Small Bowel Obstruction

Small bowel obstruction (SBO) is an interruption of the flow of the intraluminal contents through the small intestine, and is classified as mechanical (due to physical blockage) or functional (due to disruption of normal motility). The most common cause of SBO in the Western countries is post-surgical adhesions. Small bowel obstruction typically presents with nausea, vomiting, abdominal pain, distention, constipation, and/or obstipation. The diagnosis is established via imaging. Up to 80% of all cases will resolve with supportive management (bowel rest, intravenous (IV) hydration, and nasogastric decompression). However, surgery is required for persistent or complicated cases.

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Overview

Definition

Small bowel obstruction (SBO) is the interruption of the flow of intraluminal contents through the small bowel (duodenum, jejunum, or ileum).

Epidemiology

  • Average age of onset is 64 years.
  • Similar incidence in men and women
  • Accounts for 2%–4% of emergency department visits in the United States
  • Constitutes 80% of mechanical intestinal obstructions

Etiology

  • Mechanical:
    • Adhesions (post-surgical): 
      • Most common cause in the Western countries
      • Open surgery: 4-fold increase in the risk of SBO within 5 years
    • Tumors
    • Hernias
    • Crohn’s disease (inflammatory strictures or adhesions)
    • Gallstones (gallstone ileus)
    • Volvulus
    • Intussusception
    • Foreign body ingestion
  • Functional (also referred to as paralytic or adynamic ileus):
    • Surgery
    • Peritonitis
    • Trauma
    • Intestinal ischemia
    • Medications (opiates, calcium channel blockers, diuretics)
    • Electrolyte imbalance

Mnemonics

  •  3 most common causes of SBO: ABC
    • Adhesions
    • Bulge (hernias)
    • Cancer (neoplasm)
  • Causes of SBO: SHAVING
    • Stricture
    • Hernia
    • Adhesions
    • Volvulus
    • Intussusception/inflammatory bowel disease
    • Neoplasm
    • Gallstones
  • Causes of paralytic or adynamic ileus (5 Ps):
    • Postoperative
    • Peritonitis, pancreatitis
    • Potassium (low), Parkinson’s medication, painkillers
    • Pelvic and spinal fractures
    • Parturition

Pathophysiology

Classification

  • By etiology:
    • Mechanical: results from intrinsic (tumors) or extrinsic (adhesions, hernias) compression
    • Functional (ileus): 
      • Results from impaired motility 
      • Can be limited to the small bowel or include the colon as well
  • By degree of obstruction:
    • Complete: no passage of luminal contents beyond the obstruction point
    • Partial: Some intestinal contents pass through.
    • Closed loop: 
      • Segment of intestine is obstructed proximally and distally.
      • Seen in volvulus
      • Gas and fluid cannot escape → ↑ intraluminal pressure, ↑ risk of strangulation
      • Ischemia, necrosis, and perforation can develop rapidly.
  • By associated complication:
    • Simple: obstruction without ischemia
    • Complicated: Circulation is compromised, with associated ischemia, infarction, and/or perforation.

Pathophysiology

  • Early:
    • Intrinsic obstruction or extrinsic compression causes interrupted flow of bowel contents, leading to:
      • Accumulation of swallowed air and gas from bacterial fermentation
      • Progressive bowel distention (proximal to the obstruction point) leading to reduced intestinal motility
      • Fluid sequestration into the bowel lumen
      • Loss of normal intestinal absorption 
    • Emesis leads to loss of fluid and electrolytes (Na, K, H, Cl) → hypovolemia and metabolic alkalosis follow
  • Late:
    • Luminal flora changes → bacterial breakdown of stagnant contents results in feculent fluid
    • Persistent edema of the bowel wall → bowel ischemia from compression of intramural vessels
    • With ischemia → lactic acid accumulates → metabolic acidosis
    • Ischemic bowel can lead to gut translocation of bacteria, necrosis, and perforation.
Strangulated inguinal hernia

Gangrenous small bowel with closed-loop obstruction caused by an omental band adhesion

Image: “Intra-operative photograph” by the Department of Surgery, Hinchingbrooke Hospital, Hinchingbrooke Healthcare NHS Trust, Huntingdon, Cambridgeshire, UK. License: CC BY 2.0.

Clinical Presentation

Symptoms

  • Acute:
    • Abdominal pain:
      • Sudden onset (most of the time diffuse)
      • Frequently intermittent (colicky)
      • Can be temporarily relieved by vomiting
    • Severe constant pain may indicate ischemia or perforation
    • Nausea and bilious vomiting
    • Constipation
    • Obstipation (inability to pass flatus or stool)
  • Chronic:
    • Symptoms are usually milder. 
    • Post-prandial abdominal cramping/discomfort
    • Intermittent nausea/vomiting
  • Closed-loop obstruction:
    • Severe abdominal pain
    • Abdominal distention may be minimal.
    • May not have nausea or vomiting

Physical examination

  • Signs of dehydration (hallmark of SBO):
    • Tachycardia
    • Hypotension (orthostatic)
    • Low urine output
  • Abdominal exam:
    • Inspection: abdominal distention
    • Auscultation:
      • High-pitched bowel sounds (early SBO)
      • Decreased/absent bowel sounds (late or functional SBO)
    • Percussion: tympany or hyper-resonance; dullness if fluid filled
    • Palpation: 
      • Look for hernias and masses.
      • Tenderness on minimal palpation indicates possible peritonitis (indicative of complications such as ischemia or perforation).
  • Digital rectal exam:
    • To rule out fecal impaction as a cause of symptoms
    • Presence of blood would raise suspicion for a tumor.

Diagnosis

History

  • Risk factors for adhesions:
    • Prior abdominal surgeries (most important): ↑ risk of adhesions
    • Inflammatory conditions (Crohn’s disease, diverticulitis)
    • Radiation exposure
  • Other risk factors for mechanical SBO:
    • History of hernias
    • Foreign body ingestion
    • Cancer history
  • Risk factors for functional SBO:
    • Medications
    • Acute or chronic systemic illness (i.e. pneumonia)
    • Abdominal trauma
Post-surgery adhesion SBO

Adhesions that can develop post-surgery are the most common cause of bowel obstruction.

Image by Kevin Pei, PD.

Laboratory tests

  • Electrolyte abnormalities (commonly seen in acute onset):
    • Hyponatremia
    • Hypokalemia
    • High blood urea nitrogen (BUN)/creatinine
    • Hypochloremia
    • Metabolic alkalosis from vomiting (high bicarbonate)
    • Metabolic acidosis at late stages
  • Complete blood cell count (CBC):
    • Hemoconcentration
    • Leukocytosis:
      • Can be reactive
      • Can be indicative of complications
    • Anemia: if with associated etiology (Crohn’s, cancer)
  • Other:
    • Lactate: An elevated level is sensitive to bowel ischemia.
    • Blood cultures: in the setting of bacteremia

Imaging studies

  • X-ray (initial test of choice):
    • Abdominal X-ray performed in supine and upright or lateral decubitus positions
    • Performed with upright chest X-ray
    • Findings:
      • Proximal small bowel dilatation > 3 cm
      • Air-fluid levels (with stacked small bowel loops) in upright films (stepladder sign)
      • Paucity of air in the colon
      • Gasless abdomen (if all the small bowel loops are filled with fluid)
      • Ileus: can show diffuse small bowel and colon dilatation
      • Free air in case of perforation (under the diaphragm)
  • Computed tomography (CT) scan:
    • Intravenous (IV) contrast recommended (for signs of inflammation, ischemia)
    • Oral contrast is not necessary if high-grade or complete obstruction is suspected.
    • Helps identify:
      • Site of obstruction
      • Etiology (i.e. hernia, volvulus, tumor)
      • Severity (must exclude bowel compromise)
    • SBO findings:
      • Transition zone with dilation of proximal bowel and decompressed distal bowel 
      • Intraluminal contrast that does not go beyond the transition zone
      • Colon has little gas or fluid. 
    • Closed-loop obstruction findings: 
      • U-shaped or C-shaped dilated bowel loop 
      • Mesenteric vessels converging toward a torsion point
      • 2 collapsed bowel loops adjacent to the obstruction
    • Pneumoperitoneum (indicative of perforation):
      • Free air over the spleen or liver
      • Free air adjacent to the duodenum (retroperitoneum)
      • “Football sign”: supine CT
    • Strangulation/ischemia findings:
      • Thick bowel wall
      • Pneumatosis intestinalis (air in the bowel wall)
      • Portal venous gas
      • Mesenteric haziness or edema
      • IV contrast has poor uptake into the wall of the affected bowel.
      • Ascites
  • Small bowel follow-through:
    • Gastrografin (water-soluble contrast) is used in case perforation exists.
    • Generally inferior to a CT scan in acute settings but helpful in partial SBO
    • Cannot be used in bowel ischemia, pregnancy
    • Used as a diagnostic test:
      • Helps decide if surgery is still needed
      • Radiograph by 24 hours after administration: If the contrast reaches the colon, SBO is likely resolving without surgery.
    • Used as therapeutic agent in adhesive mechanical SBO: Hypertonic gastrografin reduces intestinal wall edema, thus helping peristalsis. 
  • Magnetic resonance imaging (MRI): if CT is contraindicated (children, pregnant women)

Management and Prognosis

Medical management

  • Management needed for ileus (functional SBO)
  • 75%–80% of adhesive mechanical SBOs resolve with medical management alone:
    • IV hydration
    • Correction of electrolyte abnormalities
    • Bowel rest and NPO (nil per os) status
    • Nasogastric decompression:
      • For vomiting and abdominal distention
      • Improves patient’s comfort
      • Can help improve/prevent further bowel dilatation
  • Antibiotics:
    • Not warranted in uncomplicated SBO
    • Given in bowel compromise and as a standard prophylaxis in cases where surgery is anticipated
    • Administered if etiology is an infectious cause

Surgery

  • Emergent:
    • Closed-loop obstruction
    • Intestinal ischemia/necrosis
    • Perforation
  • Surgically correctable causes:
    • Tumor
    • Foreign body/gallstone
    • Hernia
    • Intussusception/volvulus
  • For failure of medical management:
    • If SBO does not resolve in 72 hours on average
    • At any point if the patient shows clinical deterioration
  • Procedures (depend on specific etiologies):
    • Lysis of adhesions 
    • Hernia repair
    • Extraction of gallstone or foreign body
    • Small bowel resection:
      • Tumor
      • Ischemic bowel
      • Perforation
      • Crohn’s disease

Prognosis

  • < 5% perioperative mortality rate for non-strangulating SBO
  • Higher perioperative mortality rate for strangulating or complicated SBO
  • Probability of recurrence increases with each episode of SBO, until surgery is performed.

Differential Diagnosis

  • Irritable bowel syndrome (IBS): a functional bowel disorder without a clear anatomic abnormality. Irritable bowel syndrome can present with abdominal pain and constipation, but the sudden onset of symptoms is not a feature of IBS. Imaging and laboratory tests are usually normal in IBS.
  • Gastroenteritis: an acute self-limited illness usually caused by a virus. Gastroenteritis can also present with crampy abdominal pain, but the typical presentation also involves diarrhea (watery or bloody), which is infrequent with acute SBO.
  • Peptic ulcer disease (PUD): mucosal ulceration of the stomach or duodenum. Peptic ulcer disease presents with upper abdominal pain, nausea, and vomiting. Peptic ulcer disease is typically not associated with distention and constipation. Vomiting is usually non-bilious.
  • Appendicitis: inflammation of the appendix. Early appendicitis can present with diffuse colicky pain, but pain and tenderness eventually localize to the right lower quadrant. Diagnosis is established by a CT scan.
  • Pancreatitis: inflammation of the pancreas. Pancreatitis presents with epigastric pain, nausea, and vomiting. Blood work will show elevated amylase/lipase. A CT scan may show pancreatic inflammation.
  • Diverticulitis: inflammation of the colonic diverticula. Diverticulitis presents with crampy lower abdominal pain, many times with constipation. Diverticulitis frequently is also associated with fever and leukocytosis. A CT scan shows the characteristic findings.

References

  1. Bordeianou, L., & Yeh, D.D. (2019). Etiologies, clinical manifestations, and diagnosis of mechanical small bowel obstruction in adults. Retrieved 3 December 2020, from https://www.uptodate.com/contents/etiologies-clinical-manifestations-and-diagnosis-of-mechanical-small-bowel-obstruction-in-adults?search=bowel%20obstruction&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1
  2. Bordeianou, L., & Yeh, D.D. (2020). Management of small bowel obstruction in adults. Retrieved 4 December 2020, from https://www.uptodate.com/contents/overview-of-mechanical-colorectal-obstruction?search=bowel%20obstruction&source=search_result&selectedTitle=3~150&usage_type=default&display_rank=3
  3. Kulaylat, M.N., Doerr, R.J., Holzheimer, R.G., & Mannick, J.A. (Eds.) (2001) Small bowel obstruction. Surgical Treatment: Evidence-based and Problem-oriented. Munich: Zuckschwerdt. https://www.ncbi.nlm.nih.gov/books/NBK6873/
  4. Shah, V., & Gaillard, F. (2020). Paralytic ileus (mnemonic). Retrieved 5 Dec 2020, from https://radiopaedia.org/articles/paralytic-ileus-mnemonic
  5. Tavakkoli, A., Ashley, S.W., & Zinner, M.J. (2019). Small intestine. In Brunicardi, F., Andersen, D.K., Billiar, T.R., Dunn, D.L., Kao, L.S., Hunter, J.G., Matthews, J.B., & Pollock, R.E. (Eds.), Schwartz’s Principles of Surgery, 11e. McGraw-Hill.

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