Episode 1 SBO

Written By Learning Inthedark
 

Discover the 4 D’s of Radiology DETECT - DESCRIBE - DIFFERENTIAL - DECISION

DETECT

Pathophysiology.

Obstruction of the tube occurs from a number of different causes. Intraluminal pressure proximal to the obstruction increases and can lead to decreased venous flow from the bowel wall and mesentery. As pressure builds and approximates systolic pressure blood flow to the mucosa decreases, capillary rupture and hemorrhage can occur. The intestinal epithelium is sensitive to this lack of oxygen delivery and can lead to necrosis and perforation. Perforation can occur as a result of both the increased pressure and ischemia.

Clinical Presentation.

Symptoms: Colicky abdominal pain, distension, nausea and vomiting, constipation, inability to pass gas.

Signs: High pitched, tinkling or absent bowel sounds. Often non specific

Causes.

Top 3: Adhesions, Hernias, Malignancies

Framework:

  1. Intrinsic: Vascular (Ischemia, radiation enteropathy), IBD (Crohn’s disease), Neoplastic, Intussusception, Hematoma (Trauma, anticoagulants, blood dyscrasias)
  2. Extrinsic: Hernias, adhesions, endometriosis, hematomas
  3. Intraluminal: Gallstones, foreign bodies, bezoars (tightly packed collection of partially or undigested material)

Anatomy of the small bowel.

Small Bowel Wall Layers

  1. Mucosa
  2. Submucosa
  3. Muscularis (inner circular and outer longitudinal muscle)
  4. Serosa

Anatomy:

  • Duodenum:
    • Length: 20 – 25 cm
    • Arterial Blood Supply: Branches from gastroduodenal (off celiac access) and inferior pancreaticoduodenal artery (off SMA)
    • Venous Drainage: pancreaticoduodenal veins to the Superior Mesenteric Vein and then portal venous system
  • Jejunum:
    • Length: 2.5 meters
    • Arterial Blood Supply: Branches from SMA. SMA mesentery anchors the ileum and jejunum to the posterior abdominal wall.
    • Venous Drainage: Superior mesenteric vein to portal venous supply
  • Ileum:
    • Length: Distal 3 meters
    • Arterial Blood Supply: Superior Mesenteric Artery
    • Venous Drainage: Superior Mesenteric Vein

DESCRIBE

Classification Schema

Hallmark: Obstruction leading to dilation upstream and decompression downstream

Warning:

  1. Complete: nothing getting past
  2. Incomplete or Partial: some air and fluid post
  3. Closed Loop: Obstructed bowel at 2 points along the GI tract
  4. Strangulated obstruction

Imaging Workup

  1. Abdominal X -Ray
  2. CT with IV Contrast

Abdominal XRay

Variable sensitivity

Views and Findings

  1. Dependent (stuff that sinks): Supine or Prone
    • Dilated air or fluid filled small bowel loops > 3cm
    • Stretch Sign: Low density gas arranged in perpendicular stripes to the long axis of the bowel caused by gas in between the valvulae conniventes in a primarily fluid filled bowel.
    • Absence of rectal gas
    • Gasless abdomen
    • Dilated stomach
    • Dilated small bowel out of proportion to the colon (usually greater than 50% of the size of the colon)
    • Pseudo tumour sign: Obstructed, fluid filled, dilated small bowel
  2. Non-Dependent (stuff that rises): Upright or Antecubital
    • Air fluid levels > 2.5 cm in length
    • Air fluid levels at ≥2 different heights in the same loop of small bowel, 2 cm apart in vertical height
    • Multiple air fluid levels
    • String of beads sign: Fluid filled bowels with small gas pockets stuck between the valvulae conniventes

**More Common

***Ileus is associated with generalized distension also involving the large bowel. Ileus is not a high pressure system caused by blockage, just a lack of bowel movement.

Ultrasound

Utility

  • Dynamic
  • No radiation

Findings

  1. Dilation of proximal bowel segment > 3 cm
  2. Length of dilated segment ≥10 cm
  3. Increased peristaltic activity within the dilated segment
  4. Signs of infarction: Small bowel wall thickening > 3mm, free fluid between dilated segments, a peristaltic nature of dilated segments

When to use in the algorithm

  • CT is considered superior
  • US can be helpful in the pediatric population to diagnose intussusception, volvulus and other causes of SBO

CT Protocol

CT allows for visualization of the mucosa and surrounding structures – identify the location, complications and surgical indications

CT Abdo Pelvis

  • With or without IV contrast

Oral Contrast is not commonly used because:

  • Patients are often nauseous and vomiting
  • time delay waiting for 2-3 hours as contrast transits down GI tract
  • the low attenuation intraluminal fluid and gas provides a contrast already in comparison to the higher attenuating surrounding bowel wall
  • contrast does not always get to the transition point in high grade obstructions
  • If there is concern for a more proximal perforation beware as water soluble oral contrast agents should be used such as gastrograffin. Barium if used can cause localized mediastinitis

CT Findings

  1. IS THERE A BOWEL OBSTRUCTION?
    2. CT Criteria
    1. Dilated loops of small bowel >2.5cm (outer wall to outer wall) and colon not dilated (<6cm)
    2. Transition point
    3. Air fluid levels
    4. Colon decompressed
    CT Signs
    1. Small bowel fecalization – often located just proximal to the transition point
  2. WHERE IS THE OBSTRUCTION? - find transition point

    3. CT Approach:

    • Look for a change in bowel caliber with proximal dilatation and distal decompression
    • The presence of the small bowel feces sign can sometimes highlight the location of the transition point
  3. HOW SEVERE?

    4. CT Severity

    • High Grade Obstruction: 50% difference in caliber btwn proximal dilated bowel and distal collapsed bowel
    • Incomplete: When positive oral contrast is given – presence of contrast distal to the transition point
  4. WHAT IS THE CAUSE?

    5. Top 3 Causes:

    1. Adhesions: Fibrous tissue from inflammation often related to post-operative changes (open or laparoscopic) leading to abrupt obstruction, tapering and a transition point. Not often visible on CT, inferred when no other causes of obstruction are present.
    2. Hernias:
      • External Hernias: Dilated bowel going into hernia, decompressed bowel exiting hernia site. Locations are most common in the abdominal wall and inguinal canal.
      • Internal Hernias: Transmesnteric --> herniation due to congenital or acquired holes in the mesentery, common when surgery creates a Roux Limb. Paraduodenal --> Defect in the mesentery close in proximity to the duodenum leading to dilated small bowel loops between pancreatic body and posterior gastric wall. Usually towards the left (3/4 of the time). Mesenteric vessels are often crowded
    3. Malignancy: Either primary or metastatic. Concern with metastatic is that there are usually multiple metastases involved which cannot be surgically treated.
  5. WHAT ARE THE COMPLICATIONS?

    6. Complications

    1. Closed Loop Obstruction
      • What: stricture at 2 points along the course of the bowel. Stuck down bowel in between continues to secrete fluid and becomes progressively dilated decreasing venous return increasing ischemic risk
      • Risk Factors: Roux-en-Y surgeries, surgery involving disruption of the mesentery, congenital mesenteric defects
      • Risks: Strangulation and ischemia
      • CT Findings: Depend on the orientation and the length of small bowel involved
        • Within plane: U or C shaped
        • Orthogonal: 2 dilated areas close to a tethered stricture
        • Remember to look at the sagittal and coronal reformats, sometimes dilatation can be smaller in the obstruction upstream of the proximal end as compared to the bowel within the closed loop
        • Double beaked sign (two tapering ends towards the site of the obstruction)
        • Whirl Sign: Mesenteric vessels twisting around one another
    2. Ischemia
      • Clinical Significance: Immediate surgical referral – high rate of mortality. Clinical presentation can be variable and may not point towards an ischemic obstruction especially early on in its course
      • CT Findings:
        • Bowel wall thickening > 3 mm – edema, hemorrhage or both, often circumferential and can be up to 8mm in size
        • Mesenteric edema
        • Fluid in mesentery or peritoneal cavity
        • Occlusion of mesenteric vessels
        • Engorged mesenteric veins
        • Whirl sign: 60% sensitivity and 80% positive predictive value for SBO requiring surgical intervention
        • Bowel wall enhancement: Variable depending on use of IV contrast. Without IV contrast can see mucosal attenuation due to hemorrhage, with IV contrast may have relative decrease in enhancement due to poor vascular supply, targetoid appearance (enhancement of the mucosa and muscularis mucosa due to IV contrast with submucosal edema) or heterogeneous enhancement
        • Pneumatosis +/- gas in portal or mesenteric veins
    3. Perforation
      • Pathophysiology: Distension, poor venous return and compromised blood supply as the pressure from obstruction approaches systolic pressure leads to mucosal damage, ischemia and perforation
      • General CT Findings: You can see free intra-peritoneal fluid, blurred and smudged mesenteric fat, leakage of contrast (if oral contrast is given – specific finding but not sensitive), dirty fat, localized bowel wall thickening, air bubbles surrounding thickened segment
        • Ischemic: Thickened bowel wall, abnormal bowel wall enhancement, Pneumatosis intestinalis, portal venous gas, pneumoperitoneum (if on either side of the falciform ligament can indicate a more proximal obstruction), free fluid in the abdomen
        • Inflammatory: Ileal perforation associated with Crohn’s disease, with surrounding inflammatory mass or loop adhesions and possible abscess formation
      • Intra-operative: pus and fluid in the abdomen and the bowel can look edematous and dusky
    4. Volvulus
      • Definition: Bowel twisting on its mesentery
      • CT Findings:
        • Whirl sign: mesenteric vessels congested and twisted
        • Signs of malrotation: SMA/SMV opposite configuration – SMA to the right and SMV to the left, whirlpool sign, malrotated bowel configuration

DIFFERENTIAL

Diagnosis

Differential goes back to our common causes so break it down into intrinsic, extrinsic or intraluminal

Intrinsic

  • Vascular – radiation enteropathy or ischemia
  • Inflammatory: TB, Crohn’s, Eosinophilic gastroenteritis
  • Neoplasms: GIST, Lymphoma, or metastatic
  • Hematoma – trauma, dyscrasias, anticoagulation
  • Intussusception
  • Congenital

Extrinsic

  • Adhesions
  • Hernias
  • Neoplasms – metastatic peritoneal deposits
  • Hematomas
  • Endometriosis

Intraluminal

  • Foreign Bodies
    • Balloon tipped catheter
  • Gallstones
    • Rigler’s Triad: SBO, gallstone present within the lumen of the small bowel, biliary gas
  • Bezoars

DECISION

Treatment

Conservative: NG decompression, bowel rest. Surgical: Resection indicated in high grade obstruction, bowel ischemia, failure to improve with conservative management. Both clinical and radiological presentations can predict the need for surgical intervention

Clinical Predictors

  1. Clinical signs of intestinal ischemia: abdominal pain worse despite conservative management, SIRS, Peritonitis, Metabolic Acidosis
  2. Failure of conservative management

CT Indications for surgical management

  • Predictors of poor CT outcomes: 1.) Intraperitoneal free fluid 2.) Free intra-abdominal gas 3.) Duodenal distension 4.) High grade obstruction 5.) Signs of intestinal ischemia 6.) Closed Loop Obstruction
  • Surgical Correctable causes of SBO: 1.) Volvulus 2.) Incarcerated Hernia 3.) Closed Loop Obstruction 4.) Small Bowel Tumour 5.) gallstone ileus 6.) Foreign body ingestion
  • Signs of small bowel obstruction with equivocal outcomes in regard to conservative vs. surgical intervention: Mesenteric congestion, presence of a transition point, small bowel feces sign
 
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References

Paulson, E. K., & Thompson, W. M. (2015). Review of small-bowel obstruction: the diagnosis and when to worry. Radiology, 275(2), 332-342.

Silva, A. C., Pimenta, M., & Guimaraes, L. S. (2009). Small bowel obstruction: what to look for. Radiographics, 29(2), 423-439.

Collins, J. T., Nguyen, A., & Badireddy, M. (2017). Anatomy, abdomen and pelvis, small intestine.

Nicolaou, S., Kai, B., Ho, S., Su, J., & Ahamed, K. (2005). Imaging of acute small-bowel obstruction. American journal of roentgenology, 185(4), 1036-1044.

Reynolds J et al. (2017). Small Bowel. Netter Collection of Medical Illustrations: Digestive System: Part II – Lower Digestive Tract. Section 2, 31 – 114. Elsevier.

Kulaylat, M. N., & Doerr, R. J. (2001). Small bowel obstruction. In Surgical treatment: evidence-based and problem-oriented. Zuckschwerdt.

Zins, M., Millet, I., & Taourel, P. (2020). Adhesive small bowel obstruction: predictive radiology to improve patient management. Radiology, 296(3), 480-492.

Chang, W. C., Ko, K. H., Lin, C. S., Hsu, H. H., Tsai, S. H., Fan, H. L., ... & Chen, R. C. (2014). Features on MDCT that predict surgery in patients with adhesive-related small bowel obstruction. PLoS One, 9(2), e89804.

Suri, R. R., Vora, P., Kirby, J. M., & Ruo, L. (2014). Computed tomography features associated with operative management for nonstrangulating small bowel obstruction. Canadian Journal of Surgery, 57(4), 254.

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