A 58-year-old man from rural Maharashtra presents to the emergency department with acute onset severe epigastric pain radiating to the back, associated with vomiting. He has a history of chronic peptic ulcer disease and stopped his medications 3 months ago. On examination, he is in severe distress, BP 92/58 mmHg, HR 118/min, and has rigid abdomen with rebound tenderness. An upright chest X-ray is obtained. Which imaging finding is most specific for the diagnosis of perforated peptic ulcer?

Explanation

## Imaging Findings in Perforated Peptic Ulcer **Key Point:** Free air under the diaphragm on upright chest X-ray (pneumoperitoneum) is the most specific imaging finding for hollow viscus perforation, particularly perforated peptic ulcer. ### Characteristic Radiological Signs | Finding | Sensitivity | Specificity | Timing | Notes | |---------|-------------|-------------|--------|-------| | **Free air under diaphragm** | 70–80% | >95% | Early (within 6 hrs) | Best on upright CXR; may be subtle | | Ground-glass opacity | 50–60% | 40–50% | Later (>12 hrs) | Non-specific; seen in peritonitis | | Dilated bowel loops | 40–50% | Low | Later | Indicates ileus, not specific to perforation | | Rigler's triad | Variable | High | Variable | Free air + pneumoperitoneum + portal venous gas | **High-Yield:** Upright or left lateral decubitus CXR is the first-line imaging for suspected perforation because it is rapid, available, and highly specific for free air. Even 1–2 mL of free air can be detected. ### Why Free Air is Diagnostic 1. **Mechanism:** Perforation of a hollow viscus (stomach, duodenum) releases intraluminal gas into the peritoneal cavity. 2. **Visualization:** On upright CXR, free air rises and collects under the diaphragm, creating a lucent crescent between the liver dome and diaphragm. 3. **Clinical correlation:** In the setting of acute peritonitis with rigid abdomen and shock, pneumoperitoneum is pathognomonic for perforation. **Clinical Pearl:** If upright CXR is negative but clinical suspicion is high, obtain a left lateral decubitus view or CT abdomen/pelvis. CT has >95% sensitivity for detecting even small amounts of free air and can identify the site of perforation. ### Other Radiological Signs (Less Specific) - **Ground-glass opacity:** Diffuse haziness due to ascites and peritonitis; develops over hours; non-specific. - **Sentinel clot sign:** Clot in the lesser sac; seen in bleeding peptic ulcer, not perforation. - **Dilated bowel loops:** Reflect ileus secondary to peritonitis; not diagnostic of perforation itself. **Mnemonic: RIGLER'S TRIAD** — (1) Free air under diaphragm, (2) Pneumoperitoneum in peritoneal cavity, (3) Portal venous gas (rare, late finding). [cite:Harrison 21e Ch 297]