Pathophysiological mechanisms of ileus, other causes of Acute Abdomen

Acute abdomen is a sudden onset of abdominal pain caused by intra-abdominal disease, which may rapidly progress to ischemia, perforation, sepsis, or shock. This requires urgent surgical care to prevent gangrene and perforation of the gut, which can occur in less than 6h from onset of symptoms in some situation). Example includes ruptured abdominal aortic aneurysm, mesentectic ischemia, ruptured ectopic pregnancy, intestinal obstruction and appendicitis. [1]

Ileus

Ileus is the obstruction of the intestine causing arrest of intestinal peristalsis.

Paralytic ileus

Paralytic ileus is the interruption of normal passage of intestinal contents due to reduced peristalsis (absence of mechanical obstruction, often occurring post operation (should resolve within 72h). Other ethologies include inflammation of abdominal viscera, metabolic disturbances, certain medications and ischemia.

When there is irritation of the bowel, it causes sympathetic nervous system activation and releases nitric oxide locally, decreasing peristalsis and relaxes intestinal smooth muscles. This will then lead to stasis of luminal contents and gas, and bowel wall distention.

Symptoms include nausea, vomiting, abdominal pain, abdominal distention, and constipation. Usually, bowel sounds would be absent during auscultation. [2]

Mechanical ileus

Mechanical ileus is the interruption of normal passage through the bowel due to a structural barrier. [3]

  • Hernia (protrusion of intra-abdominal contents through the abdominal wall or internal passage)
    • Types - inguinal (direct, indirect), femoral, hiatal, umbilical or through previous surgical excision
    • Requires a weakened abdominal wall and increase abdominal pressure
    • Mechanical obstruction
      Traps intestine -> obstructive ileus
  • Adhesion (bands of scar like tissue that form between 2 surfaces inside the body and causes them to stick together. In this case, between intestine and nearby organs and tissues [4]
  • Volvulus is when a segment of intestine twist around itself -> bowel obstruction and cuts blood supply[5]
  • Intussusception is the invagination of a segment of intestine into the adjacent section of intestine[6]

Other causes of Acute Abdomen[7]

  • Bleeding -> hypovolemic shock
    • Severe acute blood loss e.g. ruptured abdominal aortic aneurysm, ruptured ectopic pregnancy, trauma or any other gastrointestinal bleeding
  • Perforation of gastrointestinal tract -> peritonitis -> shock
    • Often caused by peptic ulcer perforation, perforated diverticulitis, ischemic bowel and inflammatory bowel perforation
    • Perforation will cause content leak into the peritoneal cavity -> irritation -> peritonitis
  • Gastrointestinal obstruction e.g. paralytic or mechanical obstruction
  • Acute appendicitis
  • Acute pancreatitis
  • Acute cholecystitis
  • Urogenital pathologies

References

  1. ↑ Ansari, P. (2024, July). Acute abdominal pain. In MSD Manual Professional Edition. MSD Manuals.
  2. ↑ AMBOSS. (2025, October 28). Paralytic ileus. AMBOSS. From https://next.amboss.com/us/article/aI0QYh
  3. ↑ AMBOSS. (2026, March 30). Bowel obstruction. AMBOSS. From https://next.amboss.com/us/article/230T3f?q=mechanical%20ileus
  4. ↑ MedlinePlus. (2024, March 31). Adhesion. U.S. National Library of Medicine. (medlineplus.gov)
  5. ↑ Le, C. K., Anand, S., & Cooper, W. (2022, September 12). Volvulus. In StatPearls. StatPearls Publishing. NCBI Bookshelf.
  6. ↑ AMBOSS. (2025, March 5). Intussusception. AMBOSS. From https://www.amboss.com/us/knowledge/intussusception/
  7. ↑ AMBOSS. (2026, February 10). Acute abdomen. AMBOSS. From https://next.amboss.com/us/article/N50-4g