How do you differentiate a loculated (isolated) pneumothorax from a non-loculated pneumothorax on a chest x-ray (CXR)?

Differentiating Loculated from Non-loculated Pneumothorax on Chest X-ray

On chest X-ray, a loculated pneumothorax appears as a focal, well-defined air collection that does not shift with changes in patient position and often has a lenticular or rounded shape with visible margins, while a non-loculated pneumothorax appears as a peripheral lucency that follows the contour of the chest wall and shifts with patient positioning. 1

Key Radiographic Findings

Loculated Pneumothorax

• Appears as a focal, well-defined air collection
• Does not change position with gravity (non-mobile)
• Often has a lenticular or rounded appearance
• May have visible pleural thickening at margins
• Can be located in atypical locations (not just apical)
• Often associated with underlying lung disease or previous interventions
• May show internal septations or compartmentalization

Non-loculated (Free) Pneumothorax

• Appears as a peripheral lucency following chest wall contour
• Air collects at the highest point (typically apical on upright films)
• Visible visceral pleural line separated from chest wall
• Shifts with changes in patient positioning
• No lung markings visible in the pneumothorax area

Advanced Imaging Considerations

When standard chest X-ray findings are equivocal, additional imaging techniques can help differentiate:

1. Lateral decubitus views: Particularly useful for non-loculated pneumothoraces, which will shift to the non-dependent side, while loculated pneumothoraces remain fixed 1

2. CT scan: The gold standard for difficult cases

   • Can definitively differentiate loculated pneumothorax from bullae or blebs
   • Shows the exact location and extent of loculations
   • Essential for patients with underlying bullous disease 1

3. Ultrasound: Increasingly recommended as first-line when available

   • For non-loculated pneumothorax: absence of lung sliding, absence of B-lines, and presence of lung point 2
   • For loculated pneumothorax: may show complex septations and fixed position despite patient movement 3

Common Pitfalls to Avoid

• Skin folds: Can mimic the linear appearance of a pneumothorax edge

  • Look for continuation of vascular markings beyond the suspected "edge"

• Overlying tubes/lines: Can create artificial lucencies

  • Trace the entire course of any tubes/lines on the image

• Bullae vs. Pneumothorax: Both appear as air-filled spaces

  • Bullae typically have thinner walls and may show internal structures
  • CT is often needed for definitive differentiation 1

• Relying solely on single-view images: Two-view chest radiography (PA and lateral) provides greater diagnostic accuracy 1

Diagnostic Algorithm

1. Start with a standard two-view chest X-ray (PA and lateral)
2. Look for characteristic features of loculation vs. free air
3. If findings are equivocal, obtain lateral decubitus views
4. Consider lung ultrasound for bedside confirmation (when expertise available)
5. Proceed to CT scan for definitive diagnosis in difficult cases, especially with:
   • Underlying bullous disease
   • Surgical emphysema obscuring the lungs
   • Suspected complex loculations 1

Remember that loculated pneumothoraces are often associated with underlying pleural disease, previous interventions, or infections that cause pleural adhesions. They typically require more aggressive management as they often do not resolve with simple aspiration.