Which radiological sign on plain abdominal X-ray is most specific for hollow viscus perforation?

Explanation

## Rigler's Triad — The Gold Standard Sign of Perforation **Key Point:** Rigler's triad is the most specific radiological sign for hollow viscus perforation on plain abdominal radiography. ### Components of Rigler's Triad 1. **Pneumoperitoneum** — Free air in the peritoneal cavity (most sensitive but not specific) 2. **Pneumomediastinum** — Air in the mediastinum (indicates severe perforation, rare) 3. **Subcutaneous emphysema** — Air tracking into subcutaneous tissues (indicates extensive perforation) **High-Yield:** When all three components are present together, the specificity for perforation approaches 100%. However, pneumoperitoneum alone (without the other two) can occur post-operatively or from incompetent sphincters. ### Other Common Signs of Perforation (Less Specific) | Sign | Finding | Specificity | |------|---------|-------------| | **Pneumoperitoneum** | Free air under diaphragm on upright CXR | Sensitive but not specific | | **Sentinel loop** | Dilated small bowel loop near perforation site | Non-specific; seen in many acute abdominal conditions | | **String sign** | Thin, thread-like appearance of bowel | Suggests ischemia, not specific to perforation | | **Thumbprinting** | Thickened bowel wall with indentations | Suggests edema/ischemia, not perforation | **Clinical Pearl:** Free air is best visualized on **upright chest X-ray** (or left lateral decubitus view) rather than supine abdominal films. In supine position, air may collect anteriorly and be missed. **Mnemonic:** **RPE** = **R**igler's triad = **P**neumoperitoneum + **P**neumomediastinum + **E**mphysema (subcutaneous) ### Sensitivity of Pneumoperitoneum Detection - Upright CXR: ~70–80% sensitive - CT abdomen: ~95% sensitive (gold standard for detecting free air) - Supine abdominal X-ray: ~50% sensitive (air may not layer dependently) **Warning:** Absence of free air on plain radiography does NOT exclude perforation. CT is required if clinical suspicion is high.