## Rigler's Sign — Definition & Clinical Significance **Key Point:** Rigler's sign is the visualization of both sides of the bowel wall on plain radiograph, indicating free intraperitoneal air outlining the bowel loops. ### Mechanism When pneumoperitoneum is present, air outlines the visceral peritoneum (inner wall) and parietal peritoneum (outer wall) of bowel loops, making both walls visible as thin lucent lines on frontal radiographs. ### Clinical Context - **Most common cause:** Perforated peptic ulcer (80% of cases) - **Other causes:** Perforated appendicitis, perforated diverticulitis, perforated colon, iatrogenic perforation - **Sensitivity:** ~60–70% on erect CXR; lower on supine abdominal films - **Best technique:** Erect chest radiograph (air rises to lung bases) or left lateral decubitus abdominal film ### Distinction from Other Signs | Sign | Finding | Indicates | |------|---------|----------| | **Rigler's sign** | Both walls of bowel visible | Free intraperitoneal air | | **Pneumoperitoneum** | General term for free air | Free intraperitoneal air (non-specific) | | **Sentinel loop** | Single dilated small bowel loop | Localized inflammation (not free air) | | **String of pearls** | Multiple small air-fluid levels in zigzag pattern | Small bowel obstruction | **High-Yield:** Rigler's sign is the *specific radiographic sign* for free air; "pneumoperitoneum" is the *clinical finding* (free air itself), not the radiographic sign name. **Clinical Pearl:** On supine films, free air collects anteriorly and may be missed. Always request erect CXR or lateral decubitus view when pneumoperitoneum is suspected clinically. **Tip:** In exam questions, if asked for the "sign" (not the finding), Rigler's sign is the answer; if asked for the radiographic appearance of free air, pneumoperitoneum is acceptable but less specific. 
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