A 58-year-old man presents to the emergency department with acute onset severe epigastric pain and abdominal rigidity. He has a history of peptic ulcer disease managed with NSAIDs. On examination, he is febrile (38.5°C), tachycardic (110/min), and hypotensive (90/60 mmHg). His abdomen is rigid with rebound tenderness. Upright chest X-ray shows pneumoperitoneum with a lucent crescent sign under the right hemidiaphragm. What is the most likely diagnosis?

Explanation

## Clinical Presentation & Imaging Correlation **Key Point:** Pneumoperitoneum on upright chest X-ray in a patient with acute epigastric pain, peritonitis, and NSAID use is pathognomonic for perforated peptic ulcer until proven otherwise. ### Imaging Findings in Perforated Peptic Ulcer The **lucent crescent sign** (also called the **Rigler's sign**) is the classic radiological finding: - Free air appears as a radiolucent (black) crescent under the hemidiaphragm - Best seen on upright or left lateral decubitus chest X-ray - May also be visible as pneumoperitoneum outlining bowel loops on frontal views **High-Yield:** Upright CXR is the first-line imaging for suspected perforation — it is rapid, bedside-capable, and has ~70% sensitivity for pneumoperitoneum. CT with IV contrast (if patient is stable) has >95% sensitivity and can also identify the perforation site and localize free fluid. ### Pathophysiology 1. NSAID use → mucosal ulceration in antral/prepyloric region 2. Ulcer erodes through visceral peritoneum → perforation 3. Gastric/duodenal contents spill into peritoneal cavity 4. Immediate peritonitis + chemical irritation 5. Air enters peritoneum → pneumoperitoneum **Clinical Pearl:** The classic triad is **acute epigastric pain + rigid abdomen + pneumoperitoneum**. Patients often report a sudden "tearing" or "knife-like" pain. ### Management Algorithm ```mermaid flowchart TD A[Suspected perforated peptic ulcer]:::outcome --> B[Upright CXR]:::action B --> C{Pneumoperitoneum present?}:::decision C -->|Yes| D[Confirm with CT if stable]:::action C -->|No| E[CT abdomen/pelvis]:::action D --> F[Urgent surgical consultation]:::action E --> F F --> G{Hemodynamically stable?}:::decision G -->|Yes| H[Laparoscopic repair]:::action G -->|No| I[Open surgical repair]:::action ``` **Mnemonic:** **PERFORATED** — Peptic ulcer, Epigastric pain, Rigid abdomen, Free air, Onset acute, Radiating to back, Acute peritonitis, Tachycardia, Emergency surgery, Duodenum/antrum most common ### Differential Imaging Features | Finding | Perforated PU | Acute Pancreatitis | Perforated Appendix | Boerhaave | |---------|---------------|-------------------|-------------------|----------| | **Pneumoperitoneum** | Yes (early) | No | Rare (late) | Yes (esophageal) | | **Location of pain** | Epigastric → generalized | Epigastric → left flank | RLQ → generalized | Chest → abdomen | | **Rigler's sign** | Classic | Absent | Absent | May be present | | **Associated history** | NSAID/PUD | Alcohol/gallstones | Appendicitis | Vomiting/instrumentation | | **CT finding** | Perforation crater | Pancreatic edema/necrosis | Appendicolith, wall thickening | Esophageal wall disruption | **Warning:** Do NOT confuse pneumoperitoneum with **pneumatosis intestinalis** (air in bowel wall) — the latter is a sign of bowel ischemia and appears as a linear or bubbly lucency within the bowel wall, not under the diaphragm. ## Answer Justification The clinical constellation of **NSAID use + acute epigastric pain + peritonitis + pneumoperitoneum on CXR** is diagnostic of perforated peptic ulcer. The **lucent crescent sign** is the pathognomonic imaging finding. This is a surgical emergency requiring immediate repair (laparoscopic or open) to prevent sepsis and death. [cite:Robbins 10e Ch 17]