What are the best imaging modalities for diagnosing pneumothorax?

Best Imaging Modalities for Diagnosing Pneumothorax

Lung ultrasound is the most accurate first-line imaging modality for diagnosing pneumothorax, with superior sensitivity (85-95%) and specificity (94-99%) compared to conventional chest radiography. 1, 2

Diagnostic Accuracy of Different Imaging Modalities

Lung Ultrasound (LUS)

• Sensitivity: 85-95%
• Specificity: 94-99%
• Key sonographic signs:
  • Absence of lung sliding
  • Absence of B-lines
  • Absence of lung pulse
  • Presence of lung point (pathognomonic for pneumothorax)
• Advantages:
  • Bedside availability
  • No radiation exposure
  • Real-time assessment
  • Superior accuracy in critically ill patients
  • Can differentiate between small and large pneumothorax
  • Particularly valuable in unstable patients who cannot be transported

Chest Radiography (CXR)

• Sensitivity: Only 30-50% (particularly poor for supine radiographs)
• Specificity: 92-99%
• Recommendations:
  • Upright positioning when possible
  • PA and lateral views have greater sensitivity (83.9%) than single AP view (67.3%)
  • Lateral decubitus radiograph is superior when PA findings are unclear
  • Expiratory radiographs are NOT recommended for routine diagnosis 1

Computed Tomography (CT)

• Sensitivity and Specificity: Highest (gold standard)
• Indications:
  • Differentiating pneumothorax from bullous lung disease
  • When ultrasound findings are equivocal
  • Determining exact size of pneumothorax
  • Evaluating underlying lung pathology
  • When aberrant chest tube placement is suspected 1

Recommended Diagnostic Algorithm

1. First-line: Lung ultrasound

   • Fastest, most accurate bedside tool
   • Particularly valuable in emergency and ICU settings

2. Second-line: Chest radiography (PA and lateral when possible)

   • For documentation purposes
   • When ultrasound expertise is unavailable
   • For monitoring known pneumothorax over time

3. Third-line: CT scan

   • When ultrasound and X-ray findings are equivocal
   • To evaluate for underlying lung pathology
   • When precise measurement is needed

Special Considerations

Primary vs. Secondary Pneumothorax

• Secondary pneumothorax (with underlying lung disease) often produces more severe symptoms even with smaller air collections 1
• Lower threshold for advanced imaging in patients with underlying lung disease

Size Classification

• Small: visible rim <2 cm between lung margin and chest wall
• Large: visible rim >2 cm between lung margin and chest wall 2

Pitfalls to Avoid

• For Ultrasound:

  • False positives can occur with lung bullae, contusions, and adhesions
  • Technical limitations include subcutaneous emphysema and severe obesity
  • Absence of lung sliding alone is insufficient for diagnosis (can occur with atelectasis, consolidation, or lung contusion) 1

• For Chest Radiography:

  • Consistently underestimates pneumothorax size
  • Particularly poor sensitivity in supine patients
  • Most missed pneumothoraces occur with coexistent lower lobe consolidation 2

• For CT Scan:

  • Resource-intensive
  • Requires patient transport (potentially dangerous in unstable patients)
  • Higher radiation exposure

In critically ill patients, lung ultrasound has been shown to change management in up to 47% of cases, with 21% revealing findings not suspected by the primary physician, including pneumothorax 1. The evidence strongly supports using lung ultrasound as the primary diagnostic modality for suspected pneumothorax, with chest radiography and CT scan reserved for specific indications.