A 55-year-old woman with a history of peptic ulcer disease presents to the emergency department with sudden onset severe epigastric pain radiating to the right shoulder. She appears acutely ill and is in severe distress. Vital signs show BP 110/70 mmHg, HR 110/min, RR 22/min. On examination, her abdomen is rigid with severe guarding and rebound tenderness. An upright chest radiograph is obtained as part of the acute abdomen workup. What is the most likely radiological finding on the upright chest X-ray?

Explanation

## Clinical Diagnosis: Perforated Peptic Ulcer with Pneumoperitoneum ### Clinical Presentation **Key Point:** The classic presentation of **perforated peptic ulcer** includes: - **Sudden onset severe epigastric pain** (often described as "worst pain of my life") - **Radiation to the shoulder** (referred pain from diaphragmatic irritation) - **Rigid, board-like abdomen** with severe guarding and rebound tenderness - **Acute peritonitis** (generalized peritoneal inflammation from gastric/duodenal contents) **Clinical Pearl:** The shoulder pain is a **pathognomonic sign** of diaphragmatic irritation from free air or fluid irritating the phrenic nerve (C3–C5), which shares sensory innervation with the shoulder. ### Radiological Findings in Perforated Viscus **High-Yield:** The **upright chest radiograph** is the gold standard initial imaging for detecting **pneumoperitoneum** (free air in the peritoneal cavity). Free air rises and collects under the diaphragm, appearing as a **lucent crescent or band of air** between the liver/stomach and the diaphragm. ### Why Upright CXR Detects Pneumoperitoneum **Key Point:** On an **upright or semi-upright film**, free air in the peritoneal cavity rises due to gravity and collects **under the diaphragm**. This appears as: - A **lucent (dark) crescent** of air between the liver dome and the diaphragm - Best seen on the **right side** (over the liver) - May also be visible on the **left side** (over the stomach) - Requires **upright positioning** to be visible (supine films may miss pneumoperitoneum) **Mnemonic:** **"Free air rises"** — in upright position, pneumoperitoneum collects under the diaphragm and is visible on CXR. ### Comparison of Imaging Modalities | Modality | Sensitivity for Pneumoperitoneum | Advantages | Limitations | |----------|---|---|---| | **Upright CXR** | 60–80% | Readily available, quick, low cost | May miss small amounts of air | | **Supine abdominal X-ray** | 30–40% | Part of acute abdomen series | Air may not be visible under diaphragm | | **CT abdomen/pelvis** | >95% | **Gold standard**, detects small amounts | Higher cost, radiation | | **Lateral decubitus film** | 70–80% | Alternative if upright not possible | Less commonly used | **Clinical Pearl:** If **upright CXR is unavailable**, a **left lateral decubitus abdominal film** can be used as an alternative to detect free air (air collects along the left abdominal wall in this position). ### Why This Finding Confirms Perforated Peptic Ulcer **Key Point:** In the context of: - Sudden severe epigastric pain - Rigid abdomen with peritonitis - History of peptic ulcer disease - **Pneumoperitoneum on imaging** The diagnosis is **perforated peptic ulcer** until proven otherwise. This is a **surgical emergency** requiring urgent laparotomy. ### Pathophysiology 1. Peptic ulcer erodes through the full thickness of the gastric or duodenal wall 2. Gastric/duodenal contents (including air) spill into the peritoneal cavity 3. This causes acute peritonitis with severe pain and rigidity 4. Free air rises under the diaphragm and is visible on upright imaging **High-Yield:** Not all perforated ulcers show pneumoperitoneum on initial imaging (~20–30% may be missed on plain films), but when present, it is **highly specific** for perforation. [cite:Harrison 21e Ch 298; Robbins & Cotran 10e Ch 17]