(Radiographics. 2001;21:613-624.)
© RSNA, 2001
CT Evaluation of Small Bowel Obstruction1
Mourad Boudiaf, MD,
Philippe Soyer, MD, PhD,
Carine Terem, MD,
Jean Pierre Pelage, MD,
Emmanuelle Maissiat, MD and
Roland Rymer, MD
1 From the Department of Body and Vascular Imaging, Hôpital Lariboisière-AP-HP, 2 rue Ambroise Paré, 75475 Paris Cedex 10, France. Presented as a scientific exhibit at the 1999 RSNA scientific assembly. Received May 11, 2000; revision requested June 12 and received August 17; accepted August 18. Address correspondence to M.B. (e-mail: mourad.boudiaf@lrb.ap-hop-paris.fr).
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Abstract
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Although small bowel obstruction is a common occurrence, it is essential that this clinical condition be treated properly, that the site, level, and cause of obstruction be determined accurately, and that a tentative prognosis be formulated prior to surgery. The diagnosis of small bowel obstruction is based on a comprehensive approach that includes clinical background, patient history, and results of physical examination and laboratory tests. A variety of radiologic procedures are available to aid in the diagnosis of small bowel obstruction. Recent studies have demonstrated the superiority of CT in revealing the site, level, and cause of obstruction and in demonstrating threatening signs of bowel inviability. CT has proved useful in characterizing small bowel obstruction from extrinsic causes (adhesions, closed loop, strangulation, hernia, extrinsic masses), intrinsic causes (adenocarcinoma, Crohn disease, tuberculosis, radiation enteropathy, intramural hemorrhage, intussusception), intraluminal causes (eg, bezoars), or intestinal malrotation. Conventional radiography was the modality of choice for many years and should remain the initial imaging method in patients with suspected small bowel obstruction. However, the unique capabilities of CT in this setting make this modality an important additional diagnostic tool when specific disease management issues must be addressed.
Index Terms: Intestines, CT, 74.1211 • Intestines, hernia, 74.15 • Intestines, stenosis or obstruction, 74.723, 74.724 • Intussusception, 74.732
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Introduction
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Intestinal obstruction is a common clinical abnormality that is usually suspected on the basis of clinical signs and patient history. However, it is essential to manage treatment properly, to determine the site, level, and cause of obstruction, and even to try to establish a prognosis prior to surgery. A variety of diagnostic radiologic procedures are available, ranging from conventional radiography through barium studies to computed tomography (CT). For many decades, evaluation was based on findings at conventional radiography, with a sensitivity of 69% and a specificity of 57% (1). Several studies have demonstrated the value of CT in confirming the diagnosis (site and level) and revealing the cause of small bowel obstruction, with a sensitivity of 94%–100% and an accuracy of 90%–95% (1,2). In this article, we discuss and illustrate different mechanisms of small bowel obstruction as well as various causes of obstruction. These include extrinsic causes (adhesions, closed loop, strangulation, hernia, extrinsic masses [carcinoid tumors, lymphoma, peritoneal carcinomatosis, appendicitis, diverticulitis]), intrinsic causes (adenocarcinoma, Crohn disease, tuberculosis, radiation enteropathy, intramural hemorrhage, intussusception), intraluminal causes (eg, bezoars), and intestinal malrotation.
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Extrinsic Causes
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Adhesions
Subsequent surgery in patients who have undergone laparotomy at least once reveals adhesions in over 90% of cases (3). These adhesions occupy either the area of surgical intervention or the undersurface of the abdomen. However, adhesions produce symptoms in only a minority of cases. Adhesions are responsible for at least 60% of cases of small bowel obstruction; more than 80% of these lesions occur after surgery, 15% are due to inflammation, and the remaining few are due to congenital or unexplained causes (4).
The diagnosis of small bowel obstruction due to adhesions is made when all other causes of obstruction have been ruled out at CT. Bowel obstruction is considered to be present at CT when distended bowel loops are seen proximal to collapsed loops. When a point of transition from dilated small bowel to normal-caliber bowel without apparent cause is identified, adhesions are the presumed cause (Figs 1, 2).
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Figure 1. Simple complete small bowel obstruction caused by adhesions. CT scan shows the small bowel (I) with marked distention and filled with fluid and air. Totally collapsed bowel loops are seen at the transition zone (C).
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Figure 2. Small bowel obstruction caused by adhesions. CT scan shows dilated, fluid-filled loops (S) at the transition zone in proximity to a collapsed loop of distal ileum (arrows). No mass is seen. The adhesions were lysed surgically.
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Closed Loop
A closed (incarcerated) small bowel loop is a form of mechanical obstruction in which a segment of bowel is occluded at two points along its course by a single constrictive lesion occluding both the small bowel and mesentery. The closed loop is able to rotate along its axis, thereby producing a small bowel volvulus (5).
CT findings in closed-loop obstruction depend on the length, degree of distention, and orientation of the closed loop in the abdomen. When a closed small bowel loop is horizontally oriented, it has a U- or C-shaped configuration at cross-sectional imaging (Fig 3). A radial configuration with stretched mesenteric vessels converging toward the site of torsion may be detected depending on the orientation of different small bowel loops within the incarcerated bowel segment (Fig 4). At the site of obstruction, the collapsed loops are round, oval, or triangular. The "beak sign" seen at the site of torsion appears as a fusiform tapering at longitudinal bowel imaging (Fig 5). A tightly twisted mesentery is occasionally seen in patients with volvulus and has been described as the "whirl sign" (Fig 6) (6).
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Figure 3. Closed-loop small bowel obstruction. CT scan reveals a classic U-shaped loop (U) with fluid in the mesentery (F). Note the mural thickening at the transition zone (arrow).
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Figure 4. Closed-loop small bowel obstruction. Abdominal CT scan obtained after intravenous administration of iodinated contrast material shows marked dilatation of small bowel loops (S). Two adjacent collapsed loops at the transition zone demonstrate thickening and increased attenuation of the bowel wall (arrow). Vascular engorgement is seen in the regional mesentery (arrowhead).
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Figure 5. Closed-loop small bowel obstruction. Contrast material-enhanced abdominal CT scan shows dilatation of a small bowel loop (S). The beak sign is seen at the obstructed site (arrow).
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Figure 6. Closed-loop small bowel obstruction associated with intestinal volvulus. CT scan shows marked distention of the small bowel (S) in association with the whirl sign (arrow), which corresponds to twisted mesentery.
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Strangulation
The reported prevalence of strangulating small bowel obstruction ranges from 5% to 42% (7). and its mortality rate ranges from 20% to 37% (8). Strangulation is associated with adhesions and internal or external hernias.
CT findings include evidence of small bowel obstruction, a circumferentially thickened loop with high attenuation within the wall, the "target sign," and congestion or hemorrhage in the mesentery attached to the closed loop (Figs 7, 8) (6,9,10). In advanced cases, pneumatosis intestinalis may develop. Although ascites may be present in patients with closed-loop obstruction without ischemia or with simple bowel obstruction (9,10), it is more commonly seen in patients with strangulation and should be considered a suspicious finding.
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Figure 7. Strangulating small bowel obstruction. CT scan shows distended small bowel loops (S). The small bowel wall is thickened and demonstrates the target sign (arrows), findings that are consistent with submucosal edema. Hemorrhagic fluid within the mesentery (H) and peritoneal cavity fluid (F) are also seen. Hemorrhagic infarct was confirmed surgically.
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Figure 8. Strangulating small bowel obstruction caused by adhesion. CT scan shows several dilated small bowel loops with predominantly fluid contents (S). Two collapsed adjacent bowel loops featuring the target sign (arrows) are present in association with fluid in the adjacent mesentery (arrowhead).
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Hernia
Indirect inguinal hernias are the most common abdominal wall hernias. In cases of strangulated hernia, compromise of the blood supply is present, resulting in thickening of bowel loops. Adjacent inflammatory changes can be seen at CT in association with small bowel obstruction (Fig 9).
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Figure 9. Strangulated inguinal hernia. CT scan shows bowel wall thickening with increased attenuation (arrow). These findings are suggestive of strangulation, which was confirmed surgically.
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During the embryonic period, the abdominal contents herniate through the umbilicus and normally move back into the abdominal cavity by the 10th week of fetal life. In adults, umbilical hernias occur long after the closure of the umbilical ring and are considered acquired. Moreover, they do not spontaneously obliterate and have a high prevalence of incarceration (Fig 10).
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Figure 10a. Incarcerated umbilical hernia. (a) CT scan shows evidence of small bowel obstruction. (b) CT scan obtained at a lower level demonstrates an incarcerated small bowel segment in an umbilical hernia (arrow). This finding was confirmed surgically.
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Figure 10b. Incarcerated umbilical hernia. (a) CT scan shows evidence of small bowel obstruction. (b) CT scan obtained at a lower level demonstrates an incarcerated small bowel segment in an umbilical hernia (arrow). This finding was confirmed surgically.
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CT is useful in depicting the precise site and type of hernia and its contents, including spigelian, obturator, lumbar, and ventral hernias (11).
Extrinsic Masses
Carcinoid Tumors
Carcinoid tumors are usually solitary lesions but may be multiple in up to 30% of cases (12). Carcinoid tumors are primarily intrinsic lesions of the ileum; however, small bowel obstruction, which is one of the major manifestations of carcinoid tumors, is largely due to desmoplastic mechanisms operating in the mesentery. CT demonstrates the mesenteric metastasis as a nodular mass in association with retraction of surrounding bowel loops with mural thickening (13).
Lymphoma
Primary non-Hodgkin lymphomas of the small bowel rarely cause obstruction, even when they are annular, because they are soft lesions that infiltrate the bowel wall and tend to produce early cavitation. However, nodal non-Hodgkin lymphomas may arise in the mesentery and grow to invade small bowel segments, causing obstruction by compression, kinking, and infiltration (12). CT findings include obstruction signs and a growing mesenteric mass with infiltration of adjacent bowel loops (Fig 11).
Peritoneal Carcinomatosis
Ovarian carcinoma is the most frequent cause of metastatic disease of the omentum (14). Other tumors that frequently spread to the omentum include carcinoma of the colon, stomach, pancreas, breast, and endometrium. CT findings in small bowel obstruction secondary to peritoneal carcinomatosis include an omental mass in the transition zone causing obstruction (Fig 12).
Appendicitis and Diverticulitis
CT is accurate in the diagnosis of appendicitis and the detection of complications such as phlegmon, abscess, and peritonitis (15). Less commonly, small bowel obstruction may be secondary to appendicitis (Fig 13).
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Figure 13a. Small bowel obstruction secondary to appendicitis. (a) CT scan shows an inflammatory reaction in the right lower quadrant (arrow). Small bowel dilatation is also seen (S). (b) CT scan obtained at a lower level reveals a fluid collection adjacent to the right pelvic sidewall (arrowhead).
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Figure 13b. Small bowel obstruction secondary to appendicitis. (a) CT scan shows an inflammatory reaction in the right lower quadrant (arrow). Small bowel dilatation is also seen (S). (b) CT scan obtained at a lower level reveals a fluid collection adjacent to the right pelvic sidewall (arrowhead).
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CT is considered to be the method of choice for evaluating patients with suspected diverticulitis (16). CT findings in uncomplicated acute diverticulitis include colonic diverticula, mural thickening of the involved sigmoid colon, edema at the root of the mesentery, and engorged mesenteric vasculature. CT findings in complicated diverticulitis include paracolonic or pelvic abscess, peritonitis, and colovesical obstruction (17). In some cases, pericolic inflammation may result in secondary small bowel obstruction (Fig 14). Obstruction of the small bowel in cases of appendicitis and diverticulitis is probably related to the anatomic proximity of the transition zone to the sigmoid colon or appendix, which makes portions of the small bowel vulnerable to pericolic inflammatory processes centered in the sigmoid colon or appendiceal region. The bowel loops become trapped within the inflammatory processes, resulting in fixation and narrowing. Thus, a pericolic process may directly affect adjacent small bowel loops, resulting in obstruction (18).
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Figure 14a. Small bowel obstruction secondary to sigmoid diverticulitis. (a) CT scan demonstrates vascular engorgement (arrow) and fluid at the root of the mesentery (arrowhead), findings that are suggestive of sigmoid diverticulitis. (b) CT scan obtained at a lower level demonstrates small bowel dilatation (S) adjacent to the inflamed sigmoid colon.
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Figure 14b. Small bowel obstruction secondary to sigmoid diverticulitis. (a) CT scan demonstrates vascular engorgement (arrow) and fluid at the root of the mesentery (arrowhead), findings that are suggestive of sigmoid diverticulitis. (b) CT scan obtained at a lower level demonstrates small bowel dilatation (S) adjacent to the inflamed sigmoid colon.
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Intrinsic Causes
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Adenocarcinoma
Adenocarcinoma of the small bowel is seen more frequently in the duodenum and proximal jejunum than in the ileum (19). The tumor is usually detected at an advanced stage. At CT, small bowel obstruction secondary to adenocarcinoma usually manifests as mural thickening with luminal narrowing at the transition zone (Fig 15). CT may provide unique information about tumor extension and local or distant metastases.
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Figure 15. Small bowel obstruction secondary to adenocarcinoma. CT scan shows marked dilatation of fluid-filled small bowel loops (S). A focal mass involving the distal ileum is seen at the transition zone (arrow). This latter finding is suggestive of carcinoma, which was confirmed surgically.
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Crohn Disease
In the advanced, stenotic phase of Crohn disease, patients frequently present with recurrent episodes of partial small bowel obstruction. High-grade obstructions are rare (20). CT is extremely useful for determining the site, level, and cause of small bowel obstruction secondary to Crohn disease. Small bowel wall thickening and luminal narrowing may be seen. During the acute, noncicatrizing phase of Crohn disease, the small bowel shows mural stratification and often has a target-like or "double halo" appearance (Fig 16). Inflamed mucosa and serosa may markedly enhance after bolus intravenous administration of iodinated contrast material, and the intensity of enhancement correlates with the clinical activity of the disease (21). During the chronic phase, mural stratification disappears, so that the affected bowel wall typically has homogeneous attenuation at CT (Fig 17). In addition, fat deposition inthe bowel wall in Crohn disease indicates inactive disease (21,22).
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Figure 16a. Small bowel obstruction secondary to Crohn disease. CT scans show marked dilatation of fluid-filled small bowel loops (S) and a transition zone with collapsed distal loops (arrow in a). The distal ileum contains a small amount of intraluminal fluid and has a circumferentially thickened wall with the "double halo sign" (arrowheads in b). Mild fatty proliferation is apparent in the right lower quadrant. The small bowel obstruction resolved with conservative therapy.
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Figure 16b. Small bowel obstruction secondary to Crohn disease. CT scans show marked dilatation of fluid-filled small bowel loops (S) and a transition zone with collapsed distal loops (arrow in a). The distal ileum contains a small amount of intraluminal fluid and has a circumferentially thickened wall with the "double halo sign" (arrowheads in b). Mild fatty proliferation is apparent in the right lower quadrant. The small bowel obstruction resolved with conservative therapy.
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Tuberculosis
The ileocecal region is the most common site of involvement in tuberculosis of the gastrointestinal tract. Intestinal tuberculosis may cause small bowel obstruction (23). The gross morphologic features of this pathologic process are well evaluated with CT. When the inflammation is mild, CT demonstrates only slight, symmetric mural thickening and a few small regional nodes. When the pathologic process is severe and advanced, more characteristic abnormalities are evident. The bowel wall is thickened, and the inflammatory mass may have a heterogeneous appearance (Fig 18). Large regional nodes with hypoattenuating centers are present, findings that are consistent with caseating necrosis (24).
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Figure 18a. Small bowel obstruction secondary to jejunal tuberculosis. (a) CT scan shows markedly dilated jejunal loops (J). (b) CT scan obtained at a lower level reveals a heterogeneous inflammatory mass (arrow) located at the jejunum and associated with inflammatory changes within the peritoneal cavity (arrowhead).
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Figure 18b. Small bowel obstruction secondary to jejunal tuberculosis. (a) CT scan shows markedly dilated jejunal loops (J). (b) CT scan obtained at a lower level reveals a heterogeneous inflammatory mass (arrow) located at the jejunum and associated with inflammatory changes within the peritoneal cavity (arrowhead).
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Radiation Enteropathy
Radiation enteropathy may cause mild to moderate small bowel obstruction (25). In such cases, obstruction is caused by adhesions and, to a lesser extent, by luminal narrowing and dysmotility related to radiation serositis. The changes are localized to the irradiated area, usually in the pelvis. CT provides useful information regarding radiation damage to the small bowel. In cases of marked radiation-induced changes causing small bowel obstruction, CT shows the extent of mural thickening and mesenteric fibrosis (Figs 19, 20).
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Figure 19. Small bowel obstruction due to chronic radiation-induced enteritis following 60-Gy external-beam radiation therapy to the pelvis for cervical carcinoma. CT scan demonstrates small bowel wall thickening (arrow) with an increased amount of soft tissue within the mesentery (arrowhead), findings that are consistent with radiation-induced fibrosis.
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Figure 20. Small bowel obstruction due to acute radiation-induced enteritis following 50-Gy external-beam radiation therapy to the pelvis for bladder carcinoma. Contrast-enhanced abdominal CT scan shows luminal narrowing and mural thickening of the distal ileum with the halo sign (arrow).
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Intramural Intestinal Hemorrhage
In adults, intramural intestinal hemorrhage may involve any portion of the small bowel and is usually a complication of anticoagulation therapy. However, it may also be secondary to any condition that predisposes to bleeding or result from trauma or biopsy (26). Intramural intestinal hemorrhage most often involves the duodenum and jejunum, and its CT appearance depends on the age of the hematoma. Immediately after hemorrhage, the hematoma will have an attenuation similar to that of circulating blood. Soon thereafter, attenuation increases (Fig 21). As the clot lyses, the attenuation decreases, often in a centripetal fashion. The characteristic "ring sign" with a crescent of increased attenuation may be seen (Fig 22) (27,28). Therefore, without appropriate clinical history such as trauma or anticoagulation therapy, CT findings are often not specific enough to allow differentiation between intramural hemorrhage and inflammatory causes of small bowel wall thickening.
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Figure 21. Intramural hemorrhage in a patient who was receiving large doses of anticoagulants and presented with symptoms indicating sudden onset of small bowel obstruction. CT scan shows high-attenuation small bowel wall thickening (arrow). Dilatation of the small bowel is also seen (S).
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Figure 22. Small bowel obstruction secondary to intramural hematoma of the jejunum. CT scan shows the characteristic ring sign in the proximal jejunum (arrows) in association with intramesenteric hemorrhage (arrowhead). Dilatation of small bowel loops is also noted (S).
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Intussusception
Various extrinsic, intrinsic, or intraluminal processes may result in small bowel intussusception (29). The final common pathway is invagination of an entire proximal small bowel loop and part of its mesentery into the lumen of the distal small bowel. Polypoid tumors, benign or malignant, are the most common cause of small bowel intussusception in adults. Intussusception may occur in extrinsic disorders such as adhesions or duplications. Although a histologic diagnosis cannot be made with CT, a guarded differential diagnosis can be made based on clinical history, tumor location, and specific imaging features. At CT, intussusception displays one of three patterns depending on the severity and duration of the disease: the target sign, a sausage-shaped mass with alternating layers of low and high attenuation, and a reniform mass. In a small percentage of patients with metastatic melanoma, small bowel obstruction with intussusception is the initial manifestation of disease (Fig 23) (30).
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Figure 23. Jejunojejunal intussusception caused by a benign adenomatous polyp. CT scan reveals an intussusception with a target-like appearance (arrow). The hypoattenuating area represents intussuscepted mesenteric fat (arrowhead). Marked small bowel dilatation is also seen (S).
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Intraluminal Causes
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Bezoars
Bezoars are an unusual cause of acute abdomen due to small bowel obstruction. The number of cases has increased since the introduction of gastric surgery for gastroduodenal peptic ulcer (31).
Complete mechanical bowel obstruction is the most frequent clinical manifestation of bezoars. The obstruction caused by small bowel phytobezoars frequently occurs in the jejunum or proximal ileum (32). CT has become a useful method of diagnosing the presence and cause of small bowel obstruction in this setting. CT findings in obstruction due to bezoars include an intraluminal mass in the transition zone causing obstruction. Fluid in the small bowel outlines the mass, which has a mottled appearance owing to air retained in the interstices (Fig 24) (33). This characteristic appearance may be similar to that of small bowel feces described in cases of severe stasis in patients with high-grade small bowel obstruction.
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Figure 24. Small bowel obstruction caused by a bezoar. CT scan shows dilated, fluid-filled small bowel loops (S) and an intraluminal mass with air retained in the interstices (arrow). These findings are consistent with a bezoar, which was confirmed surgically.
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Other Intraluminal Causes
Gallstones, foreign bodies, retained meconium, or tangles of ascarides may obturate the small bowel lumen, resulting in obstruction.
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Intestinal Malrotation
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Intestinal malrotation is defined as an anomaly of rotation and fixation of the midgut. It is usually an isolated abnormality but can be associated with congenital heart disease or situs problems (34,35).
Intestinal malrotation in adults is usually an incidental finding at CT (36). CT findings include right-sided small bowel, left-sided colon (Fig 25), abnormal relationships between superior mesenteric vessels, and aplasia of the uncinate process. Small bowel volvulus is usually secondary to various conditions such as malrotation, congenital bands, postoperative adhesions, and internal hernias. CT signs of small bowel volvulus include (to varying degrees) a U-shaped configuration or radial distribution of distended loops, fluid-filled loops of the small bowel converging toward the point of torsion, and mesentery tightly wound around the point of torsion (whirl sign) (Fig 26).
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Figure 25a. Small bowel volvulus secondary to intestinal malrotation. (a) CT scan shows right-sided small bowel (S) and left-sided colon (C), findings that are suggestive of malrotation. (b) CT scan obtained at a lower level shows the whirl sign (arrow), a finding that is highly characteristic of intestinal volvulus.
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Figure 25b. Small bowel volvulus secondary to intestinal malrotation. (a) CT scan shows right-sided small bowel (S) and left-sided colon (C), findings that are suggestive of malrotation. (b) CT scan obtained at a lower level shows the whirl sign (arrow), a finding that is highly characteristic of intestinal volvulus.
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Figure 26a. Small bowel volvulus secondary to intestinal malrotation. (a) CT scan shows distended small bowel loops in a U-shaped configuration (S). (b) CT scan obtained at a lower level shows the characteristic whirl sign (arrow) corresponding to twisted mesentery.
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Figure 26b. Small bowel volvulus secondary to intestinal malrotation. (a) CT scan shows distended small bowel loops in a U-shaped configuration (S). (b) CT scan obtained at a lower level shows the characteristic whirl sign (arrow) corresponding to twisted mesentery.
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Conclusions
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The diagnosis of small bowel obstruction is based on a comprehensive approach that includes clinical background, patient history, and results of physical examination and laboratory tests. Conventional radiography should remain the initial method of imaging in patients with suspected small bowel obstruction. However, the ability to determine the site, level, cause, and severity of small bowel obstruction with CT makes this modality an important additional diagnostic tool when specific disease management issues must be addressed.
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Abstract
Introduction
