The chest X-ray shown above demonstrates free air under both hemidiaphragms, a finding most consistent with which of the following diagnoses?

Explanation

## Image Findings * **Free air under the right hemidiaphragm:** A distinct curvilinear lucency is visible outlining the inferior surface of the right hemidiaphragm (red arrows), clearly separating it from the liver shadow. The superior surface of the diaphragm is also distinctly visible (green arrows). * **Free air under the left hemidiaphragm:** Similar, though less extensive, curvilinear lucency is seen under the left hemidiaphragm. * **Rigler's sign (Double wall sign):** In the left upper quadrant/mid-abdomen, both the inner and outer walls of bowel loops are visible (blue arrows), indicating air both inside and outside the bowel lumen. ## Diagnosis **Key Point:** The presence of **free air under the diaphragm** is pathognomonic for pneumoperitoneum. The chest X-ray clearly demonstrates **free air under both hemidiaphragms**, a classic sign of **pneumoperitoneum**. This is seen as a crescent-shaped lucency separating the diaphragm from the underlying liver (on the right) and spleen/stomach (on the left). The visualization of both the inner and outer walls of the bowel loops (Rigler's sign) further confirms the presence of extraluminal air within the peritoneal cavity. Pneumoperitoneum typically indicates a perforation of a hollow viscus, such as the stomach or intestine. ## Differential Diagnosis | Feature | Pneumoperitoneum | Subphrenic Abscess | Diaphragmatic Rupture | Large Bowel Obstruction | | :------------------ | :---------------------------------------------- | :------------------------------------------------- | :-------------------------------------------------- | :------------------------------------------------- | | **X-ray Findings** | **Free air under diaphragm**, Rigler's sign | Air-fluid level in subphrenic space, mass effect | Bowel loops in chest, elevated hemidiaphragm | Dilated colon, air-fluid levels, haustra | | **Air Location** | Peritoneal cavity | Localized collection below diaphragm | Thoracic cavity (bowel herniation) | Intraluminal (bowel) | | **Clinical Context**| Acute abdomen, perforation | Fever, pain, history of surgery/infection | Trauma history, dyspnea | Abdominal distension, constipation, vomiting | | **Key Differentiating Feature** | **Generalized free air** | Localized collection, often with fluid | Bowel *above* diaphragm | No free peritoneal air | ## Clinical Relevance **Clinical Pearl:** The most common cause of pneumoperitoneum is a **perforated peptic ulcer**. Other causes include perforated diverticulitis, appendicitis, bowel ischemia, trauma, or iatrogenic injury (e.g., post-surgical, endoscopy). Patients typically present with sudden onset severe abdominal pain, guarding, rigidity (board-like abdomen), and signs of peritonitis. ## High-Yield for NEET PG **High-Yield:** Upright chest X-ray is the most sensitive plain film for detecting pneumoperitoneum, showing free air under the diaphragm in 70-80% of cases. CT scan is even more sensitive. **Key Point:** The **"football sign"** (large oval lucency outlining the entire abdominal cavity) is seen in massive pneumoperitoneum, typically in supine films. ## Common Traps **Warning:** Do not confuse subdiaphragmatic air with Chilaiditi's sign (interposition of colon between liver and diaphragm), which typically shows colonic haustra and is usually asymptomatic. Also, be careful not to mistake a gastric bubble or bowel gas for free air; free air outlines the diaphragm distinctly. ## Reference [cite:Harrison's Principles of Internal Medicine, Ch 318; Grainger & Allison's Diagnostic Radiology, Ch 16]