Primary spontaneous pneumothorax is the most common cause of pneumothoraces and has an estimated overall incidence of 5 to 10 cases per 100,000 per year. It occurs predominantly in young, healthy men, with an incidence in this group as high as 1 in 500 per year, and is due in most cases to rupture of apical subpleural blebs in otherwise normal lungs (Fig. 12.2). The pathogenesis of these apical blebs is unclear, although it is postulated that higher transpulmonary pressures at the apex lead to greater alveolar distending pressures. The resultant rupture of alveoli traps air between the internal and external elastic membranes of the visceral pleura. It is noted that these types of pneumothoraces most often occur in tall, thin individuals, many of whom are smokers. The lifetime risk of developing a pneumothorax in an otherwise healthy man is estimated at 0.1%, whereas it is as high as 12% in one who is a smoker. There may also be an association with connective
tissue disorders such as Marfan syndrome. A familial form of spontaneous pneumothorax has been described with autosomal-dominant inheritance and incomplete penetrance.

Spontaneous pneumothorax can also occur as a result of underlying lung disease, in which case it is referred to as a secondary spontaneous pneumothorax. There are various pulmonary disease states leading to secondary pneumothoraces (Table 12.1). Most cases are related to bullous-type disease, with hyperinflation leading ultimately to rupture and subsequent pulmonary parenchymal collapse. Diseases characterized by cystic lesions in the pulmonary parenchyma such as cystic fibrosis and lymphangioleiomyomatosis can also lead to spontaneous rupture and pneumothorax. Malignant
pulmonary nodules, both primary and metastatic, usually located subpleurally, can rupture as they expand and result in a spontaneous pneumothorax.

Spontaneous pneumothoraces that occur and recur in relation to the menstrual cycle are referred to as catamenial pneumothoraces. These episodes occur within 72 hours before or after the onset of menstruation. Three distinct mechanisms have been proposed based on metastatic, hormonal, and anatomic models. The metastatic model hypothesizes migration of endometrial tissue via the peritoneal cavity through transdiaphragmatic lymphatic channels, via diaphragmatic fenestrations, or hematogenously into the pleural space. An alternative theory suggests that endometrial tissue may be deposited in the chest cavity during embryonal development. Endometrial deposits have been identified in the pleural space in 13% to 62.5% of these cases. Hormonally regulated monthly sloughing of endometrial tissue is believed to result in pleural irritation, causing chest pain, and subpleural breaches, causing air leaks. The hormonal hypothesis suggests that high serum levels of prostaglandin F₂ at ovulation leads to vasospasm, associated ischemia with tissue injury, and alveolar rupture. The anatomic model for catamenial pneumothorax is based on the influx of air into the pleural space from the peritoneal cavity via diaphragmatic fenestrations (Fig. 12.3).

Infectious diseases of the lung in their many forms may result in secondary spontaneous pneumothorax. This is most notable in the immunocompromised host such as individuals infected with the human immunodeficiency virus (HIV) and those with manifestations of acquired immunodeficiency syndrome. Pneumothorax occurs in 1% to 2% of hospitalized patients with HIV and is associated with 34% mortality. The most common etiologic agent in these cases is Pneumocystis carinii. Other virulent or opportunistic infections are also causally related to the occurrence of spontaneous pneumothoraces, including most recently severe acute respiratory syndrome (SARS).

Pneumothoraces may also be incited by iatrogenic or traumatic events. Penetration of the pleural space by a needle during a diagnostic or therapeutic procedure, whether deliberate or accidental, may result in a so-called acquired pneumothorax. This is usually due to a breach or laceration of the visceral pleura. This can occur during procedures such as transthoracic needle aspiration, central line placement in the neck, and thoracentesis. In the latter procedure, recent studies have shown that the incidence of iatrogenic pneumothorax can be significantly reduced by using ultrasound guidance.