## Radiographic Signs of Pneumoperitoneum **Key Point:** Pneumoperitoneum is a medical emergency requiring urgent surgical evaluation. Plain radiographs can detect free air, but specific signs vary in reliability and visibility. ### Classic Signs of Pneumoperitoneum | Sign | Appearance | Reliability | Mechanism | |------|-----------|-------------|----------| | **Rigler's sign** | Air on both sides of bowel wall | High | Free air in peritoneal cavity | | **Falciform ligament sign** | Air outlining falciform ligament | High | Free air in hepatic fissure | | **Double wall sign** | Air between visceral and parietal peritoneum | High | Pneumoperitoneum separates layers | | **Haustra sign** | Prominent haustra visualization | Low/Not specific | Normal bowel anatomy, not a pneumoperitoneum sign | ### Why Haustra Sign Is Not Reliable for Pneumoperitoneum **High-Yield:** Haustra are normal anatomical features of the colon and become more visible with any air contrast in the abdomen. This is NOT a specific sign of pneumoperitoneum because: - Haustra are always present in the colon - They become visible with normal intestinal gas - They do not indicate free air in the peritoneal cavity - Their prominence is related to bowel distension, not pneumoperitoneum **Clinical Pearl:** The three true signs of pneumoperitoneum (Rigler's, Falciform ligament, Double wall) all rely on air outlining normally non-air-containing structures or spaces. Haustra are already air-containing structures and thus cannot be used to detect free air. ### Mnemonic for True Pneumoperitoneum Signs **RFD** — Rigler's sign, Falciform ligament sign, Double wall sign **Warning:** Absence of radiographic signs does NOT exclude pneumoperitoneum. Up to 25% of cases with proven free air may have negative plain films. CT is more sensitive (nearly 100%) for detecting pneumoperitoneum.
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