Causes of Spontaneous Pneumothorax
Spontaneous pneumothorax is classified into primary (occurring in otherwise healthy individuals, predominantly caused by rupture of subpleural blebs/bullae) and secondary (arising from underlying lung disease, most commonly COPD, infections, malignancies, and interstitial lung diseases). 1
Primary Spontaneous Pneumothorax (PSP)
Pathophysiology and Structural Causes
Subpleural blebs and bullae are the primary mechanism, found in up to 90% of cases at thoracoscopy or thoracotomy and in up to 80% of cases on CT scanning. 1
The etiology of these bullous changes in otherwise healthy lungs remains unclear, though multiple factors contribute. 1
Risk Factors
Smoking is the dominant modifiable risk factor: The lifetime risk of developing pneumothorax in healthy smoking men is approximately 12% compared with 0.1% in non-smoking men. 1 This trend is present, though to a lesser extent, in women. 1
Biomechanical factors play a significant role: Young male adults with PSP have characteristically taller chests, wider transversely, and flatter anteroposteriorly (low thoracic index), resulting in 20-fold higher pleural stress in the lung apex. 2
Genetic predisposition exists in familial cases: A 4-bp deletion in the FLCN gene (Birt-Hogg-Dubé gene) causes dominantly inherited PSP with 100% penetrance for bullous lung lesions. 3 Patients with familial PSP may be at greater risk of developing renal cancer and should be investigated for Birt-Hogg-Dubé syndrome. 3
Important Clinical Caveat
- Physical activity does NOT appear to be related to the onset of pneumothorax, contrary to common belief. 1
Secondary Spontaneous Pneumothorax (SSP)
Most Common Causes
Chronic obstructive pulmonary disease (COPD) remains the most common cause of secondary spontaneous pneumothorax, typically due to rupture of emphysematous bullae. 4, 5
Infectious etiologies include:
Other Pulmonary Conditions
Malignancies: Both primary lung cancers and metastatic disease can cause SSP. 6
Interstitial lung diseases: Including sarcoidosis and pulmonary fibrosis (which may present initially as spontaneous pneumothorax in young persons). 6, 5
Cystic lung diseases: Lymphangioleiomyomatosis and pulmonary histiocytosis X (particularly in young persons with multiple recurrent pneumothoraces). 6, 5
Connective tissue disorders: Various autoimmune conditions affecting the lungs. 6
Gender-Specific Consideration
- Catamenial pneumothorax secondary to thoracic endometriosis should be considered in women, particularly if pneumothorax occurs in temporal relationship to menstruation. 6
Clinical Implications
Secondary pneumothoraces present with more severe symptoms than primary pneumothoraces, with breathlessness typically out of proportion to pneumothorax size. 1
The distinction between primary and secondary pneumothorax is critical as it fundamentally alters management approach, prognosis, and need for investigation of underlying disease. 7, 1
Advanced imaging with CT is superior to chest radiography for detecting subtle parenchymal abnormalities that may reveal the underlying cause of SSP. 6 Photon-counting detector CT offers enhanced spatial resolution and may reveal causes missed on conventional CT. 6