Small Bowel Obstruction: Causes and Imaging Techniques

Introduction:

Small bowel obstruction is the blockage of the small intestine. Small bowel (intestine) obstruction can be either congenital or mechanical. Small bowel obstruction is most commonly caused by adhesions after intestine surgery. Radiological imaging of the small intestine is helpful in the detection of location, cause, and degree of obstruction. The radiological investigation is useful when the clinical examination and laboratory findings are unreliable in the detection of small bowel obstruction. They can determine whether the obstruction needs laparotomy (surgical procedure performed through the abdominal wall) or nonsurgical management.

What Are the Causes of Small Bowel Obstruction?

Congenital:

• Jejunal atresia (a condition that causes complete obstruction of the proximal aspect of the small intestine in newborns).

• Ileal atresia or stenosis (a condition in which narrowing or blockage of the part of the ileum).

• Enteric duplication cyst (a fluid-filled sac found in the gastrointestinal tract during birth).

• Midgut volvulus (a condition in which the intestine becomes twisted and results in bowel obstruction).

• A mesenteric cyst (a fluid-filled sac in any part of the mesentery [a membrane that connects the intestine to the posterior abdominal wall]).

• Meckel diverticulum (a congenital condition in which failure in the vitelline duct's obliteration results in the small pouch's formation at the small intestine's lower part).

Mechanical:

• Hernias (bulging out of organs or fatty tissues through a weak part of the muscles or tissue) - Femoral and inguinal hernias are the most common cause of small bowel obstruction.

• Intussusception (a condition in which part of the intestine telescopes or slides into a nearby part of the intestine).

• Tumor.

• Crohn's disease (chronic inflammatory bowel disease).

• Gallstones.

• Foreign bodies.

• Fibrous adhesions from prior surgery (formation of fibrous tissue or scar in the intestine after an intestine surgery).

• Radiation enteritis (swelling of the lining of the small intestine induced by radiation).

• Intestinal ischemia (a condition that stops the blood flow to the intestine and causes thickening of the small bowel wall).

• Gallstone ileus (gallstone obstructs the lumen of the small intestine bypassing through cholecysto-enteric fistula).

• Meconium ileus (a condition in which stool becomes thick and sticky that causes intestinal obstruction in children).

What Are the Imaging Techniques Used in the Diagnosis of Small Bowel Obstruction?

Small bowel obstruction can be diagnosed with the help of the following imaging techniques. They are -

1. Abdominal radiograph.

2. Ultrasound.

3. Computed tomography (CT).

4. Barium follow-through and enteroclysis.

1. Abdominal Radiograph:

The abdominal radiograph is the first choice of imaging technique advised when the patient is suspected of having a small bowel obstruction. In small bowel obstruction, the abdominal radiograph shows the following radiological findings:

• Dilatation of small bowel loops proximal to the bowel obstruction (dilatation greater than 2.5 to 3 cm).

• The string of pearl signs - A sign of small bowel obstruction in which tiny gas bubbles are found over the superior wall of the small intestine and trapped between the folds of the small intestine [valvulae conniventes]. The lower margin of the bubbles has an oval appearance.

• Thickened bowel walls.

• Strangulation - A condition in which blood supply to the hernia is reduced, which appears like any of the following signs in radiograph:

• Presence of gas in the portal vein.

• Pneumatosis intestinalis (presence of free air within the walls of the intestine).

• Edematous folds (swelling of the intestinal folds).

• As abdominal radiograph shows only high-grade obstruction, additional images are advised.

2. Ultrasound:

Ultrasound shows the following radiological findings in small bowel obstruction.

• Ineffective peristalsis (muscular contraction of the digestive tract).

• Multiple dilated bowel loops greater than 3 cm.

• Fluid-filled loops of the small intestine.

• Obstruction by tumor (cancer) or hernia is visible.

• Bowel wall perfusion might be visual in Doppler ultrasound.

• Pneumatosis intestinalis.

• Prominent valvulae conniventes (folds of the small intestine).

3. Computed Tomography (CT):

Computed tomography shows the following radiology findings in small bowel obstruction.

• Noncontrast CT.

• Small bowel feces sign - The presence of fecal-like material inside the dilated lumen of the small intestine in small bowel obstruction.

• Dilated small bowel loops greater than 2.5 cm.

• Distally collapsed loops.

• Whirl sign - When the small intestine twists around the mesentery, it causes compression of the small bowel lumen and blood vessels of the bowel wall. It is also called the whirlpool sign.

• The string of pearl signs.

• Beak sign - Tapered obstruction of the intestine resembles the bird's beak.

• Computed tomography has 81- 94 percent sensitivity and 96 percent specificity for diagnosing high-grade obstruction. But their sensitivity and specificity decrease respectively by 64 and 79 percent when considering all grades of obstruction.

• CT is not an ideal imaging technique for a low-grade or subacute small bowel obstruction.

• Contrast-enhanced CT for intestinal ischemia has a sensitivity of more than 90 percent.

• Contrast CT.

Contrast-enhanced CT shows the following signs of small intestine obstruction:

• Thickened bowel wall.

• Ascites (collection of fluid in the abdominal cavity).

• Pneumatosis intestinalis and gas in mesenteric or portal veins.

• Whirl sign.

• Tortuous engorged (enlarged) mesenteric vessels.

• Mesenteric hemorrhage (loss of blood from the ruptured mesenteric blood vessels).

• CT enteroclysis.

When CT is unreliable in diagnosing small bowel obstruction, CT enteroclysis is used. CT enteroclysis is a new diagnostic tool that uses an enteroclysis tube for contrast administration to visualize the obstruction. It has 89 percent sensitivity and 100 percent specificity for diagnosing small bowel obstruction.

4. Barium Follow-Through and Enteroclysis:

Enteroclysis is the use of an enteroclysis tube to administer oral contrast into the intestine and bypass the stomach. Because of nausea and vomiting in acute bowel obstruction, patients have poor tolerance to oral contrast. In addition, water-soluble contrast (special dye) materials may be diluted in the extended fluid-filled bowel; thus, the degree of opacification becomes insufficient. The barium enema is helpful for the detection of distal colon obstruction. It may show the following radiological findings in small bowel obstruction:

• Snakehead appearance - In small bowel obstruction, there is a bulbous appearance proximal to the obstruction resulting from peristaltic movement.

• Beak sign.

• Dilated loops of the small bowel.

• It can detect only low-grade and intermittent bowel obstruction.

Conclusion:

Abdominal radiography is the initial radiograph recommended for the diagnosis of small bowel obstruction. An abdominal radiograph is readily available and economical. It has 71 percent sensitivity for high-grade small bowel obstruction. The sensitivity of ultrasound is more than abdominal radiographs by 89 percent in detecting small bowel obstruction. Computed tomography has increased sensitivity (81 to 94 percent) compared to abdominal radiographs in detecting high-grade small bowel obstruction. CT can determine the cause of obstruction and differentiate bowel obstruction from the ileus.