Current concepts in imaging of small bowel obstruction

Transcription

1 Radiol Clin N Am 41 (2003) Current concepts in imaging of small bowel obstruction Dean D.T. Maglinte, MD a, *, Darel E. Heitkamp, MD a, Thomas J. Howard, MD, FACS b, Frederick M. Kelvin, MD c, John C. Lappas, MD a a Department of Radiology, Indiana University Medical Center, 550 North University Boulevard, UH0279, Indianapolis, IN , USA b Division of General Surgery, Indiana University School of Medicine, 545 Barnhill Drive, EH523, Indianapolis, IN 46202, USA c Department of Radiology, Methodist Hospital of Indiana, 1701 North Senate Boulevard, Indianapolis, IN 46202, USA Despite recent advances in abdominal imaging, intestinal obstruction remains a difficult disease entity to diagnose accurately and treat [1 3]. Small bowel obstruction (SBO) is a common clinical condition, often presenting with signs and symptoms similar to those seen in other acute abdominal disorders. Once intestinal obstruction is suspected based on the patient s clinical history and physical examination, diagnostic imaging is charged with the task of verifying the presence of obstruction and providing cogent information on the site, severity, and probable cause of the obstruction. By providing this broad range of anatomic information, imaging impacts directly on patient management, specifically addressing the crucial question of whether a trial of nonoperative therapy should be instituted rather than resorting to immediate surgery because of the possibility of strangulation [4,5]. Because of its ability to provide important anatomic and functional information, radiology has become a vital tool in the clinical decision making of patients with known or suspected SBO. This article examines current concepts in the imaging of SBO. Clinical considerations Small bowel obstruction is responsible for 12% to 16% of admissions to the surgical service in patients * Corresponding author. address: dmaglint@iupui.edu (D.T.T. Maglinte). with acute abdominal conditions [6]. Establishing the diagnosis in a timely manner is best accomplished by relying on the classic investigational triad of a well-taken history, a careful physical examination, and appropriate ancillary testing. The diagnosis of mechanical SBO is straightforward when the classic findings of crampy abdominal pain, distention, nausea, and vomiting are present along with abdominal radiographic (plain film) findings of small bowel distention, multiple air-fluid levels, and decreased colonic gas and stool [4]. In many cases, the diagnosis is much more subtle because most patients fail to present with a classic history and often have vague abdominal findings on physical examination. Plain abdominal radiographs have been found not to support the clinical diagnosis of obstruction in nearly one third of surgically proved cases. Based on these observations, after a complete history and physical and abdominal plain films, if the clinical suspicion for intestinal obstruction remains high, further abdominal imaging is often indicated [4,7]. The three most common causes of SBO in the western world are (1) adhesions, (2) Crohn s disease, and (3) neoplasia [8]. In the past, hernias represented a major cause of SBO in the United States, but improvements in health care availability and the increase in elective hernia repair has led to a substantial decline in the incidence of SBO related to abdominal wall hernias. Hernias, however, still represent the predominant cause of SBO in many developing countries. Crohn s disease has only recently been acknowledged in the surgical literature as a /03/$ see front matter D 2003, Elsevier Science (USA). All rights reserved. doi: /s (02)

2 264 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) leading cause of SBO, a fact that has long been suspected in many clinical radiology departments [8]. Controversy still exists surrounding the management of patients with adhesive SBO. If the obstruction is partial or early in the postoperative period ( < 6 weeks from operation), many surgeons recommend a trial of conservative treatment with intestinal decompression in the belief that, with close patient monitoring, surgery frequently can be avoided altogether [1,9 12]. Other surgeons advocate early surgical management for all patients, particularly those with complete intestinal obstruction, based on the high complication rate associated with delayed operative intervention in this group of patients [13 16]. Clinical experience has shown that simple mechanical obstruction cannot be reliably differentiated from strangulated obstruction on the basis of clinical, laboratory, or abdominal plain film findings [9,15,17 21]. Historical data in patients with surgically proved strangulation show that the preoperative diagnosis is unreliable in 50% to 85% of cases [2,9,16,22 24]. The current mortality rate of patients with adhesive intestinal obstruction is in the 1% to 2% range [25,26], suggesting that the risks associated with conservative management may be acceptable as long as emergent surgery is performed at the first sign of patient deterioration or evidence of incarceration or strangulation is found. Recent clinical series have shown that even patients with high-grade mechanical SBO can have a substantial rate of resolution with conservative nasointestinal decompression, further supporting an evenhanded approach to patients with SBO [11,15,27,28]. Abdominal radiography Despite its limitations, abdominal radiography remains the initial imaging study in patients with abdominal symptoms, particularly in those with possible intestinal obstruction. Its diagnostic value tends to be highest in patients with signs or symptoms of biliary or urinary system calculi, intestinal obstruction, perforation, or ischemia. Plain films are least helpful in patients with vague abdominal pain and nonspecific physical findings. Its role in the evaluation of calculi, perforation, or ischemia has been replaced by CT. In the setting of SBO, abdominal radiographs are diagnostic in 50% to 60% of cases [17 20,29]. In an analysis of plain film findings reported by experienced gastrointestinal radiologists, a sensitivity of only 66% was found in proved cases of SBO [7]. Twenty-one percent of patients reported as normal were in fact obstructed. Of patients whose films were interpreted as abnormal but nonspecific, 13% had low-grade and 9% had high-grade obstruction. Additionally, abdominal radiography has shown a low specificity for SBO, because mechanical and functional large bowel obstructions can mimic the radiographic findings observed in SBO [30]. Despite these acknowledged limitations of this examination, plain film radiography remains an important study in patients with suspected SBO because of its widespread availability and low cost. Although in many cases the abdominal radiographs are nondiagnostic, their findings can be valuable in guiding subsequent imaging, or following disease progression. A degree of confusion still exists among radiologists and clinicians over the meanings of common descriptors used to identify various intestinal gas patterns on abdominal radiographs [31,32]. Many physicians frequently use the term nonspecific bowel gas pattern to actually mean normal bowel gas pattern [3]. One survey showed that 70% of radiologists used the term nonspecific in their interpretations, with 65% trying to convey a normal or probably normal bowel gas pattern, 22% meaning to say that they cannot tell if it is normal or abnormal, and 13% interpreting this to mean abnormal but cannot tell if it represents mechanical obstruction or adynamic ileus. Clearly, the term nonspecific is imprecise and its use ultimately can lead to serious errors in patient management. If used at all, it should be qualified as abnormal, but nonspecific, satisfying a group of plain film findings that fits neither the normal nor definitely abnormal categories. This qualification adds its own set of clinical implications [33]. The use of ambiguous terms, such as nonobstructive gas pattern, which does not indicate whether the gas distribution is normal or abnormal, should be abandoned. The use of well-defined terms for describing bowel gas patterns is essential for generating understandable reports for clinicians and other radiologists. (1) The normal small bowel gas pattern refers to either absence of small bowel gas or small amounts of gas within up to four variably shaped nondistended (less than 2.5 cm in diameter) loops of small bowel. A normal distribution of gas and stool within a nondistended colon should also be recognized. (2) Abnormal but nonspecific gas describes a pattern of at least one loop of borderline or mildly distended small bowel (2.5 to 3 cm in diameter) with three or more air-fluid levels on upright or lateral decubitus radiographs. The colonic gas and feces distribution is either normal or displays a similar degree of borderline distention. This pattern can also be correctly labeled mild small bowel stasis, because many conditions

3 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) can produce it, including low-grade obstruction, reactive ileus, and medication-induced hypoperistalsis. (3) The probable SBO pattern consists of multiple gas- or fluid-filled loops of dilated small bowel with a moderate amount of colonic gas. The presence of colonic gas indicates early complete mechanical SBO, an incomplete SBO, or nonobstructive ileus. This pattern can be seen in several acute intra-abdominal inflammatory conditions that involve the small bowel (diverticulitis, appendicitis, or mesenteric ischemia). This diagnosis should trigger further investigation with a prompt CT enteroclysis in a patient with no objective clinical findings. (4) The definite SBO pattern shows dilated gas or fluid-filled loops of small bowel in the setting of a gasless colon. This constellation of findings is pathognomonic for SBO [4]. Various small bowel gas patterns are shown in Fig. 1A N. These patterns should be distinguished from the distended small bowel occurring secondary to left-sided colonic obstruction. In this pattern, in addition to the distended small bowel, a fluid-filled right colon and fluid and gas distended transverse colon can also be recognized (Fig. 2). The small bowel distention seen in this setting is secondary to decompression of the colonic distention through the ileocecal valve. Two findings on the upright abdominal radiograph can help differentiate high-grade obstruction from lower-grade obstruction: the presence of differential air-fluid levels in the same bowel loop, and a mean airfluid level width of at least 25 mm (see Fig. 1M). The combined presence or absence of these two radiographic findings has a strong positive (86%) and negative (83%) predictive value of the degree of patency of the small bowel lumen [34]. Although upright radiographs alone are not particularly sensitive for SBO, they may be of value in distinguishing patients with high-grade or complete obstruction from those with low-grade or partial obstruction. Because of its widespread availability, relative low cost, and high sensitivity in revealing high-grade SBO, the use of abdominal plain radiographs remains a prominent imaging tool in the evaluation of suspected SBO [29]. Barium radiography Because barium does not typically inspissate within the adynamic gut, it can be used safely to evaluate SBO [35,36]. Ingested orally, iodinated water-soluble contrast agents result in poor mucosal detail on radiography and are quite hypertonic. Although radiography using water-soluble agents was once used by some institutions to triage patients into surgical versus nonsurgical management, the widespread use of abdominal CT has largely supplanted this practice [37 39]. Despite the strong opinion of a few advocates, the use of water-soluble contrast has been shown to have no therapeutic effect in patients with postoperative SBO [40]. Barium evaluation of the small intestine can be performed by either nonintubation or intubation-infusion techniques [4]. The nonintubation methods include the retrograde small bowel enema; the per enterosotomy (colostomy, ileostomy) small bowel enema; and the small bowel follow-through. Although the small bowel follow-through is a useful technique when performed with meticulous fluoroscopy, it has known limitations in the setting of SBO [41 43]. In cases of high-grade obstruction, dilution of barium by fluid in the dilated proximal bowel typically results in incomplete small bowel opacification and poor mucosal detail. The duration of the small bowel follow-through examination is directly related to small bowel transit time, both of which are often markedly prolonged in cases of high-grade obstruction. Moreover, nonintubation barium techniques are inherently limited in their ability to assess intestinal distensibility and fixation of small bowel loops [42]. As a result, they may not detect partially obstructing lesions that produce only fleeting or inconspicuous prestenotic dilatations when viewed under fluoroscopy. Despite these limitations, intermittent fluoroscopic monitoring can often yield important information making the technique a viable alternative for radiology departments lacking sufficient expertise in performing enteroclysis [42,43]. Enteroclysis overcomes the limitations of the nonintubation techniques by challenging the distensibility of the bowel wall and exaggerating the effects of mild or subclinical mechanical obstruction (see Fig. 1B, C). Intubating the small bowel bypasses the pylorus, enabling delivery of a nondiluted barium or iodinated contrast bolus directly into the jejunum. Sequential infusion of barium and methylcellulose or iodinated contrast during CT enteroclysis promotes antegrade flow of contrast toward the site of obstruction despite the presence of diminished bowel peristalsis. The resultant luminal distention facilitates detection of both fixed and nondistensible bowel segments. Clinical studies have shown that the intubation infusion method of small bowel examination can correctly predict the presence of obstruction in 100%, the absence of obstruction in 88%, the level of obstruction in 89%, and the cause of obstruction 86% of patients [7]. SBO is excluded by enteroclysis or CT enteroclysis when unimpeded flow of contrast material is observed within normal-caliber small bowel loops

4 266 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 1. Small bowel gas patterns. (A) Normal bowel gas distribution. There is a small amount of gas in the duodenal bulb (arrow) and distal ileum (curved arrow); otherwise there should be no gas in the small bowel. There is no evidence of colonic or gastric distention. Colonic folds are apparent in intraperitoneal segments of the colon. (B) Abnormal but nonspecific gas pattern. Mildly dilated loops of small bowel are noted in the right hemiabdomen (arrows). There is no colonic distention. Gas is present in the duodenal bulb (near clips) and in distal ileum (curved arrow). (C) Enteroclysis done following (B) shows a moderately tight adhesive band obstruction (open arrow) involving a pelvic loop of ileum. Note retained fluid in dilated prestenotic (or sentinel) loop.

5 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 1. (D) Abnormal but nonspecific gas pattern. Small amounts of gas (arrows) are noted in nondistended small bowel loops in left hemiabdomen and pelvis in addition to usual gas in distal ileum in chronic renal patient presenting with abdominal pain, nausea, and vomiting who also had recent ventral herniorrhaphy and subsequent wound infection. Note semisolid fecal debris in right colon. This distribution is also known as small bowel stasis pattern. (E) Enteroclysis radiography shows no significant distention proximal to intraluminal filling defects (curved arrow) in ileum. (F) Further infusion of methylcellulose shows distal movement of intraluminal filling defects towards cecum (curved arrow).

6 268 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 1. (G) Infusion radiograph shows that the filling defects in distal ileum have been flushed into the right colon and the distal and terminal ileum (arrow) are normal confirming that the abnormal but nonspecific small bowel gas pattern is secondary to medication-related hypoperistalsis. The small bowel stasis pattern is not uncommon in hospitalized patients on analgesics or sedatives. C, cecum. (H) Probable small bowel obstruction (SBO) pattern. Upright abdominal radiograph shows air-fluid levels in multiple moderately distended loops of small bowel. Gas and fluid are present in transverse colon (arrow in a haustrum) and sigmoid. The pattern is suggestive of mechanical SBO but can be seen in sigmoid diverticulitis or appendicitis. (I) CT obtained following (H) shows a lower abdominal anterior parietal peritoneal fixation of decreased-caliber small bowel loops (arrow) secondary to dense adhesions. Note dilated small bowel proximal to adhesions. (From Maglinte DDT, Reyes BL, Harmon BH, et al. Reliability and the role of plain film radiography and CT in the diagnosis of small-bowel obstruction. AJR Am J Roentgenol 1996;167:1451 5; with permission.)

7 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 1. (J) Probable SBO pattern. Multiple dilated loops of small bowel are noted. Gas is still present in normal-caliber colon making diagnosis of mechanical SBO not unequivocal but highly suggestive. (K) CT obtained following (J) shows acute perforative appendicitis with abscess formation. Appendicolith is present, which is not seen on the plain film. Small bowel loops are dilated proximal to inflammatory changes. (L) Definite SBO pattern. Supine abdominal radiograph of a female patient with abdominal distention, nausea, and vomiting shows markedly dilated gas-filled loops of small bowel with a normal-caliber colon making diagnosis of mechanical SBO unequivocal.

8 270 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 1. (M) Upright abdominal radiograph of same patient (L) shows air-fluid levels and multiple fluid-filled loops of distal small bowel (arrow in one). The rectosigmoid appears dry. (N) Enteroclysis done following overnight long tube decompression shows fixation and decreased caliber of small bowel loops, multiple kinks, and strictures (arrow in one) from chronic radiation enteropathy in a patient with a history of carcinoma of the cervix. C, cecum. (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; with permission.) from the duodenojejunal junction to the right colon. The diagnosis of mechanical SBO is confirmed by the demonstration of a transition zone, defined as a change in the caliber of the intestinal lumen from a distended segment proximal to the site of obstruction to a segment that is either collapsed or decreased in caliber distal to the site of obstruction [4,5,7,44,45]. By enteroclysis criteria, 3 cm is the upper limit of normal for the caliber of the jejunal lumen, and 2.5 cm is the upper limit for the ileal lumen [42]. The level of obstruction is identified during the single-contrast phase of the examination, whereas the cause of the obstruction is best evaluated during the double-contrast phase of the study when observation of mucosal detail is optimal. In partial SBO, enteroclysis has been shown to be approximately 85% accurate in distinguishing adhesions from metastases, tumor recurrence, and radiation damage [46]. Obstructions can occur synchronously at multiple levels, such that if dilated fluid- or gas-filled small bowel loops are encountered distal to a transition zone, additional downstream obstructions need to be assessed. Enteroclysis is particularly helpful in patients about whom the diagnosis of low-grade SBO is clinically uncertain [47]. Its ability to distinguish low-grade obstruction from a normal examination makes it an important tool in this difficult clinical problem [47 51]. Enteroclysis can also objectively gauge the severity of intestinal obstruction, an important advantage over other imaging modalities [7,45]. In low-grade partial SBO there is no delay in the arrival of contrast to the point of obstruction, and there is sufficient flow of contrast through the point of obstruction such that fold patterns in the postobstructive loops are readily

9 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 2. Left-sided colonic obstruction plain film pattern. (A) Diffuse small bowel distention is seen. In addition, fluid is seen in the right colon (arrow) and gas- and fluid-filled transverse colon (open arrow). The rectosigmoid region is empty. (B) CT done following (A) shows obstruction of the proximal descending colon from carcinoma (arrow). The retained fluid and gas in the colon correspond to the gas and fluid distribution in (A). (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; with permission.) defined. High-grade partial SBO is diagnosed when the presence of retained fluid dilutes the barium and results in inadequate contrast density above the site of obstruction, allowing only small amounts of contrast material to pass through the obstruction into the collapsed distal loops. Complete obstruction is diagnosed when there is no passage of contrast material beyond the point of obstruction as shown on delayed radiographs obtained up to 24 hours after the start of the examination [7]. The authors have applied this severity scoring to CT enteroclysis [45]. The term closed-loop obstruction implies acute obstruction with a tendency to progress toward infarction and the need for urgent surgery. If a patient displays clinical signs of a bowel compromise including localized tenderness, fever, tachycardia, or leukocytosis, immediate resuscitation and urgent laparotomy should be done. If further anatomic information is required in a challenging patient (ie, multiple prior surgeries, dense adhesions, or morbid obesity) CT should be the initial choice of imaging. Further investigation using barium enteroclysis or positive contrast CT enteroclysis can provide complementary information about the obstruction, such as how much contrast material is moving through the transition zone [29,52]. Partial closed-loop obstruction has been demonstrated by enteroclysis in the subacute setting [53]. Because enteroclysis requires conscious sedation, nasointestinal intubation, and near-constant radiologist involvement, it can be impractical to perform adequately in the outpatient clinic setting. Many institutions also lack individuals with the proper expertise to perform the study. If expertise is lacking, a dedicated small bowel follow-through with close fluoroscopic monitoring is an acceptable substitute, provided that high-grade obstruction is not present [41,42]. In patients with complete or high-grade obstruction, dilution of barium occurs proximal to the site of obstruction and makes diagnostic evaluation suboptimal. Moreover, barium retained in the small bowel can degrade the diagnostic quality of subsequent CT examinations. In the acute setting where time is of the essence, CT should be the initial method of examination. In the authors department, CT enteroclysis with positive enteral contrast is now performed more frequently than barium enteroclysis for further assessment of clinically stable patients with SBO. CT CT has become important in the preoperative evaluation of patients with suspected intestinal

10 272 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) obstruction. Although some studies report a low overall sensitivity (63%) of CT for all grades of SBO, sensitivity improves to 81% when high-grade obstruction alone is considered. Conversely, sensitivity worsens to 48% in the detection of low-grade obstruction [45]. Although CT is accurate for highgrade SBO, it is not as sensitive for the lower grades of obstruction that present at the subacute level or in the outpatient setting [29]. The speed of multidetector row helical CT and its ability to reveal the cause of obstruction make it particularly valuable in the acute setting. CT is able to show the cause of obstruction in 93% to 95% of cases [29,45,54,55], at the same time revealing the more serious conditions of closed-loop obstruction and strangulation [27,56 64]. The exclusion of these two complications is of great concern to many surgeons, particularly those who believe a trial of conservative nonoperative management is warranted in simple mechanical SBO. Although the specificity of contrast-enhanced CT for intestinal ischemia is reported to be as low as 44%, its high sensitivity (90%) and negative predictive value (89%) [61] are quite helpful in making decisions concerning continued nonoperative management versus surgery [11]. Most cases of strangulation occur as complications of intussusception, volvulus, torsion, or other types of closed-loop obstruction. Interruption of the blood supply typically occurs either from twisting of the bowel on its mesentery or from pressure generated by markedly distended small bowel loops. Attention to the course of vascular arcades in the bowel on CT with the use of coronal mesenteric vascular mapping may help identify cases of closed-loop obstruction before they progress to strangulation. In a recent report [65] the whirl sign [60], described as the convergence of mesenteric vessels toward a twisted site [63], and the reversal of the normal relationship between the mesenteric artery and vein [66] were identified as the two most important vascular indicators of closedloop obstruction caused by midgut volvulus as seen on CT. The ability of CT reliably to show signs of closed-loop obstruction, ischemia, and infarction likely represents the most important imaging contribution to the management of acute SBO. If CT is used appropriately, its higher initial cost may result in overall cost savings within an episode of care by either expediting surgery or avoiding surgery in appropriate patients, reducing comorbidities and hospital length of stay. CT is also useful in differentiating SBO from ileus or other causes of small bowel dilatation [67,68]. In cases of high-grade obstruction, CT has a reported sensitivity of 100% for distinguishing obstruction from other causes of small bowel dilatation, as compared with 46% for that of plain radiographs [67]. The literature shows that by differentiating paralytic ileus from obstruction, CT findings modified management in 21% of patients either by changing conservative management to a surgical one (18%) or vice versa. CT can expedite the need for surgery and also avoid unnecessary laparotomy, important goals in the man- Fig. 3. Decompression-enteroclysis catheter. (A) The catheter is introduced transnasally similar to the conventional nasogastric tube. The black marker (arrow) in the proximal third of the tube when seen at the level of the external nares indicates the tube tip position in the body of the stomach and allows the tube to be positioned at bedside in the emergency department or hospital ward without fluoroscopic guidance similar to the positioning of conventional nasogastric tubes. A rubber adapter (1) allows connection of the decompression lumen (D) (also infusion lumen) to existing suction devices. A small plastic cap (2) prevents fluid from leaking out of the sump port (S) when suction is disconnected. The balloon (B) is used only during contrast material infusion and is inflated by first pressing in the balloon inflation one-way valve attachment (curved arrow). (B) A Teflon-coated stainless-steel braided torque guidewire with interchangeable ends is provided. The straight tip of the guidewire is introduced to the level of the nasal marker (arrow in A) of the suction-infusion lumen before intubation. The 45% angle proximal to the opposite tip of the 195-cm long guidewire allows the operator to change the direction of the tube tip when necessary. The angled tip is used only in occasional situations of difficult directional control and to allow atraumatic nasopharyngeal tube passage in patients with acute nasopharyngeal posterior wall angulation. The straight tip is all that is necessary to provide torque in most transgastric intubations.

11 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) agement of adhesive SBO. CT is particularly helpful and should be used as the primary imaging technique for patients in whom the obstructive symptoms are associated with specific medical conditions, such as a history of a previous malignant abdominal tumor, known inflammatory bowel disease, palpable abdominal mass, or sepsis [56]. Several caveats need to be considered in the application of CT to SBO. If the plain radiograph shows probable or definite SBO, oral contrast should not be used for the CT, because it often does not reach the site of obstruction by the time of examination. If it does, the moderately increased intraluminal attenuation created when bowel fluid dilutes the oral contrast bolus can nearly match the attenuation of a contrast-enhanced bowel wall, making it difficult to assess the bowel wall for thickening. Administration of oral contrast in the emergent setting also has the potential to cause delays in performing the CT examination. The use of water as an intraluminal contrast agent is preferred in this setting and in patients with suspected mesenteric ischemia. Positive oral contrast in this situation often interferes with vascular reconstruction algorithms. In the emergent setting, sick patients are able to tolerate water better than watersoluble contrast. With multidetector row CT, many small bowel diseases including inflammatory conditions, obstruction, or masses can be diagnosed with water as enteral contrast in conjunction with intravenous enhancement. In addition, with the use of water, further diagnostic investigations are not interfered with because of residual contrast in the bowel. Compared with barium enteroclysis or CT enteroclysis, abdominal CT is faster, more readily available, noninvasive, less contingent on technical expertise, and able to provide a more global evaluation of the abdomen and alimentary tract. This last advantage is of considerable importance, particularly in the acute setting when intestinal obstruction represents only one of many possible etiologies in patients presenting with acute abdominal conditions. The CT examination should be monitored closely and additional sections should be obtained through the transition zone if the cause of obstruction is unclear on the initial axial sections. Although identification of the transition zone is usually not difficult in higher grades of obstruction, the less distended loops found with low-grade obstruction can be quite confusing to follow on axial CT images [69]. When CT results are equivocal in the search for a transition zone, and closed-loop obstruction has been ruled out, CT enteroclysis or barium enteroclysis can often help establish the diagnosis by providing volume-challenge distention of the proximal loops. Box 1. Suggested instructions for suction with the decompression-enteroclysis catheter The following instructions are provided as a guide. This peelable suction order instruction, which is attached to the catheter box cover, can be removed and attached to the physician s orders sheet on the patient s chart. 1. Connect decompression (suction) port identified by rubber adapter) to low ( ) intermittent ( ) continuous suction. Modify as needed. 2. Remove cap from sumping port and inject 2 cc of air into the channel as soon as suction is started. Do not recap the air channel while suction is being applied. During section this port will allow air to enter and bubble back up the suction channel almost continuously. If bubbling is not observed, proceed to Step 3. Check that all connections are tight. 3. Irrigate decompression port every 4 hours with 20 cc of saline and p.r.n. to prevent clogging of the suction port. 4. Inject the sumping port with 2 cc of air every 4 hours and p.r.n. Do not aspirate this port. Steps 3 and 4 can be done at the same time. 5. Any time the decompression port is disconnected, reapply caps to both the decompression and the sump ports to prevent fluid leak. Repeat Step 2 each time the decompression tube is reconnected for suction. 6. If tube is to be anchored for more than 2 days, apply Bacitracin or Neosporin ointment to nasal cavity once daily. 7. Do not use balloon channel. This port is used only during enteroclysis. 8. Remove long tube at the discretion of the attending physician. Notify radiology if there is difficulty in removing tube. Signed:

12 274 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) CT enteroclysis has emerged as a promising new method of investigating the small bowel. In this technique, water-soluble contrast is infused through an enteroclysis catheter into the proximal small bowel, followed immediately by CT cross-sectional imaging of the distended small bowel loops. Multiplanar reconstructions of the CT data can be obtained either routinely or on an as-needed basis for problem solving in difficult cases. Theoretically, the volume challenge provided by the intubationinfusion technique of enteroclysis overcomes the unreliability of CT for diagnosing low-grade obstruction, whereas the cross-sectional imaging provided by CT complements the recognized limitations of conventional enteroclysis in assessing the gut wall and providing information on extraintestinal causes of obstruction. In addition to precise three-dimensional localization of small bowel pathology, CT enteroclysis allows objective determination of the severity of SBO as has been previously defined using standard enteroclysis criteria [70]. Initial reports indicate that the reliability of CT enteroclysis is equivalent to that of conventional enteroclysis (sensitivity 88% and specificity 82%) in patients suspected of having a low-grade partial SBO [71,72]. Other reports show that it has greater sensitivity and specificity (89% and 100%, respectively) than CT alone (50% and 94%, respectively) in patients suspected of having a partial SBO, a difference that was even greater when a history of abdominal malignancy was known or suspected [72]. CT enteroclysis is emerging as a promising tool in the further work-up of SBO. This topic is reviewed in detail elsewhere in this issue. Fig. 4. Therapeutic and diagnostic use of multipurpose long tube. (A) Axial CT of patient who had a history of prior colon resection for carcinoma who presented with abdominal pain and distention. The dirty feces sign (arrow) suggests chronic obstruction of small bowel with fluid and debris accumulating proximal to the point of obstruction. (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; with permission.) (B) Following nasogastric suction, no clinical improvement was noted. Enteroclysis and long tube decompression were requested. The nasogastric tube was replaced with the multipurpose tube and was advanced under fluoroscopic guidance to the proximal jejunum. The proximal small bowel was atonic and fluid filled. Long tube suction was done. (C) Following overnight decompression, an abdominal radiograph done before enteroclysis shows satisfactory decompression of the distended small bowel. (D) Radiograph obtained during barium enteroclysis shows the cobra head appearance (arrow) suggestive of dense adhesive band obstruction, which was confirmed at surgery. (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; and Maglinte DDT, Reyes BL, Harmon BH, et al. Reliability and the role of plain film radiography and CT in the diagnosis of small-bowel obstruction. AJR Am J Roentgenol 1996;167:1451 5; with permission.)

13 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) MR enteroclysis MR imaging has played only a limited role in the clinical evaluation of SBO. The emerging technique of MR enteroclysis, however, has the potential to change the assessment of the small bowel through its direct multiplanar imaging capabilities, its lack of ionizing radiation, and the functional information and soft tissue contrast that it can provide [73]. Compared with CT enteroclysis, MR enteroclysis provides the distinct advantages of direct imaging in the coronal plane and real-time acquisition of functional information. Additionally, the accuracy of the MR imaging technique does not rely as heavily on fluoroscopist experience as do conventional enteroclysis techniques [73]. To be the primary method of investigation for small bowel disease, MR enteroclysis has to provide reliable evidence of normalcy, allow diagnosis of early or subtle structural abnormalities, influence treatment decisions in patient management, and be cost effective [41]. Further research and experience will help clarify whether it will become a primary method for investigating the small bowel or be used solely as a problem-solving examination. This topic is discussed elsewhere in this issue. The role of radiology in the conservative management of SBO The gastrointestinal tract normally secretes up to 8.5 L of fluid daily, most of which is reabsorbed in the small intestine [74]. In cases of SBO, kinking and gas-trapping within distended loops of bowel above an obstruction impairs the ability of the small intestine to reabsorb secreted fluid and over time results in a net flux of fluid out of the bowel wall into the lumen [75,76]. The physiologic derangements of an intestinal obstruction are borne predominantly by the bowel immediately proximal to the point of occlusion [74]. As this part of the gut becomes distended, its Fig. 4 (continued).

14 276 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) increased intraluminal pressure slows capillary blood flow leading to mesenteric venous congestion putting this segment of bowel at risk for 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 tube tip and the site of the blockage, such that advancement of the tube beyond the pylorus into the small bowel significantly improves decompression efficacy over the standard gastric positioning [77]. These pathophysiologic principles explain why nasointestinal rather than nasogastric intubation is considered the optimal method of 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 is largely relieved. Because nasogastric tube decompression is limited to the stomach, a patient s abdominal pain persists until either the obstruction is relieved or effective decompression is achieved, either spontaneously or surgically [78]. Fig. 5. Radiographic demonstration of partial or incomplete closed-loop obstruction. (A) Abdominal radiograph of a 72-year-old woman who presented with abdominal pain, distention, and vomiting and a history of prior appendectomy and lysis of adhesions. Multiple distended loops of small bowel with little gas in colon are suggestive of small bowel obstruction. Clips are seen in right lower abdomen from her prior surgery. A nasogastric tube is in the stomach. (B) Intravenous contrast-enhanced axial CT image at level of lower abdomen shows mild dilatation of small bowel loops and possible edema of an ileal segment (arrow). (C) Axial CT image at level of upper pelvis shows clips (curved arrow) from prior surgery and normal-caliber loops (arrow) and some dilated loops. (D) Preliminary abdominal radiograph obtained after 12 hours of long tube decompression shows partial decompression of distended small bowel and more gas in colon. (E) Early enteroclysis radiograph shows focal narrowing with proximal dilatation of the small bowel at the level of the clips (curved arrow). The poststenotic loop containing a small amount of contrast and gas, however, is also dilated (arrow). (F) Further contrast infusion shows the dilated poststenotic segment coursing back toward the region of the clips (arrow). (G) Delayed radiograph shows two loops of small bowel obstructed at the same level (curved arrow) consistent with a partial volvulus secondary to dense adhesive band. Collapsed loops are seen distal to obstruction. This was confirmed at surgery. This is an illustration of how to diagnose multiple points of obstruction by enteroclysis. (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; with permission.)

15 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) The Salem pump nasogastric tube (Sherwood Medical, St. Louis, MO) cannot be used for enteroclysis and is too short to be advanced into the small bowel for nasoenteric decompression. Patients who initially undergo nasogastric tube placement but later require enteroclysis or CT enteroclysis for the evaluation of SBO experience the trauma of multiple intubations. The multipurpose intestinal tube (MDEC-1400, Cook, Bloomington, IN) was developed in 1992 to be used for both diagnostic and therapeutic purposes to eliminate the need for multiple intubations [79]. This multipurpose tube, a modification of the standard balloon enteroclysis catheter [80], is a 14F catheter, 155-cm long, triplelumen disposable catheter made of radiopaque polyvinyl chloride that is adapted for use with hospital Fig. 5 (continued).

16 278 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) wall suction devices. The important addition of a sumping mechanism prevents occlusion of the tube s side ports from collapse of the bowel wall against the tube during suctioning, thereby allowing effective small bowel decompression (Fig. 3) [81]. The multipurpose tube kit also includes a preprinted adhesivebacked order sheet that can be affixed to the orders page on a patient s chart (Box 1). This sheet provides unambiguous instructions to the patient s caregivers. In the nonemergent setting, if CT does not answer all questions relevant to a particular patient s management and nasogastric intubation is desired clinically, the multipurpose tube can be positioned in the stomach for initial gastric decompression. CT enteroclysis or barium enteroclysis can then be performed after advancing the tube, under fluoroscopic guidance, into the jejunum. The long tube can then be anchored in the proximal jejunum after the study for further decompression (Fig. 4) [5]. Closed-loop obstruction Prompt preoperative recognition of closed-loop obstruction is crucial, because strangulation represents a dangerous complication that carries a much higher risk of mortality than simple mechanical SBO. Accurate and early detection of strangulation can expedite surgery and significantly improve overall patient prognosis [82,83]. Most closed-loop obstructions result from entrapment of the small bowel either within an internal or external hernia. Unless the classic pseudotumor or coffee bean signs are present, plain film radiography often yields nonspecific and unreliable results [84]. CT is the imaging modality of choice for evaluating closed-loop obstruction in the acute setting, whereas CT or barium enteroclysis serve more complementary roles by establishing the presence of an incomplete closed-loop obstruction or by helping to clarify the cause of obstruction (Fig. 5) [37]. The enteroclysis findings of closed-loop obstruction are similar to those seen in single-band adhesive obstruction, except that the crossing defect traverses two adjacent segments of a single loop of bowel [85]. Volvulus is diagnosed if the afferent and efferent limbs seem to cross or intertwine with twisting of the folds at the point of obstruction. A separation between the two obstructed limbs excludes the presence of volvulus. In patients with moderate to highgrade obstruction, it may be difficult to exclude volvulus if the involved limbs appear closely approximated, tightly compressed, and angulated at the point of obstruction [85]. It is often impossible to differentiate closed-loop obstructions caused by herniation through mesenteric defects from those caused by prolapse of bowel under adhesive bands [40]. If the constriction is tight, there is usually delayed filling and delayed emptying of the contrast from the incarcerated loop [85]. CT is a particularly valuable diagnostic tool because it can establish the diagnosis of both closed-loop obstruction and strangulation [27,48, 51 53,55,56,58,60]. The appearance of closed-loop obstruction on CT depends on the length of the closed loop, the degree of bowel distention, and the three-dimensional orientation of the closed loop with respect to the axial imaging plane [52,53]. If the incarcerated loop is oriented horizontally, it appears U- or C-shaped in the axial plane. If an elongated segment of bowel is involved, sequential axial images demonstrate a characteristic radial distribution of dilated bowel loops having stretched and thickened mesenteric vessels converging to the point of obstruction (Fig. 6). The incarcerated segment of bowel appears almost entirely fluid-filled, whereas loops Fig. 6. CT of closed-loop obstruction. Axial CT of an elderly man obtained to investigate abdominal pain and distention. Intravenous contrast-enhanced CT at the level of the lower abdomen shows congested mesentery with distended small bowel loops converging toward a central point (o) adjacent to a pointed segment of bowel suggesting closed-loop obstruction. At surgery volvulus was confirmed; the mesentery was congested but there was no evidence of strangulation. Note normal enhancement of mucosa without evidence of bowel wall thickening. (From Maglinte D, Herlinger H. Plain film radiography of the small bowel. In: Herlinger H, Maglinte D, Birnbaum B, editors. Clinical imaging of the small intestine. 2nd edition. New York: Springer; p ; with permission.)

17 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) of bowel proximal to the site of obstruction contain greater amounts of air. Images obtained near the site of torsion demonstrate progressive, fusiform tapering of the afferent and efferent limbs manifested as the beak sign when imaged in longitudinal section. If a volvulus is present, the whirl sign of a tightly twisted mesentery may be seen [56]. CT signs of strangulation are related to the appearance of the incarcerated bowel wall and its mesentery [52,53]. Ischemia is suggested by the presence of circumferential wall thickening, increased mural attenuation, and the target or double halo sign seen on the intravenous contrast-enhanced examination. In the setting of examinations without intravenous contrast, increased bowel wall attenuation is suggestive of ischemia. Pneumatosis intestinalis may be seen with advanced ischemia and infarction. Mesenteric congestion and hemorrhage are important findings whose presence increases the specificity of the CT diagnosis of strangulation. Optimizing the imaging investigation of SBO Open communication among radiologists, primary care physicians, and surgeons is essential in the workup and management of SBO [86]. The selection of imaging is based on knowledge of the patient s history, physical examination, laboratory results, and abdominal plain film findings. The dilemma that radiologists face is not the use of one technique over the other, but the decision of which examination to use first in the context of the clinical presentation and abdominal plain film findings [56,85]. Fig. 7. Problem solving with CT enteroclysis. (A) Supine abdominal radiograph of a 26-year-old woman who presented with abdominal distention and vomiting following colectomy and ileoanal pouch construction. Multiple distended loops of small bowel are noted initially interpreted as consistent with mechanical small bowel obstruction. Conventional abdominal CT with intravenous contrast (not shown) was unable to differentiate between ileus and mechanical obstruction. Oral contrast given was vomited and patient refused nasogastric intubation. (B) CT enteroclysis with multipurpose long tube introduced following conscious sedation was requested. Overnight long tube decompression was performed before infusion of water-soluble contrast. The patient had a relief of the abdominal distention. Coronal CT image obtained 3 hours after initial infusion of contrast because of slow flow shows the tip of the multipurpose long tube in proximal jejunum. There is moderate distention of remaining small bowel with continuity of distention to the ileoanal pouch ( p). (C) Axial image at the level of the upper abdomen shows dilated loops with retained fluid. (D) Axial image at level of pouch ( p) shows an intact pouch without evidence of peripouch complications. Additional coronal and axial images did not show a transition point confirming a diagnosis of severe postoperative ileus. The patient responded to long tube small bowel decompression with return of small bowel peristalsis and passage of gas and contrast after the examination.

18 280 D.T.T. Maglinte et al / Radiol Clin N Am 41 (2003) Fig. 7 (continued). Definitive SBO on plain film radiography confirms the clinical diagnosis and opens the door for a decision on whether to perform surgery or use a trial of conservative nonoperative management. Factors that favor early surgical exploration include no prior history of abdominal surgery; clinical signs of bowel compromise; incarcerated hernia; or the presence of a complete SBO (obstipation). Factors that favor initial conservative management include the presence of a partial SBO; history of resected abdominal tumor; prior radiation therapy; history of inflammatory bowel disease; and early ( < 6 weeks) postoperative obstruction (see Fig. 1M, N). When initial conservative management is entertained, CT examination is helpful in evaluating the presence and extent of neoplastic or inflammatory disease and in excluding a strangulated obstruction. Postsurgical patients presenting early after operation with abdominal distention and no signs of bowel compromise (tachycardia, leukocytosis, localized tenderness, or fever) are treated conservatively for several days, with CT advised only if the clinical findings and abdominal plain films do not improve, or if signs of sepsis or bowel compromise develop. CT enteroclysis with positive enteral contrast is a good problem-solving tool and is easier to perform than barium enteroclysis particularly in the postoperative patient or those who are clinically ill (Fig. 7). CT enteroclysis should be used after the conventional CT study only if additional management questions are left unanswered [29,53]. In departments where CT enteroclysis is not practical, barium enteroclysis is the preferred investigation [5]. If the abdominal plain film shows colonic distention in addition to small bowel dilatation, a CT or contrast enema should be performed. In this clinical setting, CT is preferred in elderly or infirm patients, patients with a clinical suspicion of abscess or diverticulitis, and in patients with a history of previously resected colon carcinoma. CT is also necessary in the acute setting in patients with poor anal sphincter tone (see Fig. 2) [82]. Where CT is not readily available, the contrast enema is the method of choice. Discordance between the clinical presentation and plain film findings often requires additional radiologic imaging. In patients with acute abdominal symptoms who have normal or abnormal but nonspecific bowel gas pattern on abdominal plain films, CT is recommended (see Fig. 1J, K). CT is not only reliable in showing many of the acute abdominal conditions that