Small Bowel Obstruction

Published on 18/07/2015 by admin

Filed under Radiology

Last modified 18/07/2015

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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.

image
(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

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