Bowel Obstruction Overview

Bowel Obstruction Overview

• Bowel obstruction (intestinal obstruction) is a condition where intestinal contents cannot pass through the gastrointestinal tract.
• Obstructions can be in the small intestine (SBO) or large intestine (LBO), and can be partial or complete, or simple or strangulated.
• Partial obstructions allow some passage of fluids and gas, and often resolve with conservative treatments.
• Complete obstructions totally block the lumen and usually require surgery.
• Simple obstructions have an intact blood supply, while strangulated ones do not.

Types of Bowel Obstruction

• Mechanical Obstruction: Physical blockage of the intestinal lumen.
  • Most common in the small intestine.
  • Common causes in SBO include surgical adhesions (can occur days to years after surgery), hernias, cancers, Crohn's disease strictures, and intussusception after bariatric surgery.
  • Common causes in LBO include colorectal cancer (CRC), diverticular disease, adhesions, ischemia, volvulus, and Crohn's disease.
• Nonmechanical Obstruction: Reduced/absent peristalsis due to altered neuromuscular transmission.
  • Common cause is paralytic ileus (lack of intestinal peristalsis and sounds) often occurring after abdominal surgery, but also related to peritonitis, inflammatory responses (e.g., pancreatitis, appendicitis), electrolyte imbalances (hypokalemia), and spinal fractures (thoracic or lumbar).
  • Pseudo-obstruction is a GI motility disorder mimicking a mechanical obstruction, with symptoms but no identifiable cause on imaging, related to neurologic conditions, drugs, endocrine/metabolic problems, lung disease, trauma, and burns.
• Vascular Obstruction: Rare, resulting from impaired blood supply to a portion of the intestines.
  • Common causes include emboli and atherosclerosis of the mesenteric arteries, emboli arising from thrombi (in patients with chronic atrial fibrillation, diseased heart valves, or prosthetic valves), and venous thrombosis (in conditions like heart failure and shock).

Etiology and Pathophysiology

• About 6-8 liters of fluid enter the small intestine daily, mostly absorbed before reaching the colon.
• 75% of intestinal gas comes from swallowed air.
• Obstruction causes fluid and gas accumulation proximal to the blockage.
• Distention reduces fluid absorption, stimulates secretion, and distal bowel empties and collapses.
• Increasing intraluminal pressure causes increased capillary permeability, fluid/electrolyte leakage into the peritoneal cavity, muscle fatigue, and stoppage of peristalsis.
• Fluid retention in the intestine and peritoneum leads to reduced blood volume, hypotension, and hypovolemic shock.
• Inadequate blood flow causes ischemia, necrosis, and possible bowel perforation.
• Severe distention leads to intestinal strangulation or infarction.

Clinical Manifestations

• Key symptoms: abdominal pain, nausea/vomiting, distention, and constipation.
• Order/degree varies with cause, location, and type of obstruction.
• Colicky abdominal pain often first symptom; sudden onset for SBO, occurring at 4-5 minute intervals (proximal obstruction), less frequent in distal obstructions.
• Vomiting character: proximal obstruction leads to projectile vomiting with bile, giving temporary pain relief; distal obstruction leads to gradual, fecal, foul-smelling vomiting.
• High-pitched bowel sounds above the obstruction; absent sounds in paralytic ileus.
• Large bowel obstruction (LBO) signs: abdominal distension, obstipation/bowel changes, lack of flatus, persistent cramping pain, usually present but progressively hypoactive bowel sounds, rare vomiting.
• Strangulation symptoms: severe, constant, rapid-onset pain.
• Other general signs: tenderness/rigidity, acute illness, signs of dehydration/sepsis (tachycardia, dry mucous membranes, hypotension, temp >100°F), location of the obstruction influences acid-base and electrolyte imbalances. If high in the duodenum, metabolic alkalosis can result from vomiting/NG suction. Small intestine obstructions rapidly cause dehydration.