A 35-year-old woman with known cystic fibrosis presents with acute dyspnea. Chest X-ray shows a right-sided pneumothorax. Which imaging finding on high-resolution CT best distinguishes a tension pneumothorax from a simple (non-tension) pneumothorax?

Explanation

## Tension vs. Non-Tension Pneumothorax: Imaging Discriminators ### Critical Distinguishing Features **Key Point:** Mediastinal shift (away from the pneumothorax side) and hemidiaphragmatic flattening or inversion are the hallmark imaging signs of tension pneumothorax and distinguish it from simple pneumothorax. ### Pathophysiology Behind Imaging Findings **High-Yield:** In tension pneumothorax, positive intrapleural pressure accumulates during inspiration and fails to equilibrate during expiration. This causes: 1. Progressive mediastinal displacement away from the affected side 2. Compression of the contralateral lung 3. Flattening or paradoxical inversion of the ipsilateral hemidiaphragm 4. Hemodynamic compromise (hypotension, JVD, tachycardia) In simple pneumothorax, intrapleural pressure remains subatmospheric or atmospheric; no progressive shift occurs. ### Imaging Comparison Table | Feature | Simple Pneumothorax | Tension Pneumothorax | |---------|-------------------|---------------------| | **Mediastinal position** | Midline or minimal shift | Marked shift away from PTX side | | **Hemidiaphragm** | Normal curvature | Flattened or inverted (pushed down) | | **Contralateral lung** | Normal | Compressed | | **Pleural line** | Visible, sharp | Visible but with mass effect | | **Clinical urgency** | Managed conservatively or with aspiration | **EMERGENCY** — requires immediate decompression | | **Hemodynamic stability** | Stable | Unstable (hypotension, shock) | ### Why Mediastinal Shift Is the Key Discriminator **Clinical Pearl:** On frontal chest X-ray, measure the position of the mediastinum (trachea, heart) relative to the midline. In tension pneumothorax, the mediastinum is displaced **away** from the pneumothorax side by >2 cm. On CT, this shift is even more obvious and accompanied by hemidiaphragmatic changes. **Mnemonic:** **TENT** = **T**ension, **E**xcessive mediastinal shift, **N**egative intrapleural pressure, **T**reatment emergent (needle decompression). ### Why Other Options Are Incorrect - **Visceral pleural line:** Visible in both simple and tension pneumothorax; not a discriminator. - **Complete opacification:** Suggests hemothorax, empyema, or massive consolidation — not pneumothorax. - **Subcutaneous emphysema:** Can occur with either simple or tension pneumothorax; not specific for tension.