eMedicine Specialties > Radiology > Gastrointestinal

Small-Bowel Obstruction

Author: Ali Nawaz Khan, MBBS, FRCS, FRCP, FRCR, Consultant Radiologist, North Manchester General Hospital, The Pennine Acute NHS Trust, Manchester UK
Coauthor(s): Sumaira MacDonald, MBChB, PhD, MRCP, FRCR, Lecturer, Sheffield University Medical School; Endovascular Fellow, Sheffield Vascular Institute; John MT Howat, MB, BCh, MD, FRCS, Consultant General and Colorectal Surgeon, North Manchester General Hospital, UK
Contributor Information and Disclosures

Updated: Oct 20, 2008

Introduction

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)

Pathophysiology

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:

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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.

Presentation

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.

Differential Diagnoses

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.

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