Bowel obstruction

Bowel obstruction is the interruption of the normal passage of bowel contents either due to a functional decrease in peristalsis or mechanical obstruction. Functional bowel obstruction, or paralytic ileus, is a temporary disturbance of peristalsis in the absence of mechanical obstruction. Postoperative ileus is the most common cause of paralytic ileus, which can also be caused by metabolic disturbances (e.g., hypokalemia), endocrinopathies (e.g., hypothyroidism), and certain drugs (e.g., anticholinergics). Mechanical bowel obstruction is classified according to the location as either small bowel obstruction (SBO) or large bowel obstruction (LBO) and, depending on the severity of obstruction, as either partial or complete. The most common cause of SBO is postoperative bowel adhesions, while the most common cause of LBO is malignant tumors. Regardless of the cause, bowel obstruction typically manifests with nausea, vomiting, abdominal pain, abdominal distention, and constipation or obstipation. In paralytic ileus, bowel sounds are usually absent on auscultation, whereas a high-pitched tinkling sound would be heard in the early phase of a mechanical bowel obstruction. Bowel distention leads to third-space volume loss, resulting in dehydration and electrolyte abnormalities. Symptoms are less severe in partial bowel obstruction. Diagnosis is confirmed on imaging with contrast-enhanced CT scan and abdominal x-rays. Typical findings in mechanical bowel obstruction include dilated bowel loops proximal to the obstruction, collapse of bowel loops distal to the obstruction, and, on contrast-enhanced imaging, a cut-off or transition point at the site of obstruction. In paralytic ileus, findings include generalized dilatation of bowel loops with no transition point and air that is visible in the rectum. Additional laboratory tests include CBC and BGA for the assessment of infection, electrolyte imbalances (e.g., hypokalemia), and metabolic imbalances (e.g., alkalosis). Surgical intervention (i.e., exploratory laparotomy) is recommended for suspected closed-loop bowel obstruction, if there are signs of perforation or peritonitis, or if there is no improvement following conservative management. In all other cases, conservative treatment is usually successful and involves bowel rest, gastric decompression (nasogastric suction), fluid resuscitation, and correction of electrolyte abnormalities.

References:^[1][2][3][4]

Bowel obstruction

An interruption in the normal passage of bowel contents.

Etiology

• Mechanical bowel obstruction: an interruption in the normal passage of intestinal contents due to a structural barrier (e.g., bowel cancer, adhesions)
• Paralytic ileus: a temporary impairment of peristalsis in the absence of a mechanical obstruction

Degree of obstruction

• Partial bowel obstruction: bowel obstruction in which passage of some intestinal content through the blocked segment is possible
• Complete bowel obstruction: total interruption of the passage of intestinal contents
• Closed loop obstruction: a type of complete mechanical bowel obstruction in which a segment of bowel is occluded at two contiguous points (e.g., volvulus)

Site of obstruction

• Small bowel obstruction (SBO): obstruction occurring in the duodenum, jejunum, or ileum
• Large bowel obstruction (LBO): obstruction occurring in the cecum, colon, or rectum

Progression

• Simple bowel obstruction: obstruction without evidence of bowel ischemia
• Strangulated bowel obstruction: obstruction with compromised intestinal blood flow, resulting in bowel ischemia

Reference:^[5]

Etiology ^{[1] [6]}

Pathophysiology ^[7]

• Bowel obstruction → stasis of luminal contents and gas proximal to the obstruction → ↑ intraluminal pressure, which leads to the following:
  • Gaseous abdominal distention → sequestration of fluids within the distended bowel loops (third spacing) → dehydration and hypovolemia
  • Vomiting → loss of fluid and Na⁺, K⁺, H⁺, and Cl^- → hypokalemia, metabolic alkalosis, and hypovolemia
  • Compression of intestinal veins and lymphatics → bowel wall edema → compression of intestinal arterioles and capillaries → bowel ischemia
    • → ↑ Bowel wall permeability → translocation of intestinal microbes to the peritoneal cavity → sepsis
    • → Necrosis and perforation of the bowel wall → peritonitis
    • → Anaerobic metabolism and lysis of ischemic cells → accumulation of lactic acid and release of intracellular K⁺→ metabolic acidosis and hyperkalemia

Clinical features ^[4][8][9]

Partial bowel obstruction causes gradually progressive symptoms that are milder than those of complete obstruction. Obstipation is absent in partial bowel obstruction.

Diagnostics

In the workup of suspected mechanical bowel obstruction, imaging allows for quick confirmation of the diagnosis as well as detection of conditions requiring immediate surgery (e.g., perforation). Laboratory tests may further help to assess the severity of the condition (e.g., electrolyte imbalance due to vomiting).

Laboratory tests

• If recurrent vomiting
  • Hypochloremic hypokalemic metabolic alkalosis
  • Hyponatremia
• If bowel strangulation
  • Metabolic acidosis
  • Hyperkalemia
• Neutrophilic leukocytosis (left shift)
• If dehydration: ↑ Hct
• If sepsis: abnormal coagulation profile
• Potentially prerenal azotemia

Imaging ^{[10][11][12][13]}

Abdominal series

Consists of erect and supine abdominal x-rays and an erect chest x-ray.

• Indication: Best initial test in hemodynamically unstable patients or in resource-poor health centers
• Findings
  • Dilatation of bowel loops proximal to the obstruction
    • 3-6-9 rule to define bowel dilatation on imaging
      • Small bowel dilatation if > 3 cm
      • Large bowel dilatation if > 6 cm
      • Cecal dilatation if > 9 cm
    • In SBO: The dilated loops are predominantly central.
    • In LBO (esp. distal LBO): The dilated loops are predominantly peripheral.
  • Minimal/no air within the bowel loops distal to the obstruction
  • Stepladder sign (x-ray)
    • Multiple air-fluid levels and a stacked appearance of dilated small bowel loops
    • Best seen on an erect abdominal x-ray

CT abdomen and pelvis

More sensitive than x-ray

• Indications
  • With IV and oral contrast: Best initial test in hemodynamically stable patients with suspected partial bowel obstruction ^{[14][15] [16]}
  • With IV contrast: Indicated in patients with suspected complete bowel obstruction.
  • Non-contrast: Indicated in patients with contrast-allergy and suspected complete bowel obstruction.
• Findings
  • Transition point: sudden narrowing of bowel lumen at the site of obstruction
  • Dilatation of proximal loops
  • Signs of bowel ischemia
    • Unenhanced bowel loops
    • Pneumatosis intestinalis
    • Mesenteric fat stranding
  • Pneumoperitoneum indicates bowel perforation

MRI abdomen and pelvis (with and/or without IV contrast)^{[17] [16]}

• Indication: patients who have a contraindication for radiation exposure
• Findings: similar to CT

Abdominal ultrasound

• Indication: critically ill patients (easy bedside test) or patients with a suspected SBO and a contraindication for CT (e.g., contrast allergy) or radiation exposure (e.g., pregnancy)
• Findings
  • Stepladder sign (ultrasound): prominent plicae circulares of dilated small bowel loops
  • Pendular peristalsis: dysfunctional “to-and-fro” peristalsis
  • Keyboard sign: prominent plicae circulares within the dilated bowel loops
  • Thickening of the bowel wall

Barium or water-soluble contrast enema

• Indication: in suspected distal LBO
• Findings
  • Tapering of bowel lumen at the site of obstruction
    • Complete bowel obstruction: contrast would not be visible beyond obstruction
    • Partial bowel obstruction: a trickle of contrast would be visible beyond obstruction
  • Bird beak sign seen in volvulus
  • Apple core sign seen in colonic malignancy

When imaging with contrast (CT, enema) and perforation is expected, use water-soluble oral contrast.

Treatment

Conservative management

• Indications
  • Partial bowel obstruction cases
  • Complete bowel obstruction with no signs of ischemia/necrosis or signs of clinical deterioration
• Measures
  • Fluid resuscitation, correction of electrolyte imbalance
  • Intestinal decompression: nasogastric tube insertion
  • Bowel rest (NPO)
  • Administration of IV analgesics and antiemetics
  • Gradual increase of oral intake, starting with clear fluids, can be initiated once the abdominal pain and distention subside and bowel sounds return to normal.
• Etiology-specific treatments
  • Fecal impaction: stool evacuation (manual disimpaction, distal softening/washout with enemas or suppositories, proximal softening/washout with oral solutions such as polyethylene glycol or sodium phosphate)
  • Sigmoid volvulus with no signs of strangulation: rigid/flexible sigmoidoscopic detorsion

Peristalsis-inducing medication (e.g., metoclopramide) is contraindicated in complete mechanical bowel obstruction.

Surgery

• Indications
  • Suspected bowel obstruction and hemodynamic instability or features of sepsis
  • Complete bowel obstruction with signs of ischemia/necrosis or clinical deterioration
  • Persistent partial obstruction (> 3–5 days)
  • Closed-loop obstruction
• Procedure: exploratory laparotomy
  • Restoration of intestinal transit: depends on intraoperative findings
  • If bowel resection is required, the intervention may be carried out in a single procedure with anastomosis or permanent ostomy creation, or in a multistaged procedure with a temporary diverting ostomy.

Bowel obstruction requires a swift workup to establish whether emergent surgery is necessary!

Prognosis

• 100% mortality in cases of untreated intestinal strangulation
• Mortality rate for those undergoing surgery: 8–25%
• High risk of recurrence, particularly with chronic or recurring etiologies (Crohn disease, adhesions, radiation enteritis, volvulus, etc.)

References:^{[10][11][19][20][21]}

• Differential diagnosis of bowel obstruction
  • Bowel perforation (secondary peritonitis)
  • Mesenteric ischemia
  • Inflammatory bowel disease
  • Ovarian torsion
• Differential diagnosis of SBO
  • Acute appendicitis
  • Acute pancreatitis
  • Pelvic inflammatory disease
• Differential diagnosis of LBO
  • Diverticulitis
  • Toxic megacolon
  • Chronic megacolon
• See differential diagnoses of acute abdomen.

References:^[1][2][3][4]

The differential diagnoses listed here are not exhaustive.

• Bowel ischemia
• Bowel perforation
• Peritonitis

A change in the character of pain (colicky pain becoming continuous), rebound tenderness on examination, and/or signs of sepsis in a patient with bowel obstruction indicate the onset of complications and necessitate emergency surgical intervention!

We list the most important complications. The selection is not exhaustive.

Definition

• Paralytic ileus: temporarily impaired peristalsis of the gastrointestinal tract in the absence of mechanical obstruction

Etiology

• Intra-abdominal surgery (postoperative ileus)
• Abdominal trauma; (e.g., due to retroperitoneal hemorrhage)
• Endocrine abnormalities (e.g., hypothyroidism, porphyria, uremia)
• Electrolyte disturbances (e.g., hypokalemia)
• Neuropathy (e.g., diabetes mellitus, spinal injury)
• Neurosurgical procedures (e.g., spinal surgery)
• Vascular diseases (e.g., mesenteric ischemia)
• Peritonitis
• Inflammation of intra-abdominal organs (e.g., appendicitis, cholecystitis, pancreatitis, severe gastroenteritis)
• Medications (e.g., anticholinergics, opioids, antidepressants)

The common causes of paralytic ileus can be memorized using “5 Ps”: Peritonitis, Postoperative, low Potassium, Pelvic and spinal fractures, and Parturition.^[22]

References:^[21][23]

Pathophysiology

• Stressful stimuli to the bowel (e.g., surgery, peritonitis) → sympathetic nervous system activation → decreased/arrested peristalsis
• Inflammation or intraoperative manipulation → local release of nitric oxide → relaxation of intestinal smooth muscles → decreased/arrested peristalsis
• Decreased/arrested peristalsis → bowel wall distention → progresses as detailed above in mechanical bowel obstruction

Clinical features

• Continuous (noncolicky) abdominal pain or discomfort
• Nausea, vomiting
• Abdominal distention
• Percussion: tympany
• Palpation: no tenderness unless peritonitis is present
• Auscultation: bowel sounds are absent (silent abdomen) or decreased (early paralytic ileus)

Diagnostics

Laboratory^[24]

• Leukocytosis with left shift suggests intestinal infection or ischemia.
• Anemia may be a sign of intra-abdominal hemorrhage (e.g., in postoperative or trauma patients).
• Hypokalemia, hypomagnesemia

Imaging^[25][26]

• Abdominal x-ray: best initial test
  • Generalized small and large bowel gaseous distention
  • Visible gas shadows in the rectum
  • No transition or cut-off point on contrast x-rays, such as enteroclysis or barium/water-soluble contrast enema
  • If caused by retroperitoneal hemorrhage; : obliteration of the psoas muscle outline
• Abdominal CT: to rule out suspected mechanical bowel obstruction or if abdominal x-ray is inconclusive
  • Has the highest sensitivity and specificity for differentiating ileus from mechanical obstruction
  • Identifies uniformly distended loops with no transition point and no structural/mechanical cause

Treatment

• Conservative treatment: in patients with no signs of localized or diffuse sepsis (e.g., appendicitis, secondary peritonitis)
  • Bowel rest
  • Nasogastric tube insertion
  • IV fluids and electrolyte repletion
  • Stop or decrease causative medications (e.g., opioids).
  • Gradual increase in enteral feeding as tolerated by the patient
  • Early postoperative ambulation (although still recommended to prevent DVT) and use of prokinetics have not been proven to improve peristalsis.
• Surgical intervention: in patients with signs of peritonitis (e.g., appendectomy, exploratory laparotomy)

Reference:^[23]