AUTHOR AND EDITOR INFORMATION

Section 1 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

INTRODUCTION

Section 2 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Background

Small-bowel obstructions can result from a variety of causes. In parts of Europe, the term "ileus" is applied both to a mechanical small-bowel obstruction and to atony of the bowel related to abdominal surgery or peritonitis; however, in most English-speaking countries, the term "obstruction" is used for a mechanical bowel blockage arising from a structural abnormality that presents a physical barrier to the progression of gut contents. The term "ileus" is reserved for the paralytic or functional variety of gastric obstruction.^{1, 2, 3, 4, 5, 6}

Mechanical small-bowel obstruction can be classified into 3 main groups, according to the cause of the intestinal obstruction, as follows:

1. Intraluminal (eg, foreign bodies, bezoars, food bolus)
2. Obstruction resulting from lesions in the bowel wall (eg, tumors, Crohn disease)
3. Extrinsic (eg, adhesions, hernias, volvulus)

Related eMedicine topics:
Obstruction, Small Bowel
Small-Bowel Obstruction (Pediatrics)
Atresia, Stenosis, and Other Obstruction of the Colon (Pediatrics)
Specialty Center Gastroenterology

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Pathophysiology

In the mechanical form of small-bowel obstruction, the proximal gut is distended by swallowed gas and fluid that arises from gastric, small-bowel, pancreatic, and biliary secretions. Fluid sequestered within the small-bowel is drawn from the circulating blood volume and interstitial spaces, and copious vomiting exacerbates fluid loss and electrolytic depletion. The resultant hypovolemia may be fatal.

Prolonged small-bowel obstruction may result in the compromise of venous blood in the affected segment of bowel, edema, localized tissue anoxia with ischemia, necrosis, perforation leading to peritonitis, and death. Septicemia may occur in patients as a result of extensive aerobic and anaerobic proliferation in the lumen. The bowel beyond the obstruction collapses and empties.

In general, the higher the level of obstruction, the less the distention and the more rapid the onset of vomiting. Conversely, in patients with a distal small-bowel obstruction, central abdominal distention may be marked and vomiting is, usually, a late feature (because the bowel takes time to fill). Colicky pain is most marked in patients with a distal obstruction. Hypotension and tachycardia suggest fluid depletion, and tenderness and leukocytosis suggest strangulation. In the early stages, bowel sounds are usually high-pitched, and they occur in frequent runs as the bowel contracts in an attempt to overcome the obstruction. A silent, tender abdomen suggests perforation or peritonitis, and it is a late sign.

The etiologies of small-bowel obstruction are as follows:

• Intraluminal causes are relatively unusual.
  • Swallowed foreign bodies may be involved, although usually a foreign body that has passed the pylorus passes through the remainder of the small-bowel without difficulty (unless the small-bowel is already compromised by postoperative adhesions).
  • Bezoars are possible factors.⁷
  • Parasites such as Ascaris lumbricoides may be found.
  • Gallstones may occur with a cholecystenteric fistula.
  • A food bolus may occur, with indigestible vegetable material impacted in the terminal ileum. Patients with a food bolus will usually have undergone gastric outlet surgery.
  • Inspissated meconium resulting in obstruction of the distal ileum may be seen with cystic fibrosis in patients of any age.
• Regarding intramural causes, obstruction resulting from lesions in the wall of the small-bowel is a relatively infrequent finding.
  • Neonatal atresias and strictures may cause small-bowel obstructions.
  • Thickening of the bowel wall with luminal compromise may be seen, as in patients with Crohn disease. This thickening may occur with recurrent episodes of partial or incomplete obstruction.
  • Small-bowel tuberculosis is not uncommon in certain parts of the world.
  • Strictures may occur following ulceration induced by potassium tablets, nonsteroidal anti-inflammatory agents, and therapeutic irradiation for bladder or cervical cancer.
  • An intramural hematoma may occur in cases of trauma or in patients receiving higher doses of anticoagulant agents than are necessary.
  • Lipomas, leiomyomas, and carcinoid tumors rarely result in obstruction; however, these have been reported in association with small-bowel lymphoma and adenocarcinoma.
  • Secondary tumors (the most notable being gastric and colonic carcinomas, ovarian cancers, and malignant melanomas) may occasionally compromise the lumen of the small-bowel.
  • Any polypoid mucosal or submucosal lesion may form the head of an intussusception, which in turn can result in a small-bowel obstruction.
  • Intussusception in children younger than 2 years is a common abdominal emergency. These cases are usually idiopathic, although Meckel diverticulum, polyps, duplication cysts, and Henoch-Schönlein purpura have been implicated.
• Extramural causes may be the most common causes of small-bowel obstructions.
  • Adhesions related to previous surgery or peritonitis are frequently implicated in small-bowel obstructions. Adhesive bands occur between loops of bowel and the operative site. These adhesions can obstruct the small-bowel by causing acute angulation and kinking, often many years after the initial procedure is performed.
  • Congenital intraperitoneal bands may result in gastric obstruction.
  • Congenital malrotation results in a short mesenteric attachment and the entire midgut may undergo torsion or volvulus; these can lead not only to obstruction but also to a rapid progression to ischemia and death.
  • Hernias may cause small-bowel obstructions. A loop of small-bowel may enter any form of hernia and become obstructed by the narrow neck of a hernia, which compromises the caliber of the bowel from without.
    • Most frequently, a hernia-induced obstruction may occur as a complication of femoral, indirect inguinal, or umbilical hernias. Such an obstruction is a recognized complication of incisional and epigastric hernias and of the rare spigelian hernia.
    • Clearly, development of the obstruction is related to the width of the neck of the hernia, which to some degree reflects the nature of the boundaries of the hernial defect. Obstructions resulting from a femoral hernia are particularly unwieldy.
    • Initially, the venous return of the bowel is compromised, leading to congestion and edema with progression to localized tissue anoxia with frank ischemia, necrosis, and (in some patients) perforation.
    • The site of the obstruction is at the neck of the hernial sac, but any dead bowel is usually within the sac, which contains transudate; this sac is often tender.
    • In large, multiloculated umbilical and incisional hernias, obstruction and its sequelae may arise in a single locus, if the neck is sufficiently tight.
    • A Richter hernia occurs when only part of the circumference of the small-bowel is trapped at the hernial neck (usually femoral). This type of hernia may not result in a complete obstruction and is easy to overlook, particularly in patients with obesity, until perforation has occurred.
    • Obstruction occasionally results from incarceration of a loop of small-bowel through congenital defects in the mesentery or omentum.
    • Other so-called internal hernias may arise as a result of various degrees of malrotation occurring during fetal development. The most common are right and left paraduodenal hernias.
    • Obstruction caused by an internal hernia is clinically indistinguishable from obstruction resulting from postoperative adhesions.
• In some patients, the etiology of small-bowel obstruction may be multifactorial. For example, metastases to the small-bowel can directly invade the bowel wall, causing luminal compromise. Obstruction may be the result of extrinsic compression or kinking of the bowel when it adheres to the primary tumor or a metastatic deposit.

Frequency

United States

The frequency of small-bowel obstruction in the United States is the same as that found internationally.

International

Approximately 20% of patients admitted to the hospital with an acute abdomen have an intestinal obstruction; small-bowel obstruction is responsible for 80% of these cases. Some causes of small-bowel obstructions (eg, A lumbricoides, tuberculosis) are more common in developing countries.

Mortality/Morbidity

The mortality and morbidity rates of small-bowel obstruction depend on the etiology and the patient's age at presentation. For example, in cases of small-bowel obstruction resulting from intussusception in infants, the prognosis is favorable because reduction and relief of the obstruction are possible by using a gas enema under radiologic guidance. On the contrary, the prognosis in an obese elderly woman with an obstruction caused by a femoral hernia is poor, particularly if the diagnosis is delayed and the small-bowel obstruction is associated with protracted vomiting, fluid depletion, or intercurrent illnesses.

Strangulation with ischemia of the bowel by an adhesion or a hernia is an emergency, and any delay in surgery beyond the time required to resuscitate and optimize the patient's status is associated with an increase in mortality rates, which can be as high as 25% when the delay exceeds 36 hours. Leukocytosis, a slight amylasemia, and a tender silent abdomen may be signs of ischemia or a perforated small-bowel. These are of grave prognostic significance; in these cases, early surgery is essential.^{8, 9, 10}

Race

No racial predilection exists for small-bowel obstruction.

Age

Although small-bowel obstructions may occur in patients of any age, certain etiologies are more common in particular age groups.¹¹

Common causes of small-bowel obstruction:

Anatomy

The distal part of the duodenum (the jejunum, ileum, and colon proximal to the splenic flexure) develops from the embryologic midgut. A normal small-bowel is approximately 3-6 m in length, although the exact length is often difficult to assess because the small-bowel can become considerably elongated or foreshortened in life. In most individuals, the jejunal loops lie in the left hypochondrium, and the ileum lies in the pelvic midline. The terminal ileum is the narrowest part of the small-bowel, and peristalsis may be weaker in this segment than in more proximal segments. The valvulae conniventes are more prominent in the jejunum than in the ileum.

The intramural width of the small-bowel may be measured by taking plain abdominal radiographs of a gas-filled lumen. An intramural width of 3 cm is considered abnormal and may indicate obstruction or ileus. This measurement is reduced to 2.5 cm on computed tomography (CT) scans. In most healthy adults, the small-bowel contains little gas, and only 1 or 2 nondistended gas-filled loops may be seen in the small-bowel; however, in the supine position, the duodenum cannot maintain an efficient water trap, and air may pass into the jejunum from the stomach. This is a common observation in patients undergoing intravenous urography who are supine during the procedure. In young children, gas-filled small-bowel loops are common normal findings; however, distention of these loops by gas or fluid should be regarded as abnormal.

The depiction of the small-bowel on sonograms is improved when the small-bowel is filled with water. A water-filled small-bowel may occur normally, particularly after meals or the ingestion of a large volume of fluid. Sonography shows fluid-filled tubes lined by an echogenic mucosa that is thrown into folds or valvulae. These folds may lend a ribbed appearance to the inner wall of the bowel. The normal bowel wall is 3-5 mm thick, and normal loops of bowel are yielding and easily deformed during examination. The configuration of the loops may be altered as a result of peristalsis and distention with fluid and air. Progressive pathologic distention makes the bowel more tubular, with a loss of sharp angles between consecutive loops.

Clinical Details

Patients with mechanical small-bowel obstructions usually present with abdominal pain, vomiting, absolute constipation, and varying degrees of abdominal distention. Signs of peritonitis suggest perforation of the bowel as a result of ischemia, and they are indistinguishable from peritonitis resulting from other causes of perforated intra-abdominal viscus.

In patients with a simple obstruction with no associated vascular compromise, pain tends to be colicky and initially mild; however, it progressively increases, both in frequency and in severity. The pain may be continuous or interspersed with pain-free periods. The patient often assumes a knee-chest position, or he or she may roll around. Patients with peritonitis tend to lie still because movement exacerbates the pain.

Vomiting is an early sign in proximal or high small-bowel obstructions; however, if the obstruction is in the distal small-bowel, vomiting may be delayed. Initially, the vomitus contains gastric juice, which is soon followed by bile and, finally, the vomitus contains stale small-bowel content; this is often termed "feculent."

Hypovolemia and electrolyte depletion may rapidly occur unless the patient receives intravenous replacements. The degree and distribution of abdominal distention may reflect the level of the obstruction. With a proximal or high obstruction, distention may be minimal. On the contrary, central abdominal distention tends to be marked in patients with distal obstructions.

No definitive signs distinguish a strangulated obstruction from a simple obstruction; however, certain clinical and laboratory features may suggest strangulation (see Pathophysiology).

Examine the groin in all patients with small-bowel obstructions to exclude an inguinal or femoral hernia. Femoral hernias are easy to overlook in patients with obesity. The physician should make sure to note any scars from previous laparotomy. Blood and mucous may be passed, and an abdominal mass may occasionally be palpated in children with small-bowel obstructions secondary to intussusception.

In children with intussusception, colicky pain is a classic finding. The episodes of pain start suddenly, last several minutes, and then fade; the patient's condition appears fairly normal between these episodes. Vomiting is uncommon. Constipation is a typical finding, although a mixture of blood and mucus may be passed as the so-called "red current jelly", which is said to be pathognomonic for intussusception. Occasionally, an abdominal mass is palpable.

Preferred Examination

Certain radiologic investigations can be used to confirm the diagnosis and severity of a small-bowel obstruction, but not its etiology.¹² Others are aimed at determining the cause of small-bowel obstructions.¹³

Conventional plain radiography is the investigation of choice for patients with suspected small-bowel obstructions. This study should always be performed first.¹³

The unique capabilities of CT scanning make it an important diagnostic tool when a specific clinical answer is sought. Studies have shown the superiority of CT scanning in revealing not only the site of the obstruction but also its cause. CT scans may demonstrate signs of ischemia as well.

The relative value of performing conventional small-bowel contrast enema studies versus CT scans in a known or suspected small-bowel obstruction remains underdetermined. Small-bowel enema is preferable to CT scanning when the obstruction is intermittent or incomplete, and it may be of particular value under these circumstances if the plain radiographic findings are normal.

Although ultrasonography has no specific role in the diagnosis of an acute small-bowel obstruction, the technique is widely used in the investigation of acute abdominal pain. The radiologist must be aware of the sonographic appearance of small-bowel obstructions. Sonograms help not only in the diagnosis of an obstruction but also in defining the cause. The sonographic findings differentiate an adynamic ileus from a mechanical obstruction by depicting peristalsis. Ultrasonography has particular advantages in the diagnosis of neonatal small-bowel obstructions.

Radionuclide-labeled white blood cell scanning has no specific role in the diagnosis of acute small-bowel obstruction; however, the technique is sensitive and specific in the diagnosis of some of the causes of chronic and incomplete obstruction, such as Crohn disease. Similarly, a specific diagnosis of a carcinoid of the gastrointestinal tract associated with an incomplete obstruction may be achieved by performing imaging with a radionuclide-labeled somatostatin analog.

The diagnosis of small-bowel obstruction has been achieved by using magnetic resonance imaging (MRI) with T1-weighted sequences combined with antiperistaltic agents and retrograde insufflation. Similar results have been achieved by using subsecond rapid acquisition with relaxation enhancement (RARE) imaging for the delineation of bowel obstruction. Virtual small-bowel imaging is theoretically possible with a spiral CT scanner, but it should be noted that any digital imaging technique enables virtual-reality processing.

Limitations of Techniques

The sensitivity of plain radiographic findings in the diagnosis of small-bowel obstruction is approximately 50-66%. When little or no gas is present in the small-bowel, a diagnosis of small-bowel obstruction may be overlooked, especially if the condition is proximal. Barium studies should be avoided in patients with suspected perforations or complete small-bowel obstruction.

CT scans have poor sensitivity for low-grade, partial, or incomplete obstruction.

Although sonographic findings may suggest a diagnosis and etiology of small-bowel obstruction, the exact sensitivity of sonography is not known. In addition, bowel gas and obesity poses problems, and the technique remains operator dependent.

Radionuclide findings do not help with a specific diagnosis in bowel obstruction.

MRI is expensive and not universally available. The extent to which MRI will be used clinically in the future depends not only on the availability of an appropriate scanner but also on the overall sensitivity of the procedure compared with time-honored, simple methods, such as plain radiography. The sensitivity of MRI has yet to be established.

The requirement for nasogastric intubation and enteroclysis prior to CT scanning or MRI for virtual small-bowel imaging makes virtual imaging invasive and time consuming.

DIFFERENTIALS

Section 3 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Other Problems to be Considered

The differential diagnosis of small-bowel obstruction is usually straightforward and involves the diagnosis of the cause of the obstruction, which may be elusive. Many medical disorders can cause a symptom complex similar to that seen in patients with small-bowel obstructions, although other disorders often do not cause pain. Distention, vomiting, and constipation may be seen in patients with myocardial infarction, intracranial pathology, diabetic ketoacidosis, hyperthyroidism, uremia, and hypokalemia (among others). Tricyclic antidepressants, atropine, and comparable agents may cause similar symptoms, as may peritonitis of any cause in patients of any age; however, the pain is usually different in nature. Enterocolitis in a neonate may present a particular problem.

Perhaps the greatest problem arises in the immediate postoperative period following any abdominal surgery, in which it is difficult to determine whether the failure of gastrointestinal tract function to return to normal is merely the atony frequently seen after surgery or a supervening mechanical obstruction.

RADIOGRAPH

Section 4 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

Plain radiography

The following radiologic features may be of assistance in making a diagnosis of small-bowel obstruction:

• In complete obstruction, loops of small bowel distend within 3-5 hours.
• A bowel larger than 3 cm in diameter is often associated with obstruction. Gas and fluid is usually present in the distended small-bowel loops, and gas and fluid levels may be present at the same or different levels in the abdominal cavity.¹³
• In the past, it has been suggested that gas and fluid levels at different heights indicated a mechanical obstruction, whereas gas and fluid levels at the same height more likely indicated an adynamic ileus¹³; however, these findings are no longer considered to be reliable signs.
• A discrepancy often exists between the size of the loop of bowel proximal to the obstruction and the size of the loop of bowel distal to the obstruction, which is usually empty or of normal caliber.
• In long-standing obstruction, small-bowel dilatation may mimic a dilated colon. To differentiate the large bowel from the small bowel, an attempt should be made to identify the cecum, which is usually normal in caliber in the early stages of small-bowel obstruction. With time, however, the colon may empty completely and become difficult to depict.
• When distended, loops of small bowel are usually positioned in the center of the abdomen.
• With a proximal jejunal obstruction, a gas-fluid level usually exists in the stomach unless the patient has vomited recently.
• Occasionally, 1 or 2 loops of distended small bowel may be seen in the left upper quadrant.
• The more distal the obstruction, the more numerous the gas-fluid levels. At different heights, the levels reveal a stepladder appearance.
• With the patient in the supine position, fluid-filled loops of small bowel lie posteriorly, and gas-filled loops rise anteriorly and superiorly.
• With the patient erect, gas-filled loops are observed in the upper part of the abdomen.
• Grossly distended fluid-filled loops, particularly of the mid and distal small bowel, may appear as elongated soft-tissue masses.
• Valvulae conniventes are usually closely spaced and should be within 1-4 mm of each other; however, this distance is greater in small-bowel distention. A stretch sign may appear as a result of the erect valvulae conniventes encircling the bowel lumen. Even when distended, the valvulae conniventes in the jejunum are usually preserved. In a distended terminal ileum, however, they flatten and the bowel often appears tubelike.
• An increase in peristaltic activity can give rise to the "string-of-beads" sign, in which the "beads" represent gas trapped within the valvulae conniventes.
• In some patients, the outer border of the bowel is delineated by mesenteric fat, allowing for the differentiation of fluid-filled small-bowel loops and distention resulting from ascites.
• The small bowel may become massively dilated if the obstruction is chronic. An associated thickening of the intestinal wall often occurs, such that the distance between gas shadows in the small-bowel lumen increases.

Less common radiologic signs are seen in specific circumstances. They are described as follows:

• Most closed-loop obstructions (75%) are caused by adhesions. A closed-loop obstruction occurs when a loop of bowel is not decompressed by the caudal passage of gas and fluid. This obstruction may be associated with a U-shaped distended loop of small bowel. This loop may be fixed and does not change position over time. An increasing amount of fluid is incarcerated within the closed loop, and the coffee bean sign (a gas-filled loop) or a pseudotumor (fluid-filled loop) may be seen.
• Gallstone ileus is characterized by the presence of a calcified intraluminal stone (often in the terminal ileum), radiologic signs of a small-bowel obstruction above the ileus, and gas in the biliary tree as a result of the cholecystoduodenal fistula.¹⁴
• In children with an intussusception, plain abdominal radiographs may show a soft-tissue mass at the head of the intussusception. Usually, no gas is seen in the colon immediately proximal to the site of obstruction because the intussusceptum occupies the entire lumen. The right lower quadrant is occasionally effaced and no bowel gas is seen in the region. The intussusceptum may form a large, convex, soft-tissue mass in the region of the ascending or transverse colon, and the large bowel immediately distal to the intussusception is dilated and filled with gas. Radiologic signs of a small-bowel obstruction may be seen above the intussusception.
• Intramural gas secondary to ischemia may eventually appear in an obstruction of any etiology. This is a poor prognostic sign.

Conventional barium follow-through examination and enteroclysis

Controversy exists around the type of contrast medium used in patients with suspected small-bowel obstruction. The use of water-soluble contrast material, ionic or nonionic, has limitations because it is diluted by the intestinal fluid. As a result, making the diagnosis of obstruction and finding the site of obstruction are difficult. A barium suspension should be used only if no features of peritonism are present.

One or all of the following radiologic features may assist in the diagnosis:

• A delay in transit time on a conventional follow-through examination of greater than 12 hours is suggestive of an organic obstruction.
• A snakehead appearance may be observed, which is caused by the bulbous appearance of the bowel proximal to the site of an obstruction. The finding results from the attempts of peristaltic activity to overcome the obstruction.
• Abrupt changes may occur in the caliber of the small bowel at the site of the obstruction, with a dilated contrast-filled small bowel located proximally to the obstruction and a collapsed bowel located distally.
• The beak sign may be detected at the site of an obstruction as barium trickles through a narrow lumen.
• Fixation and kinking of the bowel may be clearly demonstrated.
• Stretched mucosal folds or valvulae conniventes may be apparent.

Small-bowel obstruction has been subdivided into 3 groups on the basis of enteroclysis findings, as follows:

• A low-grade or incomplete obstruction in which flow of contrast agent through the obstruction is sufficient to define the mucosal folds distal to the obstruction well.
• A high-grade obstruction in which stasis occurs and transit of the contrast material is delayed. Dilution of the contrast material by intestinal fluid and the passage of minimal contrast material through the obstruction indicate that no details may be defined.
• A complete small-bowel obstruction in which no contrast agent passes the obstructed segment during the 3-24 hours after the start of the examination.

Barium enema study

Barium enema has a limited role in patients with small-bowel obstruction. Although examining patients with an ileocecal intussusception or other causes of ileocecal obstruction is possible by using oral contrast material, in these patients barium enema studies are more quickly performed. Barium enema examination is particularly useful if a distal colonic obstruction cannot be excluded by using plain abdominal radiograph findings. In children with intussusception, barium enema studies are not only diagnostic but possibly therapeutic as well.

Degree of Confidence

Despite its limited sensitivity of 50-66%, plain radiography remains the most useful noninvasive procedure in the radiographic diagnosis of small-bowel obstruction. When plain radiographic findings are combined with the clinical history and the results of physical and laboratory examinations, small-bowel obstruction can usually be diagnosed with confidence.^{13, 15}

False Positives/Negatives

Differentiating an adynamic ileus from small-bowel obstruction may be particularly difficult, especially in the immediate postoperative period. Eventually, most intestinal obstructions may lead to an adynamic ileus (especially when strangulation is present). This finding is associated with perforation and peritonitis. Under these circumstances, gas may appear in the small bowel proximal to the obstruction, or it may be retained in the atonic colon, leading to diagnostic confusion. Little or no gas in the small bowel may lead to a false-negative diagnosis.

Differentiating a proximal obstruction, a Richter hernia, a gallstone ileus, or a volvulus of the midgut is notably difficult because all of these conditions are demonstrated as a relatively gasless abdomen on plain radiographs. The radiographic features of gallstone ileus are not consistent, as not all stones are radiopaque, and not all choledochoenteric fistulas allow reflux of air into the biliary tree. Gallstone ileus is one condition that causes accumulation of fluid rather than gas within the obstructed bowel; therefore, gallstone ileus should always be considered in the differential diagnosis of small-bowel obstruction accompanied by a relatively gasless abdomen.

In large-bowel obstruction, small-bowel dilatation may occur if the ileocecal valve is patent; this is a potential cause of confusion.

CT SCAN

Section 5 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

CT scanning is recommended when the clinical findings and the initial plain radiographs are inconclusive, or when strangulation is suspected. CT scans clearly demonstrate abnormalities of the bowel wall, the mesentery, the mesenteric vessels, and the peritoneum.^{1, 8, 15, 16, 17, 18, 19, 20, 21}

CT scanning should be performed with intravenous contrast enhancement. Intraluminal contrast material may not be necessary because fluid and gas accumulation in the bowel may provide sufficient contrast; however, oral administration of a 1-2% barium sulphate suspension or a 2% water-soluble contrast agent 30-120 minutes before scanning may be useful for accurately locating the site and degree of obstruction.¹⁶

• The diagnosis of obstruction is based on the identification of a dilated proximal loop and a collapsed distal loop of small bowel.
• A bowel diameter in excess of 2.5 cm is regarded as abnormal, but this criterion is insufficient to differentiate mechanical obstruction from an adynamic ileus.
• The feces sign is an uncommon but reliable sign of a mechanical obstruction. The feces sign occurs when feces and gas intermingle and are observed proximal to the obstruction.
• The level of obstruction may be determined by identifying the transition from dilated to collapsed loops of bowel. The degree of collapse and the amount of residual content in the distal bowel beyond the obstruction are useful to note. The passage of contrast material into the distant collapsed segment indicates that the obstruction is partial or incomplete.²²
• CT scans do not usually help in identifying an adhesive band. The diagnosis of adhesions is based on the abruptly changing caliber of the small-bowel lumen in the absence of radiologic evidence of another obstructive cause. Adhesions most frequently involve the terminal ileum, usually in association with the undersurface of an abdominal scar, the site of previous surgical intervention, or an inflammatory focus.¹⁷
• CT findings in a closed-loop obstruction depend on the length, the degree of distention, and the intra-abdominal orientation of the closed loop. Findings may include a U- or C-shaped loop of small bowel and a radial configuration of the mesentery, with stretched vessels converging on the site of a torsion. Tightly twisted mesentery is occasionally depicted as the whirl sign. At the site of the obstruction, collapsed loops are round, oval, or triangular. On longitudinal imaging, the beak sign appears as a fusiform tapering at the site of the obstruction.^{16, 17}
• The features of strangulation on CT scans may include evidence of small-bowel obstruction (as outlined above), a circumferential thickening of the bowel wall (with a high attenuation), the target sign, and congestion or hemorrhage in the mesentery attached to the closed loop. A serrated beak may be seen at the site of obstruction. Edema in the mesentery attached to the involved segment may result in a hazy appearance and diffuse engorgement, and the mesenteric vasculature may take an unusual course. After the intravenous administration of contrast material, the bowel wall may show delayed, poor, or no enhancement. In advanced cases, pneumatosis intestinalis may develop. A large amount of ascites may be present.¹⁷
• The CT findings of malrotation include the distribution of the small bowel to the right side of the abdomen and of the colon to the left side of it. Abnormal orientation of the superior mesenteric vessels and aplasia of the uncinate process of the pancreas may be seen. If a volvulus occurs, its radiologic appearance is similar to that of any closed-loop obstruction.^{17, 23}
• CT scanning is useful in the diagnosis of external hernia at unusual sites, particularly in patients with obesity. The technique can demonstrate visceral hernial contents and complications such as vascular compromise. The diagnosis of an internal hernia is always based on radiologic findings, and CT scanning is useful in depicting the precise site, type, and contents of the hernia.^{8, 17}
• The CT features of a paraduodenal hernia include the following:
  • A cluster of small bowel anterior and lateral to the pancreas
  • A saclike mass of small-bowel loops
  • Encapsulation of the small-bowel loops
  • A mass effect on the posterior wall of the stomach
  • Caudal displacement of the duodenal-jejunal junction
  • Stretching and/or engorgement of the mesenteric vessels, which may be displaced to the left or right of the aorta or the inferior vena cava, respectively

Less common CT findings include the following:

• An established small-bowel obstruction, caudal or dorsal displacement of the transverse colon, and medial displacement of the ascending or descending colon may be present. Similar features may be seen with the uncommon transmesenteric hernia, together with the additional finding of a deficiency in the omental fat overlying the herniated small bowel.
• Any of the less-common causes of small-bowel obstruction can be demonstrated on CT scans, and many of them have specific radiologic features.
  • In patients with Crohn disease, narrowing of the small-bowel lumen and mural thickening are well depicted. In the acute phase, the small bowel shows mural stratification and, often, it has a target-like or double-halo appearance. With intravenous contrast enhancement, inflamed mucosa and serosa show marked enhancement, and the intensity of enhancement is correlated with the activity of disease. In chronic disease, mural stratification disappears and the diseased bowel has a typically uniform attenuation. Fat deposition in the bowel wall indicates inactive disease.¹⁷
  • On CT scans, the appearance of intussusception depends on the severity and duration of the condition. On cross-sectional imaging, the intussusception may appear as a target, with alternating layers of low and high attenuation, or it may be seen as a sausage-shaped or reniform mass.¹⁷
  • When small-bowel obstruction is secondary to bowel adhering to an inflamed mass in appendicitis or diverticular disease, the CT findings may help in making the diagnosis. CT scans can demonstrate appendicitis and its complications, such as an inflammatory mass, abscess, or peritonitis. In diverticular disease, mural thickening may be demonstrated clearly together with edema of the mesentery and the complications of paracolic and pelvic abscess and peritonitis.¹⁷
  • In radiation enteropathy, CT scans demonstrate the features of small-bowel obstruction as well as the abnormalities of an irradiated segment of small bowel which, in most instances, lies within the pelvis. Mural thickening, luminal narrowing, and mesenteric fibrosis occur.¹⁷
  • In intestinal tuberculosis, CT scans may demonstrate only slight asymmetric mural thickening if the inflammation is mild; however, with gross disease the bowel wall is thickened and an inflammatory mass is demonstrated. This mass has a heterogeneous appearance. Large regional lymph nodes with low-attenuation centers may also be demonstrated.¹⁷
  • In gallstone ileus, CT scans may demonstrate the gallstone and gas within the shrunken gallbladder or biliary tree, in addition to small-bowel obstruction.¹⁴
  • The CT appearance of an intramural hematoma is nonspecific and depends on the age of the hematoma. Soon after hemorrhage occurs, the attenuation is low; however, as time passes, its attenuation increases. Once lysis of the clot begins, the attenuation again decreases. It may also be centripetal, which gives rise to the ring sign, with a crescent of high attenuation.¹⁷
  • If a bezoar is the cause of small-bowel obstruction, CT scanning may demonstrate the bezoar as a mass in the obstructed segment of bowel. The bezoar may be outlined by fluid in the proximally dilated small bowel, and the mass may be mottled as a result of air trapped within it.^{17, 24}
  • Malignancy involving the small bowel may have a variety of CT appearances.
    • Although rare, adenocarcinoma may be seen, particularly in the duodenum and proximal jejunum. The tumor is usually detected only at an advanced stage, and in patients with small-bowel obstruction, the mass may be seen as mural thickening with luminal compromise in a transitional zone between dilated and collapsed bowel.¹⁷
    • Primary non-Hodgkin small-bowel lymphoma rarely causes obstruction; however, nodal lymphoma may arise in the mesentery and invade the small bowel by means of direct infiltration to cause small-bowel obstruction. A mesenteric mass invading, kinking, or compressing the small bowel may be seen on CT scans.¹⁷
    • Small-bowel obstruction is a major complication of a carcinoid tumor, one largely caused by the desmoplastic reaction that may occur in the mesentery; therefore, mural thickening and retraction of bowel loops around a segment of mesentery may occur. Metastatic nodular masses may be present.¹⁷
    • Peritoneal carcinomatosis may be recognized by the demonstration of omental masses kinking or compromising the small bowel, thereby resulting in small-bowel obstruction.¹⁷
    • In all malignant tumors causing small-bowel obstruction, CT scans are useful in depicting local extent of the disease and the presence of distant metastases (when appropriate).

With the advent of multisection CT, CT angiography may provide a future alternative to angiography for assessing intestinal ischemia.

Degree of Confidence

The reported sensitivity of CT scanning for detecting small-bowel obstruction is 78-100% in high-grade or complete obstruction. If the obstruction is partial or intermittent, the accuracy is low.^{16, 17, 25}

False Positives/Negatives

If the entire small bowel is distended, a barium enema study should be performed to exclude a large-bowel obstruction and a patent ileocecal valve. A distended loop of small bowel may migrate from its expected anatomic location; therefore, determining the site of obstruction on the basis of a transitional zone alone may be misleading. Carcinoid tumors, intestinal tuberculosis, and desmoid tumors all may have CT features similar to those of peritoneal carcinomatosis. Although the presence of ascites suggests a strangulated obstruction, intra-abdominal fluid may occur in less complicated forms of obstruction. Feces in the small bowel can mimic the mottled appearance of a bezoar if stasis above a complete blockage is severe.¹⁷

MRI

Section 6 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

Although MRI does have a role in imaging of the small bowel, the plethora of contrast media and techniques available indicates that its role is not yet fully established and further clinical trials are required. The technique has been used to evaluate chronic mesenteric ischemia; however, to the author's knowledge, no human studies have been performed in patients with acute ischemia and strangulation. The results of animal studies suggest that a lack of enhancement after the intravenous injection of contrast material may be a useful early sign of acute ischemia.^{26, 27, 28}

Because MRI can be used to demonstrate vessels, quantitate blood flow, assess oxygenation, and depict associated morphologic changes, it may become the modality of choice in detecting acute ischemia in the future. MRI will therefore have a role in the assessment of small-bowel obstruction associated with strangulation. Recently, small-bowel obstruction has been diagnosed by using T1-weighted MRI sequences combined with antiperistaltic agents and retrograde insufflation. Subsecond RARE imaging may produce similar results.^{1, 23, 26}

Degree of Confidence

Clinical experience with MRI in the diagnosis of acute small-bowel disease is limited, and its use in this capacity depends on the availability of suitable equipment.

ULTRASOUND

Section 7 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

Ultrasonography may demonstrate the features and causes of small-bowel obstruction, and it is of particular value in looking at the dynamics of the small bowel.¹

In patients with small-bowel obstruction, sonography may clearly demonstrate loops of distended bowel. The loops have a rounded contour and are not deformed by the pressure of adjacent bowel loops. The loss of definition and prominence of the valvulae conniventes are demonstrated. Sonography can help accurately predict the level of small-bowel obstruction by helping locate the dilated loops and analyze the fold patterns.

Unlike other radiologic techniques, ultrasonography may be used to assess peristalsis. Close to the site of an obstruction, the distended bowel loses its muscular activity; more proximally, the peristaltic activity may be marked in cases of acute obstruction. These features help to differentiate a mechanical obstruction from the adynamic ileus of the postoperative state or peritonitis.

Occasionally, the cause of a small-bowel obstruction can be identified directly. Gallstones, foreign bodies, and mural thickening from inflammatory or neoplastic disease may be revealed. A bezoar may appear as an echogenic intraluminal mass. If a bezoar contains air or is surrounded by fluid, it may cast an acoustic shadow.

The sonographic appearance of intussusception is specific and a target, atypical target, or doughnut appearance on the transverse section of the bowel may be seen as a complex mass of concentric rings of alternating hypoechoic and hyperechoic layers surrounding a highly reflective center. The head of the intussusception may be visible within a dilated loop of small bowel, and left-right inversion of the superior mesenteric vessels may be evident. The presence of free peritoneal fluid is a less specific sonographic finding. Certain sonographic appearances have been proposed as predictors of the irreducibility of an intussusception. These appearances include a large amount of fluid within the intussusceptum and the presence of a doughnut-like appearance with a thick, echo-poor rim.

Color-flow Doppler images depict the viability of the bowel; therefore, the absence of flow suggests necrosis. This finding has been linked to the success rate of hydrostatic intussusception reduction. When blood flow is present, the success rate is in excess of 90%, but the rate decreases to only 30% when no blood flow is demonstrated.

The sonographic diagnosis of midgut malrotation is based on the inversion of the relationship of the superior mesenteric artery and vein; however, this finding is not sensitive or specific for this diagnosis. A hyperpulsatile superior mesenteric artery and a thickened and highly reflective small-bowel wall suggest volvulus.

With gallstone ileus, the classic triad of a calcified gallstone in an ectopic position, gas in the biliary tree, and small-bowel obstruction may be demonstrated.

Regarding intestinal ascariasis, individual worms may appear as hypoechoic longitudinal tubular structures with echogenic walls. If the alimentary tract of the worm is empty, the worm may appear as an echogenic line. If its alimentary tract is distended, the worm can appear as parallel hypoechoic bands. On transverse sections, the worm appears as a target sign with an echogenic body wall and a central dot representing its gut. Prolonged scanning may demonstrate movement of the worm. A tangle of worms mixed with feces and gas may appear as a complex intraluminal mass.

With a small-bowel lymphoma, eccentric mural thickening by lymphomatous infiltrate and aneurysmal dilatation of the bowel lumen and mesenteric lymphadenopathy may produce a so-called sandwich sign; this distinguishes lymphoma from Crohn disease. A lymphomatous infiltrate is usually anechoic, but hemorrhage and clotting may give rise to echogenic areas. Anechoic deposits may mimic duplication cysts.²³

In patients with jejunoileal atresia and meconium ileus, sonography may demonstrate dilated loops of bowel containing gas and fluid proximal to an atretic segment. Peristalsis may be reduced or increased. In meconium ileus, multiple loops of bowel are filled with highly echogenic material.²³

Degree of Confidence

Sonography is able to depict small-bowel obstruction well because intraluminal fluid is a natural contrast medium. Numerous studies have shown the value and effectiveness of sonography in the diagnosis of small-bowel obstruction, but its exact sensitivity remains uncertain. In patients with intussusception, the sensitivity is 95-100% with a positive predictive value of 100%. Because sonographic findings in patients with malrotation are nonspecific, barium examination remains the preferred study in patients in whom malrotation is suspected.¹⁵

False Positives/Negatives

When intraluminal air is associated with small-bowel obstruction, the air may obscure the sonographer's field of view; however, by paying attention to detail and using graded compression, an examination of all regions of the abdomen is possible. Scanning in the coronal plane through the flank may also be of value. The variation that may occur in peristalsis in patients with small-bowel obstruction may cause confusion. Occasionally, differentiating an ileus from an obstruction is difficult, particularly if peristalsis is reduced or absent in the bowel immediately proximal to an obstruction.

Fluid trapped within an intussuscepted mesentery can mimic duplication cysts or hematomas. The target appearance of small bowel referred to above is not specific and may occur in a multitude of inflammatory and neoplastic disorders. Vessel inversion can be found in patients with normal gut anatomy, and documented malrotations have occurred with normally related superior mesenteric vessels.

NUCLEAR MEDICINE

Section 8 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

Radionuclide scanning has no specific role in the diagnosis of small-bowel obstruction, with an exception for white blood cell scanning for the detection and localization of intra-abdominal inflammatory disease that may result in obstruction. Scanning for technetium-99m hexamethylpropyleneamine oxime (^99mTc HMPOA) uptake is useful for demonstrating disease activity in patients with Crohn disease.¹

Degree of Confidence

^99mTc HMPOA granuloscintigraphy has been shown to have a sensitivity of 79% and a specificity of 98% for small-bowel obstruction.

False Positives/Negatives

^99mTc HMPOA uptake is not specific for Crohn disease, and activity may be observed with other causes of inflammation or infection.

ANGIOGRAPHY

Section 9 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Findings

Superior mesenteric angiography has been used in the diagnosis of internal herniation, intussusception, volvulus, malrotation, and adhesions. Findings are of particular importance when vascular compromise is associated.

Degree of Confidence

Although angiography is a sensitive method for demonstrating vascular occlusion, it is invasive, and its role is limited in patients who are ill, particularly because alternative and less invasive techniques are available. Color Doppler imaging and CT angiography may provide the same information by alternative means.

INTERVENTION

Section 10 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Many patients with small-bowel obstruction respond to an initial trial of decompression of the stomach and small bowel by fasting and nasogastric aspiration, with intravenous fluid and electrolyte replacement. This treatment is particularly useful in patients in whom the etiology of the small-bowel obstruction is believed to be postoperative adhesions; in these patients, the obstruction is often only partial or incomplete. This treatment should be abandoned if clinical improvement does not occur fairly rapidly, as judged by the bowel sounds, the diminution of the nasogastric aspirate, the passage of flatus, and the resolution of any distention. Surgery is mandatory in these patients. The role of radiologic interventions in the management of small-bowel obstruction is limited and confined to the management of intussusception.

Most institutions choose to perform air-contrast enemas in the evaluation and treatment of intussusception. A barium enema reduction is an alternate choice. One may also choose to perform the reduction under sonographic guidance as opposed to fluoroscopic guidance. Regardless of the technique, reduction of intussusception has a high success rate, with minimal risks. The overall success rate is 80-90% with a complication rate of around 1%. Intestinal perforation is the gravest complication, but the most common complication is a recurrence of intussusception within 72 hours. A recurrent intussusception can be reduced again with an air enema.

Because of the risk of perforation or unsuccessful air enema reduction, the most important consideration in deciding to perform an air enema reduction is to ensure that a pediatric surgeon has examined the child and is readily available. In either case, the child will need to be transported the operating room. The child should be clinically stable, with adequate intravenous access. Contraindications to air reduction include evidence of free air on abdominal radiographs or signs of peritonitis on physical examination. A prolonged process increases the likelihood of ischemia, which decreases the rate of successful reduction and increases the likelihood of a perforation.

A commercial kit is available for performing air reduction (Shiels intussusception air reduction system). A tape plug and rubber disk around the enema tip help to create a tight seal. A tight seal is the key to a successful reduction. A generous amount of athletic-grade tape is necessary. The buttocks are used to create an adequate seal. If air leaks from the rectum, the procedure must be terminated and the enema tip reinserted. Alternatively, manual squeezing of the buttocks may help in minimizing the air leak.

The pressure in the colon should be monitored throughout the examination, and the pressure should not exceed 120 mm Hg. The child is not sedated for the examination because crying increases the intra-abdominal pressure and aids the reduction. The intussusception mass is visualized and, therefore, direct observation of the reduction can be achieved under fluoroscopic guidance. Complete reduction is confirmed with reflux of air into the small bowel. As an alternative, barium enema may be used for the reduction of intussusception. The barium bag should be elevated no more than 3 feet above the rectum to limit the colonic pressure. This leads to the "Rule of Threes," which states that only 3 attempts be made and that 3 minutes of rest should be allowed between each attempt.

Medical/Legal Pitfalls

• The most serious consequences of small-bowel obstruction are strangulation with ischemia, necrosis, perforation, peritonitis, and death. The prevalence of strangulated small-bowel obstruction is 5-42% of all patients. Expeditiously investigate clinical suspected strangulation because a delay may be lethal.
• The use of barium suspensions for enteroclysis or retrograde examination may cause inspissation of the small bowel and convert a partial obstruction to a complete obstruction; therefore, if CT is available, CT scans should be obtained as an alternative to conventional contrast-enhanced studies.
• The presence of a closed-loop obstruction and features of ischemia can be missed, even on CT scans. If an obvious discrepancy exists between CT and clinical findings in patients with obstruction, urgent surgery is mandatory.

Related Medscape topic:
Medical Malpractice and Legal Issues

FURTHER READING

Section 11 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Suspected small bowel obstruction.
American College of Radiology.  1996 (revised 2005).  5 pages.  NGC:004782

MULTIMEDIA

Section 12 of 13  

• Authors and Editors
• Introduction
• Differentials
• Radiograph
• CT SCAN
• MRI
• Ultrasound
• Nuclear Medicine
• Angiography
• Intervention
• Further Reading
• Multimedia
• References

Media file 1:  Plain abdominal radiograph in a 6-year-old boy who presented with vomiting and acute pain in the right iliac fossa. The loops of the small bowel are dilated, and associated with small calcific nodules in the right iliac fossa are seen. These findings are suggestive of appendicoliths. At laparotomy, an appendiceal mass was found; this caused the small-bowel obstruction.
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Media file 2:  Plain abdominal radiograph in a 9-year-old patient with a past history of appendicitis complicated by peritonitis who presented with abdominal pain and vomiting. The clinical diagnosis was small-bowel obstruction resulting from adhesions. This radiograph shows markedly distended loops of small bowel, with effacement of the valvulae in the mid abdomen. The child recovered with conservative treatment.
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Media file 3:  Lateral decubitus radiograph of a newborn who presented with features of bowel obstruction. The radiograph shows markedly distended loops of small bowel (same patient as in Image 4).
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Media file 4:  Plain abdominal radiograph of the abdomen in a newborn after 10 days of conservative treatment (same patient as in Image 3). The radiograph shows only mild dilatation of the loops of the small bowel. Note the fine grainy calcification in the right iliac fossa. Meconium peritonitis (arrow), caused by the small-bowel obstruction, was diagnosed.
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Media file 5:  An erect plain abdominal radiograph in a 4-year-old patient from a region in which roundworms are known to be endemic. The radiograph shows multiple dilated loops of small bowel, with air-fluid levels and tangled roundworms (arrow).
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Media file 6:  This plain abdominal radiograph of a 55-year-old woman presenting with features of intestinal obstruction shows dilated loops of the small bowel associated with thickened edematous valvulae conniventes and a strangulated left inguinal hernia (arrow).
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Media file 7:  Plain abdominal radiograph of the abdomen in a 72-year-old man with a history of intermittent abdominal pain over several years that settled spontaneously. On this occasion, his pain was severe. The abdominal radiograph shows dilated loops of small bowel, but the concentration of several loops of small bowel in the right hypochondrium should be noted as this finding is suggestive of malrotation. At surgery, a midgut volvulus caused by malrotation was found. The patient's age is unusual for those presenting with a midgut volvulus.
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Media file 8:  An upper GI barium series in a 32-year-old man with a history of intermittent small-bowel volvulus that lasted several years and settled spontaneously. The barium study shows the jejunal loops in the right hypochondrium; this finding is suggestive of bowel malrotation.
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Media file 9:  An erect plain abdominal radiograph of the abdomen in a 72-year-old woman that shows markedly distended jejunal loops with air-fluid levels. The air overlying the liver and a large laminated gallstone in the right iliac fossa (arrow) should be noted. The features are those of a gallstone ileus.
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Media file 10:  Plain abdominal radiograph of the abdomen in a 52-year-old woman presenting with features of small-bowel obstruction. At laparotomy, a cecal volvulus was found to be the cause of the obstruction. Note that the cecum occupies the middle of the abdomen and that the cecal pole is directed laterally.
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Media file 11:  A plain abdominal radiograph of the abdomen in a 36-year-old man shows features of mid small-bowel obstruction caused by a paraumbilical hernia (arrow).
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