Synopsis
Recognise free intraperitoneal gas and indirect perforation signs, understand the limits of plain radiography, use CT to localise the source, and escalate clinical peritonitis despite negative imaging.
- Pneumoperitoneum is a radiological sign; perforation remains a clinical diagnosis when imaging is negative or equivocal.
- CT is preferred when promptly available because it detects small gas volumes, maps fluid and inflammation, and may suggest the perforation site.
- Erect chest or abdominal radiography is useful when CT is not promptly available, but negative plain imaging cannot exclude perforation.
Key red flags
Rigid guarding, rebound, shock, rising lactate or organ dysfunction is urgent even when no pneumoperitoneum is visible.
Free intraperitoneal gas with acute pain usually represents perforation unless a recent procedure provides a convincing alternative explanation.
Portal venous gas, pneumatosis, reduced bowel enhancement or mesenteric vascular occlusion suggests bowel ischaemia and threatened viability.
Retroperitoneal duodenal or colonic perforation and contained leaks may not create classic subdiaphragmatic gas.
A crescent of lucency beneath a hemidiaphragm on an adequately positioned erect film supports free intraperitoneal gas.
Sealed, small or retroperitoneal leaks may show only focal fluid, fat stranding or subtle gas; negative plain films are insufficient.
Pneumatosis with portal venous gas, reduced enhancement, vascular occlusion or shock raises concern for necrosis and urgent source control.
Investigation priorities
Detect extraluminal gas, localise source and assess contamination, abscess, obstruction and ischaemia.
Management branches
A stable adult develops sudden severe epigastric pain and generalised peritonism.
- Context: identify shock, sepsis, ulcer risk, recent procedures, medication, previous surgery and the time symptoms began.
- Reasoning: arrange urgent CT if immediately available because it can detect and localise perforation better than plain radiography.