What are the criteria for measuring pneumothorax?

Criteria to Measure Pneumothorax

Pneumothorax size is measured by the visible rim of air between the lung margin and chest wall on PA chest radiograph, with <2 cm classified as "small" and ≥2 cm classified as "large"—this simple measurement system has replaced older volume-based calculations and directly guides treatment decisions. 1, 2

Primary Measurement Method: The 2 cm Rule

• The British Thoracic Society classification uses a straightforward approach: measure the distance between the lung margin and chest wall at the widest point on a PA chest radiograph 1, 3
• Small pneumothorax = visible rim <2 cm 1, 2
• Large pneumothorax = visible rim ≥2 cm 1, 2
• This measurement is taken perpendicular to the chest wall at the level of the hilum on upright PA radiograph 1

Important Limitations of Plain Radiography

• Plain PA chest radiographs consistently underestimate pneumothorax volume, so clinical symptoms must take precedence over radiographic size when making treatment decisions 1, 3, 4
• The correlation between chest X-ray measurements and actual pneumothorax volume is poor (r = 0.71), meaning radiographs can significantly underestimate the true size 4
• A pneumothorax that appears "small" on chest X-ray may actually represent a much larger volume of air in the pleural space 1, 4

When to Use CT Scanning for Measurement

CT scanning is the most accurate method for quantifying pneumothorax size but should be reserved for specific situations: 1, 3, 5

• Complex bullous lung disease where differentiation between bullae and pneumothorax is critical (prevents dangerous aspiration attempts) 3, 5
• Extensive surgical emphysema obscuring the lungs on plain radiograph 1, 5
• When exact size quantification is required for clinical decision-making 1, 5
• Suspected aberrant chest tube placement 5

CT scanning should NOT be used routinely, as it adds little to management in straightforward cases 5

Alternative Measurement Systems

The 35 mm Rule (Trauma Context)

• In trauma patients, a radial measurement of ≤35 mm from chest wall to lung parenchyma on CT predicts successful observation with 90.8% positive predictive value 6
• This measurement is taken perpendicular to the chest wall at the largest air pocket on axial CT imaging 6
• Approximately 38 mm on chest X-ray correlates with the 35 mm CT cutoff 7
• This rule applies specifically to trauma patients and has not been validated for spontaneous pneumothorax 6, 8

Collins Equation (Historical)

• The Collins equation estimates pneumothorax volume as a percentage of hemithorax volume 2
• A pneumothorax >50% by Collins equation generally requires chest tube drainage 2
• This method has been largely replaced by the simpler 2 cm rule in current guidelines 1, 2

Critical Clinical Caveat

Never rely solely on radiographic size—marked breathlessness with a "small" pneumothorax may herald tension pneumothorax and requires immediate intervention regardless of measured size 1, 3

• Breathless patients require intervention regardless of radiographic appearance 1, 3
• Secondary pneumothorax patients develop symptoms out of proportion to radiographic size due to poor lung reserve 3
• Clinical symptoms are more important than measurements in determining need for intervention 9