How is the size of a pneumothorax (collapsed lung) defined?

Pneumothorax Size Definition

The size of a pneumothorax is defined as "small" or "large" depending on the presence of a visible rim of <2 cm or >2 cm between the lung margin and the chest wall on a plain PA chest radiograph. 1

Measurement Methods and Classification

Standard Radiographic Assessment

• Small pneumothorax: Visible rim of <2 cm between the lung margin and chest wall 1
• Large pneumothorax: Visible rim of >2 cm between the lung margin and chest wall 1

Alternative Measurement Methods

• CT scan measurement: Most accurate method for exact size estimation 1
  • In trauma cases, a pneumothorax >35 mm on CT (measured radially from chest wall to lung parenchyma) typically requires intervention 2
• Percentage-based assessment:
  • Some clinicians use percentage of hemithorax volume
  • Pneumothoraces >20% of thoracic volume on chest X-ray often require intervention 2
  • Pneumothoraces ≥50% have higher rates of prolonged air leak and recurrence, potentially indicating need for surgical intervention 3

Imaging Considerations

Plain Radiography Limitations

• Plain PA radiographs typically underestimate pneumothorax size 1
• Expiratory radiographs add little value and are not recommended as routine investigation 1
• When standard PA views are inconclusive:
  • Lateral radiographs can provide additional information in up to 14% of cases 1
  • Lateral decubitus radiographs are superior to erect or supine views and approach CT sensitivity 1

CT Scanning Indications

• Most robust approach for exact size estimation 1
• Recommended only for specific situations:
  • When lungs are obscured by surgical emphysema
  • To differentiate pneumothorax from bullae in complex cystic lung disease
  • In cases where management decisions depend on precise size measurement 1

Clinical Implications of Size Classification

Treatment Approach Based on Size

• Small primary pneumothorax (<2 cm) with minimal symptoms:

  • Observation is the treatment of choice 1
  • Can be considered for discharge with early outpatient review
  • Patient should receive clear instructions to return if breathlessness worsens 1

• Small secondary pneumothorax (<1 cm depth):

  • Observation may be appropriate in asymptomatic patients
  • Hospitalization is recommended for monitoring 1

• Large pneumothorax (>2 cm) or symptomatic patients:

  • Active intervention required (aspiration or chest drain insertion)
  • Simple aspiration is first-line for primary pneumothoraces 1
  • Secondary pneumothoraces more likely require chest tube drainage 1

Important Caveats

• Symptoms override size classification: Breathless patients should not be left without intervention regardless of pneumothorax size on chest radiograph 1
• Resolution rate: Spontaneous pneumothoraces reabsorb at approximately 1.25-1.8% of hemithorax volume per 24 hours 1
• Supplemental oxygen: High-flow oxygen (10 L/min) can increase reabsorption rate four-fold and should be administered to hospitalized patients 1
• Secondary pneumothorax considerations: Even small secondary pneumothoraces may have significant implications and require more aggressive management 1
• Automated detection: Modern AI systems can detect moderate and large pneumothoraces with reasonable accuracy, but may miss small pneumothoraces 4

While some case reports suggest that even large traumatic pneumothoraces may resolve spontaneously in stable patients 5, the standard of care remains intervention for large pneumothoraces, particularly in patients with underlying lung disease.