Synopsis
Differentiate small- from large-bowel obstruction on imaging, identify the transition and likely cause, recognise strangulation or perforation, and understand when negative radiography cannot defer CT or surgical review.
- Obstruction imaging must answer level, transition point, severity, cause and complications rather than merely confirm dilated bowel.
- Small bowel is usually central with valvulae crossing the lumen; colon is more peripheral with haustra that do not fully traverse it.
- Plain radiography may support obstruction but CT abdomen and pelvis is usually required in acute adult SBO to define cause and threatened bowel.
Key red flags
Pain out of proportion, continuous pain, fever, tachycardia, acidosis, rising lactate or peritonism suggests ischaemia or perforation.
CT evidence of closed loop, reduced or absent mural enhancement, pneumatosis, portal venous gas, mesenteric congestion or free fluid requires urgent surgical review.
A competent ileocaecal valve can create a closed-loop large-bowel obstruction with dangerous caecal dilatation.
A normal abdominal radiograph does not exclude early, fluid-filled, partial or closed-loop obstruction.
Two adjacent transition points isolate a segment; C- or U-shaped loops, converging mesenteric vessels and twisting raise urgent concern.
Reduced enhancement, wall thickening, mesenteric oedema, haemorrhage, pneumatosis, portal venous gas or free fluid may indicate compromised bowel.
Investigation priorities
Locate transition, define cause and assess bowel wall, mesentery and complications.
Management branches
An adult with previous laparotomy develops colicky pain, vomiting and abdominal distension.
- Context: assess physiology, peritonism, herniae, last flatus, prior operations and whether pain has become continuous.
- Reasoning: use CT to distinguish mechanical obstruction from ileus, locate the transition and look specifically for a closed loop or reduced enhancement.