Small Bowel Obstruction

Published on 18/07/2015 by admin

Filed under Radiology

Last modified 22/04/2025

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 Air-fluid levels on upright or decubitus radiograph

• Transition zone between dilated and collapsed bowel is critical to define presence, site, and cause of obstruction 

image All better determined on CT than on plain films (accuracy near 100% for high-grade SBO)
• Small bowel feces  sign: Gas bubbles mixed with particulate matter in dilated loops just proximal to site of obstruction
• “Positive” oral contrast medium for CT is rarely useful
• Closed loop obstruction

image SB segments are markedly distended (> 4 cm) by fluid, little gas
image Whirl sign due to tightly twisted mesenteric vessels
image “Balloons-on-strings”:  Dilated SB tethered by stretched mesenteric vessels
• Strangulating SBO: Impaired blood supply to SB

image Absent, decreased, or delayed bowel wall enhancement
image Bowel wall thickening (edema or hemorrhage)
image Mesenteric and interloop edema ± ascites
image Vessels: Congested, thrombosed, or obscured
image Obscured margins among affected SB segments

TOP DIFFERENTIAL DIAGNOSES

• Adynamic or paralytic ileus
• Aerophagia
• Colonic obstruction
• Cystic fibrosis

CLINICAL ISSUES

• Most common causes: Adhesions (∼ 60%), hernias (15%), tumors (∼ 15%; metastases > primary tumor)
• Up to 80% of adhesive SBOs resolve spontaneously
• Mortality > 25% if symptoms persist and surgery postponed > 36 hours
• Mortality is 100% for untreated strangulated SBOs

DIAGNOSTIC CHECKLIST

• CT diagnosis of closed loop or strangulated (ischemic) SBO is crucial for directing prompt surgical intervention
image
(Left) Anteroposterior graphic depiction of a small bowel obstruction (SBO) due to an adhesive band. Note the dilation of the proximal small bowel image, as well as the adhesive band image.

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(Right) In this patient with abdominal pain, distention, and nausea, a supine film of the abdomen shows no obvious dilation of small bowel (SB).
image
(Left) An upright film in the same patient shows a string-of-pearls sign image, indicating gas within fluid-distended, obstructed segments of SB.

image
(Right) Axial CT section in the same patient shows collapsed distal SB image, but massive dilation of proximal SB segments image with only small bubbles of intraluminal air image, accounting for the string-of-pearls sign. An adhesive SBO was confirmed at surgery.

TERMINOLOGY

Abbreviations

• Small bowel obstruction (SBO)

Definitions

• Obstruction or blockage of ≥ 1 SB segments by intrinsic or extrinsic narrowing of SB lumen

IMAGING

General Features

• Best diagnostic clue

image Identification of transition zone between dilated and collapsed bowel is critical to define presence, site, and cause of obstruction (all better determined on CT than on plain films)
• Size

image Small bowel loops > 3 cm diameter on radiographs, 2.5 cm on CT (magnification effect on plain films)

Radiographic Findings

• Radiography

image Supine abdomen with upright or decubitus views

– Dilated SB loops with air-fluid levels on upright or decubitus radiograph
image Can miss SBO (fluid-distended bowel not evident on plain films)
image String-of-pearls sign: Small air bubbles within fluid-distended bowel seen on supine view

Fluoroscopic Findings

• Enteroclysis or SB series

image Passage of enteric contrast into colon excludes complete SBO
image Transition may define location, degree, cause of obstruction

– e.g., angulated segment with distortion of folds suggests adhesive SBO

CT Findings

• Dilated SB loops > 2.5 cm diameter ± air-fluid levels
• Small bowel feces sign: Gas bubbles mixed with particulate matter in dilated loops just proximal to site of obstruction
• Extrinsic lesions

image Adhesions

– At transition, angulation of course of SB, minimal mural thickening
– Adhesions themselves are not identified on CT
– Adhesive SBO is diagnosis of exclusion; no hernia or mass
image Hernia

– External hernias (inguinal, femoral, Spigelian, obturator, etc.)

image Most common type of hernia to cause SBO
– Internal hernia: Cluster of dilated SB segments; crowding, twisting, displacement of mesenteric vessels
– Dilated segment of SB leading into hernia; collapsed segment leaving hernia
– Strangulated hernia: Thickened bowel wall ± intramural hemorrhage
image Peritoneal carcinomatosis: Omental and peritoneal masses, dilated bowel loops, multiple transition zones

– Metastases may cause luminal obstruction or functional obstruction due to serosal coating (impairs peristalsis)
image Other inflammatory causes (appendicitis, diverticulitis, etc.)
• Intrinsic lesions

image Malignant tumor (adenocarcinoma, GIST, carcinoid, etc.)

– Thickened enhancing wall and luminal narrowing at transition zone
image Crohn disease

– Mucosal hyperenhancement, submucosal edema over long segment of distal SB
image Intussusception

– Bowel-within-bowel
– Layers of bowel wall interspersed with mesenteric fat and vessels
image Other infectious, ischemic, or inflammatory

– e.g., radiation or ischemic stricture, tuberculous enterocolitis
• Intraluminal lesions: Gallstones, foreign bodies, bezoars, Ascaris

image Classic triad: Ectopic calcified stone and gas in gall bladder/biliary tree and SBO = gallstone ileus
image Bezoar: Intraluminal mass with air in interstices at point of transition
• Closed loop obstruction: Obstruction at 2 points, involves mesentery

image Affected SB segments are markedly distended (> 4 cm) by fluid, little gas
image Relatively little dilatation of bowel proximal to closed loop obstruction
image Stretched mesenteric vessels converging toward site of torsion
image Beak sign: Fusiform tapering at point of torsion/obstruction
image Volvulus: C-shaped, U-shaped, or “coffee bean” SB configuration

– Whirl sign due to tightly twisted mesenteric vessels
– “Balloons-on-strings”: Appearance of dilated SB tethered by stretched mesenteric vessels
• Strangulating SBO: Blood flow to affected SB is blocked

image Absent, decreased, or delayed bowel wall enhancement in affected SB
image Bowel wall thickening (edema or hemorrhage)

– High density of SB wall on NECT = hemorrhage = ischemia
image Mesenteric and interloop edema ± ascites
image Combination of factors obscures margins among affected SB segments
image Mesenteric vessels: Congested, thrombosed or obscured by adjacent edema

Imaging Recommendations

• Best imaging tool

image Multiplanar CECT: Sensitivity 95%, specificity 96% in high-grade SBO

– CT evaluation is mandatory in patients with

image Suspected bowel ischemia
image Suspected abdominal sepsis
image History of inflammatory bowel disease
image Known or suspected cancer
image Enteroclysis: Most sensitive test for patients with intermittent, low-grade SBO
• Protocol advice

image Administration of positive oral contrast medium is rarely usefu l and may impede CT detection of complications of SBO, such as ischemia

DIFFERENTIAL DIAGNOSIS

Adynamic or Paralytic Ileus

• Dilated SB and colon with no transition point
• Ileus plus ascites mimics SBO

image Gas collects in nondependent SB and transverse colon
image Minimal gas in dependent colon and rectum

Aerophagia

• Excessive air swallowing: Belching, flatulence, distention
• Excess gas in stomach, SB, and colon without much dilation or air-fluid levels

Colonic Obstruction

• Dilation of colon due to mechanical causes (cancer, volvulus)

Cystic Fibrosis (CF)

• “DIOS”: Distal intestinal obstruction syndrome
• Functional obstruction of SB due to thick, viscous bowel contents
• Small bowel feces sign in distal ileum
• Look for lipomatous pseudohypertrophy of pancreas, lung damage from CF

PATHOLOGY

General Features

• Etiology

image Most common: Adhesions (∼ 60%), hernias (15%), tumors (∼ 15%; metastases > primary tumor)
image Extrinsic lesions: Adhesions, external and internal hernias, tumor, abscess, aneurysm

– Adhesions: Postsurgical, inflammatory, congenital
image Intrinsic lesions: Tumors, inflammatory, vascular (ischemic), metabolic, radiation induced
image Intraluminal lesions: Gallstones, bezoars, foreign bodies, Ascaris
image Pathogenesis: Obstruction of SB → proximal dilatation due to accumulation of GI secretions and swallowed air

– Bowel dilatation stimulates secretory activity resulting in increased fluid accumulation
image Etiology of ischemia in SBO

– Twisting and occlusion of mesenteric vessels (common in closed loop SBO)
– Luminal distention causes occlusion of small vessels in SB wall

Staging, Grading, & Classification

• Classification based on degree of obstruction

image Simple

– Intermittent, incomplete, or partial low-grade obstruction
– Prolonged, complete, or high-grade obstruction
image Complicated

– Closed loop or incarcerated obstruction: Adhesive bands > internal or external hernia
– Strangulation: Most common complication of closed loop obstruction, indicates vascular compromise

Gross Pathologic & Surgical Features

• Dilated proximal loop, distal collapsed loop, and transition point

CLINICAL ISSUES

Presentation

• Most common signs/symptoms

image Variable: Mild to severe abdominal pain with vomiting

– Abdominal distention, tenderness, guarding
– Bowel sounds high pitched (early) or absent (late sign)

Demographics

• Epidemiology

image Accounts for 20% of acute abdomen presentations

Natural History & Prognosis

• Complications

image Bowel strangulation, infarction, gangrene, perforation, peritonitis, and sepsis
• Prognosis

image Up to 80% of adhesive SBOs resolve spontaneously
image Mortality > 25% if symptoms persist and surgery postponed > 36 hours
image Mortality decreased to 8% if surgery performed ≤ 36 hours
image Mortality is 100% for untreated strangulated (ischemic) SBOs

Treatment

• Nasogastric suction, decompression, IV fluids
• Nonoperative treatment for incomplete SBO with early resolution of symptoms
• Immediate surgery for complete SBO or closed loop obstruction

DIAGNOSTIC CHECKLIST

Consider

• CT diagnosis of closed loop or strangulated (ischemic) SBO is crucial for directing prompt surgical intervention

Image Interpretation Pearls

• Know the CT signs for closed loop or ischemic SBO
•  

image
(Left) This patient had a prior surgical history and now presents with nausea and vomiting. Axial CT shows the dilated proximal SB image and decompressed bowel image distal to the transition site.
image
(Right) A coronal-reformatted CT image in the same patient shows the transition image from dilated to collapsed SB. No mass, hernia, or other specific cause was identified, leading to the inference that this was an adhesive SBO, subsequently confirmed at surgery.
image
(Left) This elderly woman has abdominal pain, distention, and vomiting. Axial CT shows markedly dilated proximal SB segments image, while distal SB and colon are decompressed image.

image
(Right) A more caudal CT section in the same patient shows dilated SB image entering a right femoral hernia.
image
(Left) A more caudal CT section in the same patient shows a “knuckle” of SB image trapped within the femoral hernia, establishing this as the transition point and etiology of the SBO.

image
(Right) A coronal-reformatted CT section in the same patient shows the transition point, as the dilated SB image enters the right femoral hernia image. Hernias are the 2nd most common etiology for SBO, though much less common than adhesive SBO.

image
(Left) This woman has metastatic ovarian carcinoma and has had prior bowel resection and ileostomy for SBO. CT shows the ileostomy image and SB segments that are matted together image without definable mesenteric fat between SB segments.
image
(Right) Another CT section in the same patient shows SB loops that are encased and angulated image due to serosal implants of recurrent ovarian carcinoma. Almost 1/2 of all women with ovarian carcinoma will develop symptoms of SBO.
image
(Left) This young man has cystic fibrosis with recurrent abdominal pain and distention. CT shows contrast material in decompressed colon image, the result of prior attempts at colon cleansing by water-soluble contrast enema. The SB is markedly distended with a small bowel feces sign in the distal ileum image. This is a classic example of DIOS (distal intestinal obstruction syndrome) in a patient with cystic fibrosis.

image
(Right) A coronal image in the same patient shows the collapsed colon and the dilated SB with DIOS image.
image
(Left) This young man has longstanding Crohn disease with progressive abdominal distention and pain. Axial CT shows massive dilation of some SB segments, some of which have air-fluid levels image. The colon image is decompressed.

image
(Right) Coronal CT section in the same patient shows the dilated, more proximal SB image with the transition point being more distal SB, featuring mucosal hyperenhancement, submucosal edema, and luminal narrowing image. Mesenteric lymphadenopathy image is another typical feature of Crohn disease.

image
(Left) This elderly man has abdominal distention and pain. A supine film of the abdomen shows moderate gas dilation of proximal SB segments image, but minimal gas in the distal SB and colon. Instead, there is a suggestion of a lower abdominal mass image.
image
(Right) Axial NECT in the same patient shows dilated mid SB segments image that are almost entirely fluid-filled with little gas, accounting for the mass seen on the supine film. Mesenteric vessels image to these segments are engorged and crowded with blurred margins. Ascites is present image.
image
(Left) Another CT section in this patient shows a whirl sign image, twisting of the SB mesenteric vessels.

image
(Right) Another CT section in this patient shows poor definition of the margins among the dilated SB segments image, owing to infiltration of the SB mesentery and localized ascites between the affected SB loops. The mesenteric vessels image are acutely angulated.
image
(Left) A coronal CT section in the same patient shows a classic “balloons-on-strings” appearance of the fluid-distended SB segments image tethered by their stretched mesenteric vessels.

image
(Right) A sagittal CT section in the same patient also demonstrates the “balloons-on-strings” appearance image, with the “strings” being the mesenteric vessels and the “balloons” the SB segments within a closed loop SBO. This patient has many classic CT features of a closed loop SBO with ischemia, confirmed at surgery.
image
Axial CECT shows closed loop small bowel obstruction. Note the U-shaped appearance of dilated ileum image and adjacent intraloop ascites image. Bowel wall enhancement suggests the viability of bowel.

image
Axial CECT at more cranial level in the same patient demonstrates the “whirl” sign of mesenteric vessels image.
image
Axial CECT of SBO from a study performed with oral contrast illustrates dilated unopacified loop with the “small bowel feces” sign image.
image
Axial CECT at a more caudal level in the same patient demonstrates collapsed distal bowel loops image.
image
Axial CECT shows dilated small bowel loops image extending to ventral hernia, incarcerated loops within a hernia sac image, and decompressed loops image distal to obstruction.
image
Coronal CECT in the same patient shows denser oral contrast in the proximal bowel of LUQ image, less dense in distal obstructed segments image, reflecting slow transit time, and contrast dilution secondary to fluid-filled segments in SBO.
image
Frontal scout film from CECT shows dilated small bowel image disproportionate to colon image.
image
Axial CECT shows massively dilated fluid-filled small bowel loops image. Note the decompressed colon image. At surgery, a chronic stricture was resected.
image
Upright radiograph shows dilated small bowel with air-fluid levels and no colonic gas.
image
Supine radiograph shows dilated small bowel and no colonic gas. Note the surgical clips in pelvis as clue to prior surgery and probable adhesions.
image
Small bowel follow through shows barium distended proximal to the small bowel, collapsed distal bowel, and colon.
image
Coronal reconstruction of CECT shows diluted fluid-filled small bowel (not evident on supine radiographs). Note the acutely angulated distal SB image due to adhesions, which caused SBO.
image
Axial CECT shows abrupt transition image from dilated to nondilated bowel. This small bowel obstruction was due to adhesions.
image
Axial CECT shows Spigelian hernia image with dilated bowel image entering through the defect and collapsed bowel image exiting the hernia.
image
CECT in a patient with closed loop obstruction due to midgut volvulus shows fluid-distended small bowel, ascites, and twisting of the root of the mesentery image.
image
CECT in a patient with closed loop obstruction shows a cluster of dilated fluid-distended loops of bowel image with mesenteric infiltration and blurred engorged blood vessels. Plain radiographs were “normal.”

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