The axial CT scan of the chest shown above demonstrates which of the following conditions?

Explanation

## Image Findings * Presence of extensive **free air** within the **mediastinal compartment**. * Air is seen **outlining the trachea**, making its walls distinctly visible. * Air is also seen **outlining the great vessels** (e.g., aorta) within the mediastinum. * Evidence of **air extending into the supraclavicular soft tissues**, indicating **subcutaneous emphysema**. ## Diagnosis **Key Point:** **Pneumomediastinum** is diagnosed by the presence of free air within the mediastinal space. The axial CT scan clearly demonstrates multiple **lucent (dark) areas** representing **free air** within the **mediastinum**. This air dissects along the fascial planes, distinctly **outlining the contours of mediastinal structures** such as the **trachea** and **great vessels** (e.g., aorta). The additional finding of **subcutaneous emphysema**, characterized by air in the soft tissues of the neck and supraclavicular regions, is a frequent association and further supports the diagnosis of pneumomediastinum. ## Differential Diagnosis | Feature | Correct Dx: Pneumomediastinum | Alt 1: Pneumothorax | Alt 2: Mediastinal Lymphadenopathy | Alt 3: Aortic Dissection | | :-------------------- | :---------------------------------------------------------- | :---------------------------------------------------- | :------------------------------------------------------- | :-------------------------------------------------------- | | **Location of Air/Pathology** | Air within the **mediastinal compartment** (central chest) | Air within the **pleural space** (between lung and chest wall) | Enlarged **lymph nodes** in the mediastinum (soft tissue density) | **Intimal flap** in the aorta, often with **pericardial/pleural fluid** | | **Imaging Appearance** | **Lucency** outlining mediastinal structures (trachea, vessels) | **Lucency** separating visceral pleura from parietal pleura, often with **collapsed lung** | **Soft tissue masses** (enlarged nodes) in mediastinum | **Double lumen** in aorta, **intimal flap**, sometimes **mediastinal hematoma** | | **Clinical Presentation** | Chest pain, dyspnea, **Hamman's sign**, subcutaneous emphysema | Acute dyspnea, pleuritic chest pain, reduced breath sounds | Often asymptomatic, or symptoms related to compression/underlying disease | Sudden severe tearing chest pain radiating to back, pulse deficit | ## Clinical Relevance **Clinical Pearl:** **Hamman's sign**, a crunching sound synchronous with the heartbeat (best heard in the left lateral decubitus position), is a classic physical finding in pneumomediastinum. ## High-Yield for NEET PG **High-Yield:** The most common cause of **spontaneous pneumomediastinum** is a **Valsalva maneuver** (e.g., severe coughing, vomiting, childbirth), leading to alveolar rupture and air dissection into the mediastinum. **Key Point:** Other important causes include trauma, iatrogenic injury (e.g., endoscopy, mechanical ventilation), esophageal rupture (**Boerhaave syndrome**), and asthma exacerbations. ## Common Traps **Warning:** Confusing **pneumomediastinum** with **pneumothorax** is a common trap. Remember, pneumomediastinum involves air *within* the mediastinum, outlining central structures, while pneumothorax involves air *in the pleural space*, separating the lung from the chest wall. ## Reference [cite:Harrison's Principles of Internal Medicine Ch 269]