Small-Bowel Obstruction: State-of-the-Art Imaging and Its Role in Clinical Management

CLINICAL GASTROENTEROLOGY AND HEPATOLOGY 2008;6:130 –139

CLINICAL IMAGING Small-Bowel Obstruction: State-of-the-Art Imaging and Its Role in Clinical Management DEAN D. T. MAGLINTE,* THOMAS J. HOWARD,‡ KEITH D. LILLEMOE,‡ KUMAR SANDRASEGARAN,* and DOUGLAS K. REX§ *Department of Radiology, ‡Division of General Surgery, and the §Division of Gastroenterology, Indiana University Medical Center, Indianapolis, Indiana

Small-bowel obstruction (SBO) is a common clinical condition with signs and symptoms similar to other acute abdominal disorders. The radiologic investigation of patients with SBO as well as the indications and timing of surgical intervention have changed over the past 2 decades. This review focuses on modern imaging techniques and their role in both the diagnosis and treatment of patients with SBO.

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mall-bowel obstruction (SBO) remains a difficult clinical entity to accurately diagnose and treat.1,2 Advances in abdominal imaging coupled with the development of more versatile nasointestinal tubes have led to innovative changes in the evaluation and treatment of patients in whom SBO is suspected.3 Although controversies remain in both the diagnosis and treatment of patients with SBO, radiologic imaging remains a cornerstone in guiding clinical decision making.3–7 This review article focuses on advanced radiologic methods of examination and their use in both the diagnosis and treatment of patients with SBO.

Clinical Overview The etiology of SBO has shifted over the past 5 decades from predominately hernias to adhesions, Crohn’s disease, and malignancy as the top 3 causes of SBO in Western society.8 –11 Hernias still represent the predominant cause of SBO in some developing countries. Crohn’s disease just recently has been recognized as a leading cause of SBO in the surgical literature, a fact long suspected by many radiologists and gastroenterologists.10,11 Patients with SBO present clinically with abdominal pain, distension, nausea, and vomiting. These signs and symptoms mimic other intra-abdominal emergencies from which SBO must be distinguished.4,5,8,12–16 Physical examination often reveals mild to moderate dehydration, tachycardia, abdominal distension with tympany to percussion, and high-pitched tinkling or rushing bowel sounds. Mild abdominal tenderness is common, but localized tenderness or peritoneal signs are indicative of intestinal compromise and these patients are best treated by emergent surgery. Laboratory investigations reveal a mild to moderate leukocytosis, electrolyte level abnormalities, and an increased blood urea nitrogen\creatinine ratio, and occasionally increased amylase levels, which can mimic acute pancreatitis.

In patients with suspected SBO, radiographic imaging plays a central role in confirming the diagnosis and based on these findings; in helping to choose the appropriate therapy. Simple mechanical SBO can be treated safely using a trial of medical management that includes nasoenteric decompression, intravenous fluid hydration, and serial abdominal radiography. Historical data in patients with surgically proven strangulation showed that the preoperative diagnosis based on clinical and laboratory data, abdominal plain films, and monoslice computerized tomography (CT) scanning is reliable in only 15% to 50% of patients.15,17–19 Based on these reports, there have been advocates for early surgical management in all patients with SBO because of the uncertainty in diagnosing strangulation and the high complication rate associated with delayed surgical intervention in this setting.10,20,21 Despite these concerns, current mortality rates for patients with SBO is only 1% to 2%, suggesting that the vast majority of patients do not have strangulated obstruction and the risks associated with nonsurgical management are acceptable provided immediate surgery is available if a patient fails to improve or develops signs and symptoms of intestinal compromise.22–27 Recent literature has shown that even patients with high-grade mechanical SBO can resolve spontaneously with nonsurgical management using nasointestinal decompression, further supporting an even-handed approach to this complex problem.28 –31

Radiologic Investigations Plan Film Examination (Abdominal Radiography) In SBO, abdominal radiographs are diagnostic in only 50% to 60% of cases.1,2,32 A critical analysis of plain film findings shows a sensitivity of only 66% in proven cases of SBO, and in 21% of these patients their plain films were reportedly normal.1,32,33 In patients, identified as having abnormal but nonspecific plain film findings, 13% had low-grade and 9% had high-grade obstructions, findings that have been confirmed in a recent investigation.34 Despite these limitations, plain film radiographs remain the initial imaging study in almost all patients with suspected SBO.2,33 Abdominal radiography has a low Abbreviations used in this paper: CT, computerized tomography; MR, magnetic resonance; SBO, small-bowel obstruction. © 2008 by the AGA Institute 1542-3565/08/$34.00 doi:10.1016/j.cgh.2007.11.025

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specificity for SBO because both mechanical obstructions and functional bowel disorders can have the same radiographic findings.1,32 Despite the frequent use of CT as the initial radiologic investigation performed in the emergency room for patients with acute abdominal pain, a cost-effective approach using plain film radiographs coupled with careful clinical examination has been advocated.35 This approach, however, relies on direct communication between the radiologist and clinician to interpret the radiographic findings in the context of the patient’s clinical presentation to make the appropriate management decisions. Plain film radiography at times does not corroborate the clinical suspicion of SBO and, conversely, abnormalities on plain films may not have associated clinical findings. In these instances, abdominal CT with intravenous contrast is the imaging study of choice.36 – 43 Consistency in the terms used by radiologists to describe intestinal gas patterns when reading plain abdominal films has been emphasized.44 The normal small-bowel gas pattern is defined as the absence of small-bowel gas or the presence of small amounts of gas within up to 4 nondistended (diameter, ⬍2.5 cm) loops of small bowel. These radiographs have a normal distribution of gas and stool within a nondistended colon. The abnormal but nonspecific gas pattern has at least one loop of small bowel that is borderline or mildly distended small bowel (diameter, 2.5–3.0 cm) with 3 or more air-fluid levels on upright or lateral decubitus films. The colonic gas pattern and fecal distribution are normal or display a similar degree of borderline distention. This pattern is a reflection of mild small-bowel stasis, a condition not specific for obstruction and often seen in several disease states including partial obstruction, reactive or reflex ileus, and medication-induced hypoperistalsis. The probable SBO plain film pattern has an abnormal gas distribution consisting of multiple gas- or fluid-filled loops of dilated small bowel with a relatively small or moderate amount of colonic gas. This pattern, although sensitive, is not specific, and can be seen in several acute intra-abdominal inflammatory conditions such as appendicitis, diverticulitis, or mesenteric ischemia. An unequivocal SBO pattern has dilated gas- or fluid-filled loops of small bowel in the setting of a gasless or dry nondistended colon; a combination of findings that is considered pathognomonic.1,3,33,35 If high-grade or complete SBO is suspected on abdominal radiography, immediate surgical evaluation is essential because the need for urgent surgery makes further diagnostic imaging unnecessary. When plain film radiography shows both colonic and smallbowel gaseous distention, adynamic ileus may be difficult to distinguish from an evolving high-grade SBO or colonic obstruction. The approach to additional imaging in this setting is modified by several factors. First, if an nonobstructive colonic ileus or distal colonic obstruction with an incompetent ileocecal valve is suspected, CT scanning is both fast and well tolerated, making it the preferred imaging method. Barium enema also is accurate if CT is not available. Second, for patients within the immediate postoperative period, CT with intravenous contrast is the imaging method of choice because of its ability to show mural and extra-intestinal abnormalities, particularly if neutral oral contrast (water or dilute barium) is used. Third, in patients without a prior history of abdominal surgery with an unequivocal SBO pattern but no indication for urgent surgery, further investigation is required before exploratory surgery, particularly if a laparoscopic approach is contemplated.

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Abdominal Ultrasound Sonography is used in many countries as the primary imaging test in the investigation of patients with presumed SBO and recent articles have confirmed the value of this technique.45–52 In the United States, however, abdominal sonography is used uncommonly for the evaluation of SBO because our preference is for CT imaging owing to less interobserver variability and its common availability. Expert sonologists are not readily available in many practices for emergency cases and patients with SBO have increased gas that, similar to obesity, limits its accuracy. The features of SBO on sonography are identified readily by expert sonologists.51 Meticulous sonographic examination through the flanks to circumvent bowel gas by an expert sonologist can detect the presence, level, and cause of the SBO.52 In cases of proximal SBO with normal plain films, sonography can determine accurately the level and cause of the obstruction. Peristaltic movement of the bowel is observed readily by ultrasound and differentiating mechanical obstruction from paralytic ileus can be performed relatively easily.45 The level of SBO can be determined by sonographic assessment of the valvulae conniventes of the dilated bowel at the transition point between dilated and collapsed segments. Adhesions or an internal hernia may be suggested as the cause of obstruction when no apparent alternative etiology such as a mass or inflammation can be seen. The presence of free fluid between dilated small-bowel loops on sonography suggests worsening mechanical obstruction and the need for surgery rather than medical treatment.52 Similar to other imaging tests that do not assess the distensibility and fixation of the bowel segments, sonography may not be sensitive enough to diagnose lower grades of SBO.3 Triage of patients to undergo further imaging, either emergent CT or elective CT enteroclysis, can be aided by the findings on abdominal sonography.52

Barium and Water-Soluble Contrast Oral Small-Bowel Series Radiography using water-soluble contrast agents has been used to determine surgical versus nonsurgical management in patients with suspected SBO.53–55 The widespread use of abdominal CT has supplanted this practice in the United States.3 Despite the purported therapeutic benefits of using water-soluble contrast agents in patients with postoperative SBO, recent evidence has disputed these claims.56 –59 Disadvantages of using barium or water-soluble contrast agents include the inability of patients to ingest large amounts of unpalatable contrast, difficulty in assessing distensibility and fixation of the small bowel, flocculation and dilution of barium leading to incomplete bowel opacification or poor mucosal detail, and the logistics of repeated patient transport to the radiology suite to follow the course of contrast in a patient with obstruction.

Enteroclysis Examinations: Infusion-Distended Small-Bowel Examination The intubation infusion methods of small-bowel examination (enteroclysis) overcome the limitations of orally ingested contrast techniques by challenging the distensibility of the bowel wall and exaggerating the effects of mild or subclinical mechanical obstruction.60 In addition, polyps, masses, or inflammatory changes are diagnosed more reliably using this method. Intubation of the small bowel bypasses the stomach and allows direct delivery of positive enteral (barium) contrast

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directly into the jejunum. Advantages include the following: active contrast infusion toward the site of obstruction overcomes diminished bowel peristalsis, luminal distention facilitates detection of fixed and nondistensible bowel segments, and the ability to detect multiple levels of obstruction. The diagnosis of SBO can be excluded safely by enteroclysis when unimpeded flow of contrast is observed through normal-caliber small-bowel loops from the duodenum to the right colon.1 The diagnosis of mechanical obstruction is made by showing a transition zone or gradient, defined as a change in the caliber of the small-bowel lumen from distended bowel proximal to the site of obstruction to collapsed bowel distal to the site of obstruction.3,32,61 Intubation infusion methods objectively gauge the severity of intestinal obstruction, an important advantage over other imaging modalities.6,32 When used appropriately, this technique has 100% sensitivity, 88% specificity, and 86% accuracy in determining the cause of the obstruction.1,3,60 In low-grade partial SBO, there is no delay in the arrival of contrast to the point of obstruction. Sufficient flow of contrast is seen during fluoroscopy such that the fold patterns of the postobstructive loops are seen. High-grade partial SBO occurs when proximal retained fluid dilutes the infused water-soluble contrast, resulting in inadequate contrast density above the site of obstruction. In this situation, only small amounts of contrast material pass through the obstruction into the collapsed distal loops. Complete obstruction is defined arbitrarily when no infused contrast material passes beyond the point of obstruction as shown on delayed radiographs obtained 3 to 24 hours after the start of the examination.1,3 Disadvantages to infusion enteroclysis are the need for radiologist involvement, which often is impractical in a busy outpatient clinic or emergency room coupled with a lack of expertise on the part of many radiologists in performing these studies. In addition, particularly when using barium, it will preclude the performance of emergent CT should it be needed because of artifacts. Although nasointestinal intubation can be an unpleasant procedure, the newer tubes used for enteroclysis and long-tube decompression are better tolerated than conventional nasogastric tubes.24,46 The use of conscious sedation has made enteroclysis and its newer modifications a well-tolerated procedure.61,62 In our practice, conscious sedation is administered by a trained nurse. We commonly use 2 to 7 mg of midazolam (Versed; Abraxis, Schamburg, IL) and 50 to 150 ␮g of fentanyl citrate (Hospira, Lake Forste, IL) titrated to individual patient needs.

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intestinal ischemia is reportedly low (44%), its high sensitivity (90%) and negative predictive values (89%) are helpful in decision making concerning surgical versus nonsurgical management. Most cases of strangulation occur in the setting of intussusception, volvulus, or internal hernia where the arterial supply or venous drainage of a segment of small bowel is impaired. Simple mechanical obstruction as a result of adhesions rarely results in infarction unless the intraluminal pressure exceeds the venous hydrostatic pressure, resulting in bowel wall ischemia. Axial CT signs of closed-loop obstruction have been described in detail.66 Multiplanar reconstructions facilitate the identification of closed loop and the associated swirling mesenteric vessels. Coronal mesenteric vascular mapping can identify closed-loop obstructions with vascular compromise before they progress to strangulation.41– 44 Faster multichannel CT imaging with multiplanar reformatting can identify arterial and venous bowel-wall enhancement, which facilitates the identification of ischemia (Figure 1). A sign we have found to be highly specific for bowel ischemia is decreased enhancement of a focal segment of the bowel wall in the arterial phase and increased enhancement in the venous phase.61,67 Early signs of strangulation described during the era of monoslice CT such as bowelwall thickening, mesenteric vessel blurring, or the halo sign from submucosal edema are nonspecific. CT is particularly helpful in patients with a history of abdominal malignancy, inflammatory bowel disease, or a palpable abdominal mass because it can distinguish SBO from ileus, inflammatory conditions such as appendicitis, and other causes of small-bowel dilatation such as the blind pouch syndrome.7,67 The demonstration of feces-appearing material in dilated small bowel (small-bowel feces sign) aids in the determination of the transition point. This sign reflects inspissated debris in dilated hypoperistaltic bowel. This, however, is not specific for mechanical obstruction because this can be seen in adult patients with

Multichannel Computerized Tomography, Computerized Tomography Enterography, and Computerized Tomography Enteroclysis Initial studies using conventional CT in SBO reported a sensitivity of 96%, a specificity of 96%, and an accuracy of 95%.63,64 Most of these studies were performed in patients with high-grade SBO. A critical analysis of this reliability in a population of patients with both high- and low-grade obstructions found less favorable results.65 In this mixed population of patients, the overall sensitivity was 63%, the specificity was 78%, and the accuracy was 66%. In addition to identifying the severity and location of SBO, CT is useful in identifying closed-loop obstructions and intestinal strangulation.36 – 45 Recognition of these 2 entities is of great concern for surgeons, particularly when considering a trial of nonsurgical management.22,23,28 Although the specificity of contrast-enhanced monoslice CT for

Figure 1. CT diagnosis of strangulation. Transverse CT image of a middle-aged patient who presented with severe acute abdominal pain shows a long distended small-bowel loop containing small-bowel folds (arrow) diagnostic of intussusception (bowel within bowel). Ascitic fluid with increased attenuation (arrowheads) compared with water is seen, which is suggestive of hemorrhage. There are patchy segments of decreased mucosal enhancement (curved arrow) indicating diminished perfusion. The diagnosis of a strangulated small-bowel intussusception without a lead point was confirmed at surgery.

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Table. Comparison of Orally Ingested Contrast Small-Bowel Examination (Abdominal CT) Versus Fluoroscopically Guided Enteral Infusion–Distended (Enteroclysis) Examination in the Diagnosis of Obstruction From Active Inflammation and Stenosis in Small-Bowel Crohn’s Disease CTa

Ulcerations Mural edema Obstruction Stenosis Sinus tract Fistula Abscess Extraintestinal disease Total number of sites

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radiographic examination that adds diagnostic confidence particularly in lower grades of obstruction in which a gradient or transition point may not be seen with a single multislice CT acquisition. This concentration allows the radiologist to adjust infusion flow speed, which is of significance in patients who may vomit during the procedure. It also can be performed using

Enteroclysisb

n

%

n

%

7 15 6 4 2 3 10 3ⴱ 47

19 41ⴱ 16 11 5 8 27ⴱ 0

29 17 17 14 10 9 7 0 54

78 ¢ 46 46 ¢ 38 ¢ 27 ¢ 24 ¢ 19 0

The superiority of infusion-distended small-bowel examination (enteroclysis) over the orally ingested small-bowel examination (abdominal CT) in the diagnosis of stenosis and other features of Crohn’s disease is apparent except for the demonstration of abscess and extraintestinal disease. The combination of the advantages of infusion distention and the multiplanar capability of multidetector CT technology is the underlying principle for the advantages of CT enteroclysis. b ⫹ a ⫽ added value of CT enteroclysis. Modified from Maglinte et al.85

cystic fibrosis or other conditions associated with small-bowel stasis resulting in inspissation of particulate debris. Controversy exists as to which oral contrast is practical to use when abdominal CT is performed for acute abdominal pain.3 In the era of monoslice CT, positive (water soluble) oral contrast was used for all examinations. Large volumes (2.5 L) of water or dilute barium (Volumen; EZEM Inc, Lake Success, NY) as neutral contrast agents are gaining popularity with the use of multichannel CT technology (CT enterography). Neutral oral contrast improves the visualization of the bowel wall, allowing accurate determinations of bowel-wall thickness, assessment of mucosal hyperemia, and clearer multiplanar reformatted images for mesenteric vessel assessment when intravenous contrast is used. With intravenous contrast, admixture defects that result in pseudo-masses or pseudo-fold thickening of the small bowel are minimized. However, with the large volume of oral contrast needed for small-bowel filling it is not well tolerated by patients with obstructive symptoms who already are nauseated. This makes CT enteroclysis the optimum method of examination in patients with obstructive symptoms. CT enteroclysis is a hybrid technique that combines the advantages of barium enteroclysis and conventional CT and overcomes individual deficiencies of each technique (Table). It has come of age with the use of multislice CT.60 It can be performed with positive enteral (water-soluble) contrast in patients with a history of vomiting in which fluoroscopy is used to control infusion (Figure 2). The use of positive enteral contrast is ideal for showing adhesions and fistula, and is ideal for use in patients with high-grade obstruction as well as those with contraindications to intravenous contrast. In our practice we use 11% diatrizoate meglumine (Reno-Dip; Bracco Diagnostics, Princeton, NJ). This concentration allows a diagnostic fluoroscopic (real time) and

Figure 2. Abdominal/CT versus CT enteroclysis in the diagnosis of low-grade SBO. (A) Abdominal CT transverse image shows no evidence of distended small bowel and was interpreted as showing no evidence of mechanical SBO. This was the third unremarkable abdominal CT in 15 months in this 38-year-old man presenting with recurrent abdominal pain after remote appendicectomy. A small-bowel followthrough also was reported as normal. (B) Transverse image of CT enteroclysis with positive enteral contrast, performed 3 days after the last abdominal CT, shows distended proximal bowel loop with abrupt tapering of caliber (arrowhead) adjacent to the anterior parietal peritoneum. The distal small bowel contains enteral contrast but is nondistended (arrow), indicating a significant gradient (ie, obstruction). Lowgrade obstruction secondary to anterior enteroparietal peritoneal and enteroenteric (interloop) adhesions were diagnosed and proved on subsequent laparoscopic adhesiolysis. Enteral infusion challenges the distensibility of the bowel wall and exaggerates prestenotic dilation of partial obstruction and facilitates demonstration of small intraluminal or mural masses. Its negative predictive value is high compared with other imaging methods, which is important in an organ with a low incidence of disease whose symptoms are nonspecific.

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neutral enteral contrast (water or dilute barium) infused at a high rate blindly combined with intravenous contrast, which allows characterization of bowel-wall abnormalities (Figure 3). The use of large volumes of enteral contrast (4 L) allows simultaneous evaluation of the small bowel and colon which is of value in patients with inflammatory bowel disease. Long nasointestinal tubes can both decompress the small bowel and also be used for enteroclysis, an advantage over simple nasogastric tubes.61,68 In challenging patients with obstructive symptoms, CT enteroclysis often can help establish the diagnosis by providing distention of small-bowel loops.61,69 In this technique, water-soluble contrast is infused through an enteroclysis catheter into the duodenum or proximal small bowel followed by CT image acquisition during continued enteral contrast infusion. Important functional information on the bowel is obtained during fluoroscopy when an appropriate concentration (10%–15%) of water-soluble contrast agent is used. Postprocessing of fluoroscopically obtained images adds confidence to the CT diagnostic findings. Multiplanar reformats are obtained routinely to precisely define the site and cause of obstruction. Our experience has suggested that the precise localization of sites of adhesive obstruction and the decompression of distended small bowel by long-tube suction facilitates a laparoscopic approach to surgical management.61,68 Infusion with luminal distention overcomes the unreliability of abdominal CT in diagnosing lower grades of SBO. Initial reports placed the reliability of CT enteroclysis as equivalent to that of barium enteroclysis (sensitivity, 88%; specificity, 82%) in patients with suspected low-grade partial SBO.61 Other reports have shown greater sensitivity and specificity (89% and 100%, respectively) with CT enteroclysis than when using CT alone (50% and 94%, respectively) in patients with suspected partial SBO. These differences are magnified in patients with a history of abdominal malignancy70 (Figure 4). The presence of peritoneal nodules, asymmetric thickening, enhancement of the transition site, and mesenteric whirling all are useful features in distinguishing benign from malignant etiologies of SBO.

Figure 3. Differentiation of active inflammatory stenotic and fibrostenotic phenotypes of Crohn’s disease with partial mechanical SBO using CT enteroclysis with neutral enteral with intravenous contrast. (A) Coronal reformation of neutral enteral contrast CT enteroclysis in a 59-yearold woman shows mucosal hyperenhancement (black arrow), thickened wall (asterisk), luminal narrowing of terminal ileum, and prominent vasa recta (black arrowhead), the comb sign. Proximal small bowel was distended (white arrow). Findings indicate active inflammatory subtype of Crohn’s disease with secondary obstruction. Note mild, irregular bile duct dilation (white arrowheads) indicating associated sclerosing cholangitis. The patient was treated with immunomodulatory therapy. (B) A 49-year-old woman with Crohn’s disease presented with chronic abdominal pain and vomiting. Coronal reformation of CT enteroclysis showed stricture of terminal ileum (arrowhead). The thickened small bowel wall did not significantly enhance and there was no engorgement of the vasa recta. Mild mucosal hyperenhancement may be seen in patients with fibrostenosis. Colonoscopic dilation was performed. Biopsy specimens did not show active inflammation. The use of intravenous contrast allows accurate characterization of Crohn’s disease phenotypes and adjacent soft-tissue abnormalities. Neutral enteral contrast, however, cannot be used in patients with a history of vomiting because infusion cannot be monitored by fluoroscopy compared with positive enteral contrast because it is not visible fluoroscopically and is infused blindly at a high rate to distend bowel.

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Magnetic Resonance Imaging and Enteroclysis Magnetic resonance (MR) imaging to date has played a limited role in the clinical evaluation of mechanical SBO. With increasingly fast sequences (many now completed within a single breath-hold), it now is possible to image the entire abdomen and pelvis within 10 minutes.67 A recent study based on a small number of patients reported that half-Fourier acquisition imaging in 3 planes is superior to helical CT in diagnosing SBO.71 This report, however, did not mention the severity of obstruction that was being investigated. MR enteroclysis has great potential because of its direct multiplanar imaging capabilities, lack of ionizing radiation, and the functional information and soft-tissue contrast that it can provide.72 Compared with CT enteroclysis, MR enteroclysis provides distinct advantages of direct imaging in the coronal plane and real-time acquisition of functional information. In addition, the accuracy of MR imaging technique does not rely as heavily on the experience of the fluoroscopist as do barium enteroclysis techniques. Further research and experience may help clarify whether MR imaging and enteroclysis will become integral parts in the investigation of SBO or will be used solely as a problem-solving examination.72

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enteroclysis) is a 14F, 155-cm long, triple-lumen disposable catheter made of radiopaque polyvinyl chloride and has been adapted for use with wall mechanical suction devices currently used in hospitals. The smaller tube and the ease with which it can be advanced under fluoroscopy into the small bowel have made it a practical alternative to the currently used nasointestinal tubes for decompression and subsequent diagnostic studies. The important addition of sump ports to the Maglinte multipurpose long tube prevents intestinal debris and collapsed bowel from occluding the decompression side ports and thereby permits effective decompression compared with other long tubes (Figure 5). Complications reported with other nasointestinal tubes,80 such as perforation, have not been reported

Nasogastric and Nasointestinal (Long-Lube) Decompression in the Nonsurgical Management of Small-Bowel Obstruction The use of nasogastric versus nasoenteric (long) tubes for the decompression of distended small bowel have been debated.22,23,28 The gastrointestinal tract normally secretes 8.5 L of fluid daily, most of which is reabsorbed in the small intestine.73 In cases of SBO, there is an impaired ability of the small intestine to reabsorb secreted fluid above an obstruction, which, over time, results in a net flux of fluid out of the bowel wall and into the lumen.74,75 The functional and physiologic derangements of intestinal obstruction are borne predominantly by the bowel immediately proximal to the point of occlusion. As progressive distention occurs, these segments become at risk for the development of ischemia, gangrene, and perforation. With an intact pylorus, nasogastric tubes cannot decompress the small bowel until the pressure of backed-up intestinal fluid and gas is strong enough to overcome the strength of the pyloric sphincter. The results of several studies have shown that the efficacy of decompression is inversely proportional to the distance between the decompressing tube tip and the site of the blockage. Consequently, advancement of the decompression tube beyond the pylorus into the small bowel significantly improves decompressive efficacy over standard nasogastric tube positioning.75 This explains why nasointestinal rather than nasogastric intubation is considered the optimal method for decompressing the distended small bowel. An added advantage to using a long tube is that as soon as the tube passes the pylorus and begins to decompress the small bowel, the colicky pain of obstruction largely is relieved.76,77 Because the Salem sump nasogastric tube (Sherwood Medical, St. Louis, MO), currently the most commonly used nasogastric decompression tube, is too short to be advanced into the small bowel, a multipurpose catheter (diagnostic and therapeutic) was introduced (Maglinte Decompression Enteroclysis Catheter, Cook, Inc, Bloomington, IN) in 1992.78 This long intestinal tube (nasogastric, nasoenteric decompression, and

Figure 4. Imaging of SBO from partially obstructing masses. An 80year-old man with a history of colon cancer presented with nausea and vomiting. (A) Conventional CT with oral and intravenous contrast shows distended small-bowel loops (arrow), but no bowel mass or etiology of dilatation seen. (B) Positive enteral contrast CT enteroclysis performed 2 weeks later shows multiple masses in small-bowel wall (arrowheads) consistent with serosal metastases with secondary multiple points of obstruction (arrow). Metastases were proven to be the cause of bowel obstruction at surgery.

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in the literature with this tube since its introduction. The availability of this long tube has provided our surgeons with the option to use long-tube decompression of the distended small bowel before surgery and to perform immediate CT enteroclysis to answer management-relevant questions.61,68 Controversy remains regarding the use of nasogastric tubes versus long nasointestinal tubes in the nonsurgical management of SBO. In one series, 81% of patients treated with nasogastric tubes and only 40% of patients treated with long nasointestinal tubes successfully were managed nonsurgically.28 Careful scrutiny of this data reveals that patients in this nonrandomized series treated with long tubes had more severe obstruction, making nonsurgical management less successful. Furthermore, the need for surgery in patients managed nonsurgically appears to be unrelated to whether or not a long tube is positioned beyond the pylorus.22,28 Accurate comparisons between short versus long tubes for decompression in SBO have been hampered by the difficulty in advancing the long tubes into the small bowel.78 It is because of this difficulty that many surgeons have accepted nasogastric intubation as equivalent to long tube intubation without the benefit of good level I data.79 In institutions where accurate placement of a long intestinal tube is possible, their use continues to be advocated.61,68 A recent study using a long tube has shown an overall cost effectiveness when compared with the use of nasogastric tubes, but a controlled randomized clinical trial is needed to properly compare the efficacy of these 2 methods of intestinal decompression.81

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and strangulating obstruction. When the CT examination is not diagnostic and clinical questions remain unanswered, elective CT or MR enteroclysis can be performed. CT enteroclysis is the best initial imaging technique in patients with a history of laparotomy or complaints of mild intermittent abdominal pain who have few physical findings and normal or abnormal but nonspecific findings on abdominal plain film (Figure 1). The possibility of low-grade mechanical SBO should be considered in any case of undiagnosed acute abdominal pain.1 The long intestinal tube can be used for both diagnosis and decompression.79 Low-grade intermittent obstructions and intraluminal tumors can be detected and better evaluated using this technique. MR enteroclysis provides additional information, particularly in patients with inflammatory bowel disease or in pregnant patients with suspected SBO. Patients with high-grade

Comments In patients with SBO when nonsurgical management is considered, emergent CT is helpful to exclude strangulated obstruction. Surgical patients presenting early after surgery with abdominal distention and no signs of bowel compromise (tachycardia, leukocytosis, localized tenderness, or fever) can be safely treated nonsurgically. CT is recommended only if the clinical findings or small-bowel distention on abdominal plain films does not improve or if signs of intra-abdominal abscess or bowel compromise develop. CT enteroclysis with positive enteral contrast is an excellent problem-solving tool and is easier to perform than barium enteroclysis in the postoperative patient or in a patient who is critically ill.61 If the abdominal plain film shows colonic distention in addition to small-bowel dilatation, abdominal CT or a contrast enema is needed to exclude colonic obstruction. In this setting, CT is preferred in elderly or infirm patients, patients with a clinical suspicion of abscess or diverticulitis, patients with a history of previously resected colon cancer, or in patients with poor anal sphincter tone.82 When CT is not available, a contrast enema is reliable in excluding colonic obstruction. In countries where sonographic expertise is available, sonography may follow plain film examination to evaluate patients for SBO. Discordance between the clinical presentation and plain film or sonographic findings often requires additional radiologic imaging. In patients with acute abdominal symptoms and an abnormal but nonspecific bowel gas pattern on plain films, CT using water or dilute barium as an oral contrast agent with intravenous contrast enhancement is recommended. CT not only is reliable in showing many of the acute abdominal conditions that can mimic SBO, but also has a high sensitivity for high-grade or complete obstruction and can reveal closed-loop

Figure 5. Maglinte long tube. (A) Image of Maglinte long tube with stiffening wire that helps maneuver the tube into the proximal jejunum. The adapter (in the middle of the circle formed by the guidewire) facilitates its connection to wall suction. The straight and curved ends of the guidewire allow directional change during fluoroscopic advancement. (B) Line diagram of the tube. B ⫽ balloon port, D ⫽ drainage port, S ⫽ sump port (helps prevent the tube from becoming obstructed by debris). The inset figures show the cross-sectional appearances of the tube at positions a and b. The tube is 13.5F and is better tolerated than the smallest nasogastric tube.

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SBO are investigated using conventional abdominal CT with intravenous contrast in the emergency setting. The all too frequent and erroneous application of conventional imaging methods with low sensitivity is frequent in clinical practice because of the mistaken notion of cost containment. In patients with SBO, this often delays diagnosis, results in inappropriate treatment, a prolonged hospital stay, and frequently can lead to an overall increase in cost. The continued use of conventional methods of investigation with poor specificity without exploring newer methods of investigation will result in ineffective patient care.3 Active collaboration among gastroenterologists, surgeons, emergency physicians, and radiologists is necessary to optimize the diagnostic evaluation and management of SBO, which remains a common and potentially dangerous clinical problem.83

Conclusions The dictum, “never let the sun rise or set on smallbowel obstruction,” once was popular among general surgeons because of the feared complication of strangulation and the difficulty associated with its preoperative recognition.84 Recent refinements primarily in CT technology, enteroclysis, and the introduction of the multifunctional long tubes have changed the approach to the evaluation and management of patients with suspected SBO in our practice.61 The modern era of radiologic management of SBO in the United States relies on understanding the limitations of plain film radiography, judicious selection of multidetector CT, CT enteroclysis, and nasogastric/ nasoenteric decompression. These techniques have replaced the traditional radiologic approach that depended on plain films and oral barium or water-soluble contrast examination and conventional monoslice CT technology. Barium enteroclysis has been advocated over the past 2 decades as the definitive imaging for suspected SBO. With multidetector CT technology and multifunctional long tubes, CT enteroclysis, which combines the advantages of barium enteroclysis and abdominal CT into one technique, now is easier to perform and provides useful and relevant information to assist in the management of patients with SBO.1 References 1. Shrake PD, Rex DK, Lappas JC, et al. Radiographic evaluation of suspected small bowel obstruction. Am J Gastroenterol 1991; 86:175–178. 2. Suh RS, Maglinte DDT, Lavonas EEJ, et al. Emergency abdominal radiography: discrepancies of preliminary and final interpretation and management relevance. Emerg Radiol 1995;2:1– 4. 3. Maglinte DDT, Balthazar EJ, Kelvin FM, et al. The role of radiology in the diagnosis of small bowel obstruction. AJR Am J Roentgenol 1997;168:1171–1180. 4. Markogiannakis H, Messaris E, Dardamanis D, et al. Acute mechanical obstruction: clinical presentation, etiology, management and outcome. World J Gastroenterol 2007;13:432– 437. 5. Pickleman J, Lee RM. The management of patients with suspected early postoperative small bowel obstruction. Ann Surg 1989;210:216 –219. 6. Herlinger H, Maglinte DDT. Small bowel obstruction. In: Herlinger H, Maglinte DOT, eds. Clinical radiology of the small Intestine. Philadelphia: WB Saunders, 1989:479 –507. 7. Megibow AJ. Bowel obstruction: evaluation with CT. Radiol Clin North Am 1994;32:861– 870. 8. Mucha P Jr. Small intestinal obstruction. Surg Clin North Am 1987;67:597– 620.

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Address requests for reprints to: Dean D. T. Maglinte, MD, Department of Radiology, Indiana University Hospital, 550 North University Boulevard, UH 0276, Indianapolis, Indiana 46202-5253. e-mail: [email protected]; fax: (317) 274-1848. D.D.T.M. and K.S. are consultants for COOK Inc., Bloomington, IN, and EZEM Inc., Lake Success, NY.