Residency · Residency · Diagnostic Radiology

Bowel Obstruction: Small and Large Bowel

Small Bowel Obstruction (SBO)

Epidemiology and Causes

Small bowel obstruction accounts for approximately 15% of emergency department visits for acute abdominal pain. The most common cause is adhesions from prior surgery, responsible for 60-75% of cases. The second most common cause is hernias (inguinal, ventral, or internal), accounting for 10-20%. Other causes include neoplasm, Crohn disease, intussusception, gallstone ileus, foreign body, volvulus, and radiation enteritis.

Imaging Approach

The abdominal radiograph serves as an initial screening tool but has limited sensitivity at 60-70%. Findings include dilated small bowel loops exceeding 3 cm with air-fluid levels on the upright view, the string of pearls sign (small trapped gas bubbles in a line within fluid-filled loops), and paucity of distal colonic gas. CT of the abdomen and pelvis with intravenous contrast is the modality of choice, with sensitivity and specificity both exceeding 90%. CT identifies the transition point, the degree and cause of obstruction, and detects complications such as strangulation, closed-loop obstruction, and perforation.

CT Findings

The hallmark findings include dilated proximal small bowel exceeding 3 cm in diameter and decompressed distal small bowel and colon, creating a caliber discrepancy at the transition point. The transition point itself, defined as the abrupt change from dilated to decompressed bowel, is the key finding to identify and report. The small bowel feces sign, which is particulate material in dilated small bowel proximal to the obstruction, suggests prolonged obstruction. An adhesive band may be visible as a sharp angle or beak at the transition point with no identifiable mass.

Grading

In partial (low-grade) obstruction, contrast or air passes beyond the transition point and some distal bowel gas is present. In complete (high-grade) obstruction, no contrast or air passes the transition point and the distal bowel is decompressed. Complete obstruction is more likely to require surgical intervention.

Closed-Loop Obstruction

Definition and Significance

A closed-loop obstruction occurs when a segment of bowel is obstructed at two adjacent points, forming a closed loop. This configuration carries a high risk for strangulation and ischemia because the mesenteric vessels are compromised, making it a surgical emergency.

CT Signs

The characteristic findings include a U-shaped or C-shaped dilated loop with two ends converging toward a single point (the site of torsion or adhesion), a radial distribution of mesenteric vessels converging toward a central point (the whirl sign if twisted mesentery is present), the beak sign (tapered narrowing of the bowel at the point of obstruction), and two adjacent collapsed segments leading to the point of obstruction.

Strangulation

Definition

Strangulation represents compromise of the mesenteric blood supply to the obstructed bowel segment, leading to ischemia, necrosis, perforation, and peritonitis if untreated. Mortality increases significantly with delayed diagnosis.

CT Signs of Strangulation

Decreased or absent bowel wall enhancement indicates ischemia and is the most direct sign of vascular compromise. Increased bowel wall thickening from congestion and edema may be seen in early venous compromise. Mesenteric haziness and edema reflect engorgement and hemorrhage in the mesentery, while mesenteric vessel engorgement indicates venous congestion. Hemorrhagic ascites, defined as high-attenuation free fluid exceeding 20 HU between loops, is an ominous finding. Pneumatosis intestinalis, or gas within the bowel wall, is a late sign indicating necrosis. Portomesenteric venous gas, which is gas in the portal vein or mesenteric veins, is another ominous sign. Free intraperitoneal air indicates perforation.

Large Bowel Obstruction (LBO)

Causes

Colorectal carcinoma is the most common cause, accounting for 60% of cases. Volvulus, including sigmoid (most common) and cecal variants, is the second most common cause. Diverticular disease, specifically stricture from chronic diverticulitis, is another important etiology. Other causes include hernia, adhesions (less common than in SBO), fecal impaction, and extrinsic compression.

CT Findings

CT demonstrates a dilated colon (greater than 6 cm, with a cecum greater than 9 cm being at risk for perforation), a transition point with decompressed distal colon or rectum, and potentially a mass at the transition point such as the apple-core lesion of carcinoma. An important consideration is the competency of the ileocecal valve. A competent valve creates a closed-loop obstruction between the valve and the obstructing lesion, causing the cecum to become maximally dilated and at high risk for perforation. An incompetent valve allows retrograde decompression into the small bowel, lowering the risk of cecal perforation.

Volvulus

Sigmoid Volvulus

Sigmoid volvulus is the most common type of colonic volvulus, representing 70-80% of cases. It typically affects elderly, institutionalized, and neuropsychiatric patients, with chronic constipation being a predisposing factor. On the abdominal radiograph, the classic finding is the coffee bean sign, an inverted U-shaped dilated sigmoid loop arising from the pelvis and extending toward the diaphragm, with loss of haustral markings. CT findings include the whirl sign (twisted mesentery and vessels at the point of torsion), a markedly dilated sigmoid loop, the bird beak sign (convergence of afferent and efferent limbs at the twist point), and a transition point in the pelvis. Management involves endoscopic decompression with sigmoidoscopy if there are no signs of ischemia, followed by surgical resection for recurrent volvulus or if ischemia is suspected.

Cecal Volvulus

In cecal volvulus, the cecum and ascending colon twist on the mesentery. There are two types: axial torsion (true volvulus) and cecal bascule (anterior folding). CT findings include an ectopic cecum displaced to the left upper quadrant or midline, the whirl sign at the point of torsion, a dilated cecum, and small bowel obstruction. The coffee bean sign may also be seen, with its apex pointing toward the left upper quadrant. Management is surgical (right hemicolectomy), as endoscopic decompression is generally not effective.

Cecal Bascule

Cecal bascule involves anterior and superior folding of the cecum over the ascending colon without true axial twist. The dilated cecum is positioned anterior and superior to its normal location, and the condition may be intermittent.

Ileus vs. Mechanical Obstruction

Distinguishing Features on CT

Ileus (adynamic or paralytic) is characterized by generalized dilation of both small and large bowel with no transition point, no caliber discrepancy, and less prominent air-fluid levels compared to mechanical obstruction. The clinical context is important, as ileus most commonly follows surgery and is also associated with electrolyte abnormalities, medications (particularly opioids), and peritonitis. Mechanical obstruction, by contrast, shows a clear transition point with proximal dilation and distal decompression, along with prominent air-fluid levels at different heights (differential air-fluid levels on the upright film).

<image>An axial CT diagram showing the transition point in a small bowel obstruction due to an adhesive band. Dilated proximal small bowel loops (greater than 3 cm) filled with fluid are shown on the left side of the abdomen. The transition point is marked with an arrow showing an abrupt change to decompressed small bowel loops on the right. The beak sign is annotated at the transition point where the dilated loop tapers sharply. The colon is decompressed. A small callout box defines the key measurements: normal small bowel less than 3 cm, normal colon less than 6 cm, cecum at risk if greater than 9 cm.</image>

<image>A series of three images demonstrating closed-loop obstruction and strangulation on CT. Image 1: axial CT showing a U-shaped dilated loop of small bowel with two limbs converging toward a single point. Image 2: the whirl sign showing twisted mesenteric vessels and fat at the point of obstruction. Image 3: signs of strangulation including thickened non-enhancing bowel wall, mesenteric haziness with engorged vessels, and high-attenuation free fluid between loops representing hemorrhagic ascites. Each finding is labeled with arrows.</image>

<image>A comparison of sigmoid volvulus and cecal volvulus on abdominal radiograph and CT. The left panels show sigmoid volvulus: a radiograph with the classic coffee bean sign (massively dilated sigmoid loop rising from the pelvis toward the diaphragm) and an axial CT with the whirl sign and bird beak sign at the point of torsion in the pelvis. The right panels show cecal volvulus: a radiograph with a dilated cecum displaced to the left upper quadrant and an axial CT showing the ectopic dilated cecum with the whirl sign in the right lower quadrant. Key differentiating features are listed below each pair.</image>

Clinical Pearls

The transition point must always be identified and reported in bowel obstruction, as it determines the cause and guides surgical planning. Closed-loop obstruction is a surgical emergency due to its high risk of strangulation, and the radiologist should look for the U- or C-shaped dilated loop, whirl sign, and converging mesenteric vessels. CT signs of strangulation, including absent wall enhancement, mesenteric hemorrhage, pneumatosis, and portal venous gas, mandate urgent surgical consultation. In large bowel obstruction, the competency of the ileocecal valve must be assessed because a competent valve creates a closed-loop situation with the cecum at risk for perforation when it exceeds 9 cm. The coffee bean sign on abdominal radiograph can be seen in both sigmoid and cecal volvulus, and the location of the apex (toward the right upper quadrant in sigmoid volvulus, toward the left upper quadrant in cecal volvulus) helps differentiate them. Ileus should not be confused with mechanical obstruction, as ileus causes generalized bowel dilation without a transition point and typically follows surgery, medications, or metabolic derangement.

References

• Silva AC, et al. "Small Bowel Obstruction: CT Features and Clinical Correlation." RadioGraphics, 2009
• Peterson CM, et al. "Volvulus of the Gastrointestinal Tract: Appearances at Multimodality Imaging." RadioGraphics, 2009
• ACR Appropriateness Criteria: Suspected Small Bowel Obstruction, 2019

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