisch Medisch Centrum, Universiteit van Amsterdam, Amsterdam, the Netherlands.)

Fig 9 (a, b) Axial CT images obtained after intravenous, oral, and rectal contrast material administration in 46-year-old man with 2-year history of abdominal pain and recent progressive acute abdominal pain. He had not defecated for the past 2 days and had experienced weight loss of 12 kg during the past year. Acute diverticulitis was clinically suspected, with colorectal cancer as a differential diagnosis. Images show (a) apple-core stenosis (arrow) of the sigmoid colon caused by colorectal cancer and (b) proximal prestenotic dilatation of descending colon and cecum (arrow).

Acute Cholecystitis Cholecystolithiasis is the main cause of acute cholecystitis, for which an estimated 120 000 cholecystectomies are performed annually in the United States (60). The prevalence of acute cholecysti- tis is approximately 5% in patients who present with acute abdominal pain to the ED. Traditionally, the diagnosis has been based on the clinical triad of right upper quadrant tenderness, elevated body tem- perature, and elevated white blood cell count. In a prospective series of patients with acute cholecystitis (61), however, this triad was present in only 8% of pa- tients. Relatively recently published Tokyo guidelines introduced diagnostic and severity assessment criteria (62). The diagnostic criteria for acute cholecystitis are one local sign of inflammation (Mur- phy sign; mass, pain, and/or tenderness in right upper quadrant), one systemic sign of inflammation (fever, elevated C- reactive protein level, elevated white blood cell count), and confirmatory imaging findings. Cholecystitis severity is clas- sified as mild, moderate, or severe (stages I, II, and III, respectively). Mild cholecystitis is defined as cholecystitis in a patient who has mild inflammatory changes adjacent to the gallbladder with- out organ dysfunction. Findings of moderate cholecystitis are elevated white blood cell count, palpable tender mass in the right upper quadrant, duration of complaints longer than 72 hours, and marked local inflammation. Severe chole- cystitis is defined as cholecystitis combined with multiple organ dysfunction syndrome. Radiologic findings have an important influence on treatment management in patients with cholecystitis and organ fail- ure due to sepsis. Percutaneous drainage of the inflamed gallbladder with delayed cholecystectomy can be a safe option. In all other cases that do not involve severe inflammation or surrounding infiltration at imaging, laparoscopic cholecystectomy should be

performed within 96 hours af- ter the start of the complaints. Imaging findings are therefore essential in making decisions regarding treatment for chole- cystitis. Several imaging techniques are avail- able for the evaluation of suspected acute cholecystitis. US is the most frequently performed modality for right upper quad- rant pain and yields a sensitivity of 88% and a specificity of 80% in the diagnosis of acute cholecystitis (63). Features of cho- lecystitis include gallbladder wall thicken- ing; enlarged tender, noncompressible gallbladder; and adjacent infiltration or fluid collections (Figs 10, 11). According to ACR appropriateness criteria, US is considered the most appropriate imaging modality for patients suspected of having acute calculous cholecystitis (64). In a highly select study sample, CT also showed good accuracy, with a sensitivity of 92% and a specificity of 99% (65). In patients with acute abdominal pain, CT has demonstrated

Fig 10 US image obtained in 79-year-old man with 4-day history of right upper quadrant pain, nausea, and vomiting shows a thickened gallbladder wall (arrowheads) and an obstructing gallstone (arrow), which was position indepen- dent. The patient was initially treated with percuta- neous gallbladder drainage.

Fig 11 Axial CT image obtained after ad- ministration of oral and intravenous contrast me- dia in 73year-old obese woman with abdominal pain, fever, elevated C-reactive protein level (400 mg/L), and a normal white blood cell count shows cholecystitis with wall thickening, radio-opaque gallstones (arrow), and some adjacent fat infiltra- tion. The broad clinical differential diagnosis in this patient included cholecystitis, diverticulitis, and appendicitis. The acute cholecystitis was treated with percutaneous drainage because of this patient’s critical condition. accuracy comparable to that of US

in the diagnosis of acute cholecystitis (61). US should be consid- ered the primary imaging technique for patients clinically suspected of having acute cholecystitis (63).

Bowel Obstruction Bowel obstruction is a relatively frequent cause of acute abdominal pain. The majority of patients found to have bowel obstruction after they present to the ED have an SBO. SBO Disease SBO is primarily caused by postoperative adhesions. The combination of vomiting, distended abdomen, and increased bowel sounds is suggestive of SBO and has a positive predictive value of 64% (65). Other patient characteristics and risk fac- tors associated with bowel obstruction are previous abdominal surgery, age older than 50 years, and history of consti- pation (65). In patients with SBO compli- cated by ischemia (strangulated hernia), immediate surgery is warranted, whereas many other patients with low-grade ob- struction can be treated conservatively with a nasogastric tube and bowel rest. Seventy-three percent of all patients who are treated conservatively will not be re- admitted. In one series, however, 19% of the patients were readmitted for recur- rent obstruction—one-third of these sub- jects underwent surgery—and 8% of the patients died (66). Approximately one quarter of patients who are initially found to have SBO in the ED will undergo sur- gery. For adequate treatment, it is important to identify the cause (eg, adhe- sion, neoplasm, or hernia) and severity of he obstruction. An obstruction can be partial or complete and complicated by ischemia, especially in the case of closed- loop obstruction (strangulation). Unlike adhesions, internal hernia is an uncom- mon cause of bowel obstruction in the Western world. Accurate diagnosis is mandatory because of the risk of strangu- lation. Because clinical evaluation has lim- itations in the diagnosis of bowel obstruc- tion (65), imaging is routinely performed to identify the site, cause, and severity (high- vs low-grade) of the obstruction. All of these are important parameters to help guide patient treatment. Radiography has long been the pri- mary imaging modality of choice for pa- tients suspected of having bowel obstruc- tion. Radiography has been reported to have 69% sensitivity and 57% specificity in the diagnosis of bowel obstruction (67) (Fig 12).

Fig 12 Upright conventional abdominal radiograph obtained in 59-year-old man who had

abdominal pain and a distended abdomen at clini- cal evaluation, as well as a history of SBO 3 years ago, for which he was treated conservatively, shows distended small-bowel loops and air-fluid levels (arrowheads), consistent with SBO. The previous obstruction was most likely caused by adhesions because the patient had previously undergone appendectomy. This patient was again treated conservatively.

CT has the best reported accuracy for the diagnosis of SBO, with a sensitiv- ity of 94% and a specificity of 96% (68). In one study, the cause of the obstruc- tion was correctly identified at CT in 85% of patients with a clinically equivo- cal diagnosis of bowel obstruction. The clinical management was correctly altered for 23% of patients—mainly from conservative to surgical treat- ment. Evaluating the location and differ- entiating the common causes of bowel obstruction are more difficult on ab- dominal radiographs (67). CT can also enable the differentiation between high- and low-grade obstruction (70). How- ever, the sensitivity of CT is markedly lower for the diagnosis of low-grade (64%) SBO than for the diagnosis of high-grade SBO (71). An important CT finding that may suggest SBO is a clear change in bowel diameter. With SBO, loops proximal to the transition point are distended, whereas loops distal to the transition point are collapsed. A helpful sign for identifying the point of obstruction is the small-bowel feces sign—that is, feces-like material in the distended small bowel (72) (Fig 13). The transition point should be scrutinized for the cause of the ob- struction (adhesion, neoplasm, hernia, or inflammatory disorder).

Fig 13 Sagittal reconstructed CT image in 47-year-old woman who had a history of lysis of adhesions and presented with cramping pain of 2 days duration, nausea, and vomiting shows the transition point (arrow) and the small-bowel feces sign (arrowheads) proximal to the transition point. No mass is visible, and the diagnosis is obstruc- tion by adhesions. The patient was treated conser- vatively with a successful outcome.

Because SBO is most often due to ad- hesions, which are usually not visible at CT, in most patients, no cause will be identified at CT. In this setting, the diag- nosis of SBO due to adhesions is made by

means of exclusion. In some patients with adhesions, an adhesive band may be sug- gested if extraluminal compression and kinking (acute angle) of the bowel are present (73). Internal hernias can be difficult to identify. The sensitivity and specificity of CT in the diagnosis of internal hernias are moderate, 63% and 73%, respectively (74). A cluster of small-bowel loops is a CT finding strongly associated with the diagnosis of internal hernia (odds ratio, 7.9) (75). The ACR proposes that abdominopel- vic CT with intravenous contrast medium is the most appropriate imaging examina- tion when complete or high-grade SBO is suspected (76). If low-grade or intermit- tent SBO is suspected, several CT tech- niques (barium or water as the contrast agent, enteroclysis) or small-bowel exam- inations (follow-through, enteroclysis) are more or less equally appropriate, according to the ACR. Other imaging modalities such as US and MR imaging are not widely used, al- though US reportedly has good accuracy (81%) in the diagnosis of bowel obstruc- tion (75). Fluid-filled loops are easily visu- alized at US, and one can easily differen- tiate between mechanical obstruction and paralytic ileus by visualizing peristaltic movement. US has important limita- tions: Gas-filled loops may obscure the underlying abnormality, which has im- portant treatment management–related implications, and the obstruction is difficult to stage accurately. The ACR states that US is the least appropriate imaging modality when high- or low-grade SBO is suspected (76). Evidence supporting the accuracy of MR imaging in the diagnosis of bowel obstruction is limited, but results are promising. A sensitivity of 95% and a specificity of 100% were reported in a small study by Beall et al (77). Overall, CT can be considered the primary imaging technique for patients suspected of having SBO. Large-Bowel Obstruction Large-bowel obstruction (LBO) is most commonly caused by colorectal cancer (60% of cases), sigmoid volvulus (10%–15% of cases), and diverticulitis (10% of cases). The clinical features of LBO— abdominal pain, constipation, and abdominal distention—are not very spe- cific. Therefore, the clinical diagnosis is often incorrect. Traditionally, conven- tional radiography has been the initial imaging examination performed. At present, CT is more often used to identify the cause of the obstruction, the level of obstruction, and the presence of a com- plicated obstruction (eg, strangulation). LBO is diagnosed at CT if the colon is dilated (colon diameter 5.5 cm, cecum diameter 10 cm) and filled with feces, gas, and fluid proximal to an abrupt tran- sition point, after which the colon is col- lapsed distally. The accuracy of CT in the diagnosis of LBO has been reported in only one study—that by Frager et al (78). In that study, CT had a sensitivity of 96% and a specificity of 93%. LBO can be di- agnosed with barium enema examina- tion. Although obstruction can often be adequately detected with barium enema examination, unlike CT, barium enema examination generally does not enable the visualization of mural changes and ex- tracolonic abnormalities. CT is the imag- ing modality of choice in the diagnosis of LBO in patients. For short luminal seg- mental obstruction caused by colorectal cancer, a colonic stent can be placed as either a palliative treatment or a “bridge” to elective surgery (79). Volvulus of the large bowel can be reduced endoscopi- cally. Perforated Viscus

Acute abdominal pain as a result of gas- trointestinal tract perforation is most commonly caused by a perforated gas- troduodenal peptic ulcer or diverticulitis. Less frequent causes include carcinoma and bowel ischemia. Perforated viscus is a generally recognized diagnosis, al- though the incidence of this abnormality with free perforation is low (1%) in the ED (Table E1 [online]). Perforation of a peptic ulcer is now less frequent because of the availability of adequate medical therapy for peptic ulcer disease. Among patients who are evaluated for possible acute diver- ticulitis, only 1%–2% have free perforation. Most perforated diverticula are con- tained perforations (Fig 14). Because the clinical symptoms of free perforation are associated with the underlying cause of the perforation, the clinical presentations of patients with perforated viscus are quite variable. Be- sides the variable symptoms of the underlying mechanism, a rigid abdomen usually is present. Recognizing a perfo- ration and establishing the cause and site of the perforation can yield crucial information for the surgeon. Formerly, suspected free intraperitoneal air was always an indication to perform sur- gery. Currently, with the increased use of CT, contained perforations are more commonly diagnosed, and the initial treatment for these may be conserva- tive. For example, contained perforated peptic ulcers and Hinchey type 2 diver- ticulitis with peridiverticular air bubbles (Fig 14) are often treated with conser- vative management.

Fig 14 Axial CT images obtained in (a) abdominal and (b) lung window settings after intravenous contrast medium administration in 71-year-old woman who had a 2-day history of left lower quadrant pain and was suspected of having diverticulitis show diverticulitis of the sigmoid colon with a contained perforation (arrow) and infiltration of pericolic fat. The patient was treated conservatively with antibiotics.

Upright posteroanterior chest radiog- raphy traditionally has been used for the initial examination of patients suspected of having pneumoperitoneum. Pneumo- peritoneum is visualized as a translucent crescent or area below the diaphragm. Left lateral decubitus radiography is an alternative in patients who are not able to stand upright. CT is currently replacing conventional radiography for this indication. This reflects the fact that multisec- tion CT is more sensitive for the detection of smaller amounts of free intraperitoneal air. Conventional radiography is insensi- tive for the detection of air pockets smaller than 1 mm and only 33% sensi- tive for the detection of 1–13-mm pockets (80). The major advantage of CT, as compared with radiography and US, is that it can correctly depict the actual site of per- foration in 86% of cases (81). A concentration of extraluminal air bubbles, a focal defect of the bowel wall, and segmental bowel wall thickening are CT findings substantially associated with correct iden- tification of the location of a perforation (Fig 15). The location of the

Fig 15 Axial CT images obtained after intravenous administration of contrast medium in 54-year-old woman who presented to the ED with acute periumbilical abdominal pain that radiated to the back. The ab- dominal pain started after the woman ingested a nonsteroidal antiinflammatory drug. (a) Image obtained in lung window setting shows free intraperitoneal air (arrow). (b) Image shows wall thickening at the duodenal bulb and evidence of perforation (arrow), with adjacent soft-tissue infiltration and air bubbles (arrowhead). A diagnosis of perforated duodenal ulcer was made and confirmed at surgery.

free air is a useful indicator of the site of the perfora- tion. If free air is located around the liver and stomach, this most likely indicates a gastroduodenal perforation. Free air de- tected predominantly in the pelvis and supramesocolic and inframesocolic re- gions makes perforation of the colon or appendix more likely (81). Multiplanar reformations at CT are helpful for identi- fying perforations (82). A perforation can be diagnosed at US when echogenic lines or spots with comet-tail reverberation ar- tifacts representing free intraperitoneal air are seen adjacent to the abdominal wall in a supine patient. A sensitivity of 92% and a specificity of 53% have been reported for the detection of perforation with US and constitute an overall accu- racy of 88% (83). It is important to note that establishing the cause and location of the perforation is difficult with US. Intestinal ischemia Mesenteric ischemia leading to bowel infarction is a relatively common catastrophic occurrence in the elderly. In this condition the diagnosis may be difficult, but time is of the essence for sur- vival, because the prognosis is poor, and the treatment is almost inconsequential if performed too late [84-86]. Acute mesenteric ischemia is a true surgical emergency. Risk factors for acute mesenteric ischemia in patients of advanced age include athero- sclerosis, arrhythmias, hypovolemia, congestive heart failure, recent myocardial infarction, valvular disease, deep venous thrombosis, intraabdominal malignancy, and the use of medica- tions with vasoconstrictive effect on the splanchnic vascular dis- trict, such as digitalis, beta-blockers, somatostatin, and vasoactive amines. The diagnosis of acute mesenteric ischemia may be over- looked because of the vague nature of the

patient's symptoms [87-90]. Patients may present with recurrent episodes of post- prandial abdominal pain (intestinal angina), but often the clinical presentation consists of vomiting or diarrhoea with occasional blood in the stool and localized or generalized abdominal pain of either acute or sub-acute onset. The characteristic of this disease is that symptoms typically are out of proportion to findings. Acute mesenteric ischemia is a syndrome in which inadequate blood flow through the mesenteric circulation causes ischemia and eventual gangrene of the bowel wall. The aetiology could be arterial or venous; the arterial disease can be subdivided into non-occlusive and occlusive ischemia [91,92]. Occlusion of the superior mesen- teric artery may be caused by embolism or thrombosis. Emboli may occlude the proximal portion or one of the distal branches of the superior mesenteric artery, whereas thrombosis more frequently involves the origin of superior mesenteric artery, where wall aortic atheromatous apposition may cause partial obstruction of the orifice. When the superior mesenteric artery is obstructed at the origin by an embolus, most of the small intestine and the right colon are subject to ischemia [93,94]. Non-occlusive mesenteric ischemia, most frequent in the elderly, results from decreased arterial perfusion that is not related to the presence of endovascular obstruction but rather is caused by an insufficient cardiac output from congestive heart failure, from myocardial infarction, or from a decreased blood pressure with a low-flow state caused by hypo- volemia or shock. Occlusion of the superior mesenteric vein may be caused by thrombosis at the origin or in its distal branches. It can be primary, or it can be observed in cirrhotic patients who have portal hypertension or in patients who have coagulation disorders. Oc- clusion of the superior mesenteric vein leads to intramural hae- morrhage from impaired venous drainage of the bowel wall. Chronic mesenteric ischemia may be the precursor of any of these conditions. When the arterial lumen is narrowed secondary to atherosclerosis, an increased metabolic demand (e.g., in digestion) exceeds the possible blood supply and can result in severe abdominal pain and possibly infarction. The superior mesenteric vessels are involved more frequently than the inferior mesenteric vessels, but blockage of the latter often is silent because of better collateral circulation. Damage to the affected bowel portion may range from reversible ischemia to trans-mural infarction with ne- crosis and perforation.

Foreign bodies The ingestion of foreign bodies occurs involuntarily while eating; meat boluses are the most common foreign bodies ingested in western countries, fish bones in oriental countries [95e97]. However, in 1% of cases, it causes complications such as acute abdomen due to intestinal perforation. In some cases, it can cause severe complications and even death; in the USA,1,500 people die annually from foreign body ingestion [98]. In the early pa- tients this condition is more frequent because persisted two predisposing risk factor for foreign body ingestion: co-morbid condition and the use of dentures, because they reduce the sensi- tivity of the palate [99,100]. Bowel perforation by a foreign body is less common, as the majority of foreign bodies uneventfully pass to the feces and only 1% of them (the sharper and more elongated objects) will perforate the gastrointestinal tract, usually at the level of the ileum [101]. The complications of foreign bodies ingestion with perforation include the formation of localized abdominal ab- scesses, colorectal, and entero-vascular fistulas, inflammatory masses or omental pseudotumors, and endocarditis.

The intestinal tract, where perforations by foreign bodies are most frequent, include the ileocecal and rectosigmoid regions, because the intestinal lumen narrows and the digestive tract is angulated in these sites. Sites where impaction is most likely, include zones with adhesions, areas containing a diverticular pro- cess, or surgical anastomoses. Treatment consists of surgery (from primary suture to rectosigmoid resection with colostomy, removal of the foreign body, and abdominal cavity lavage), and antibiotics. The patient presented, at the emergency department with diffuse abdominal pain with peritoneal irritation and vomiting of 24 h' duration. Laboratory tests showed generally leukocytosis and increased C-reactive protein. The supine plain abdominal radio- graph demonstrated signs of small bowel obstruction but do not always shows a radiopaque foreign body or pneumoperitoneum. This finding is not surprising because for example fish bones have variable radio-opacity depending on the fish species; in general, the foreign bodies are minimally radiopaque and can rarely be detected on plain films, especially if they are masse by coexistent inflam- matory tissue, fluid or abscesses [102,103]. Moreover, signs of pneu- moperitoneum are not usually observed in plain films because impaction of the foreign body into the intestinal wall is gradual, allowing the perforation site to seal with omentum or adjacent loops and limiting the amount of gas or fluid in the peritoneal cavity. MDCT is currently considered the method of choice for the evaluation of patients with acute abdominal pain and the depiction of foreign bodies due to MDCT's ability to generate highresolution, thin-collimation, multi-planar reconstructions, which allow the GI tract to be examined in all projections. Abdominal CT showed generally a foreign body in the small bowel, with pneumoperitoneum and fluid within the abdominal cavity.

Colorectal cancer There are many hereditary pathologies, such as genetic factors, that predispose onset of colorectal carcinoma, among these the syndromes characterized by the occurrence of polyps [104e106]. Bowel obstruction is the most commonly observed complication of colon cancer. The reported frequency of patients with obstructive colon cancer has ranged from 8% to 29% [107]. Left-sided colon malignancies are more prone to obstruct the colon lumen than are the right-sided malignancies. This is because the diameter of the left colon is smaller than that of the right colon. CT is a sensitive imaging modality for detecting bowel obstruction, and the multi- planar reconstruction images can provide additional information on the transition point in problematic cases. Identifying the transitional zones and an obstructing lesion on CT, and these usu- ally appear as irregular circumferential thickening of the colon, is important to differentiate this entity from other benign conditions such as adynamic ileus, colonic pseudoobstruction and stercoral colitis, and all these maladies can present with colonic dilatation. Because of the relatively larger diameter of the cecal lumen, cecal adenocarcinomas have a tendency to grow without displaying clinical manifestations for a long time [108]. Therefore, cecal cancer only infrequently presents as bowel obstruction; as the initial manifestation, distal small bowel obstruction has been reported to occur in 1.5e8.1% of the patients with cecal cancer [109]. Addition- ally, adenocarcinoma developing near the ileocecal valve

area can cause distal small bowel obstruction even though the tumor is small. CT is also useful for examining cecal cancer patients who present with small bowel obstruction. Careful inspection of the cecum on the CT, and especially the ileocecal valve area, is needed to diagnose this rare condition. Closed-loop obstruction is a unique form of mechanical obstruction in which two points of a bowel segment are occluded, and this most frequently occurs in the small bowel [110,111]. On rare occasion an obstructing colon cancer with a competent ileocecal valve can lead to the condition in which the intraluminal pressure of the colon proximal to the obstructing mass increases due to failure of decompression through the ileocecal valve; this produces effects that are similar to those of a closed-loop obstruction in the small bowel [112]. On CT, this condition can be identified as an obstructive colon mass that causes severe dilatation of the proximal colon. The affected colon is usually filled with fecal material, and the small bowel is not dilated due to a competent ileocecal valve. Radiologists and surgeons should be aware of a closed-loop obstruction associated with colon cancer because this is an urgent surgical situation that can lead to a perforated colon [113]. Common presenting symptoms of CRC include abdominal pain, change in bowel habits, rectal bleeding, anaemia and weight loss [114]. A less frequent presentation is perforation and abscess formation, which is usually intraperitoneal, but may occasionally be located in extraperitoneal spaces. With contained perforation and abscess formation, the clinical picture can closely resemble complicated diverticulitis, whether on clinical examination or on radiological imaging such as Computed Tomography (CT) scans. Patients typically present fever, abdominal pain and leukocytosis, and CT scans show a pericolic or intra-abdominal abscess. Perfo- ration in association with a colonic tumour is uncommon as a primary presentation, with incidences ranging from 2.6%4e10% [115]. Perforation of the colon can be diagnosed by CT with the demonstration of a focal defect in the colon wall that may be accompanied by a fluid-density abscess, free air or stranding of the pericolic fat. Abscess formation occurs in 0.3%e0.4% of colonic carcinomas and it's the second most common complication of perforated lesions. Abscesses commonly remain localised in the paracolic region or may develop into a pelvic abscess, but they can also track along various tissue planes and have been reported to present as a flank abscess, psoas abscess [116], or even a subcu- taneous abscess on the trunk. Perforations can also occur proxi- mally to an obstructing primary lesion, for example, a perforated caecum secondary to a closed loop obstruction with a competent ileocecal valve in an obstructed carcinoma of the sigmoid or descending colon [117,118]. It is important that the diagnosis of perforated colonic carcinoma is considered as a differential diag- nosis whenever a patient presents with an intra-abdominal abscess with the presumptive diagnosis of perforated diverticular disease. All patients who present with complicated “diverticular disease” and intraabdominal abscessdespecially those that do not respond to conservative treatmentdshould be offered surgery with resec- tion of the involved colon and removal of the abscess for histo- logical evaluation [119].

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