What is the pathophysiology of spontaneous pneumothorax?

Updated: Apr 28, 2020
  • Author: Brian J Daley, MD, MBA, FACS, FCCP, CNSC; Chief Editor: Mary C Mancini, MD, PhD, MMM  more...
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Answer

The underlying pathophysiology of pneumothorax is reviewed in this section.

Spontaneous pneumothorax

Spontaneous pneumothorax in most patients occurs from the rupture of blebs and bullae. Although PSP is defined as occurring in patients without underlying pulmonary disease, these patients have asymptomatic blebs and bullae detected on computed tomography scans or during thoracotomy. PSP is typically observed in tall, young people without parenchymal lung disease and is thought to be related to increased shear forces in the apex.

Although PSP is associated with the presence of apical pleural blebs, the exact anatomic site of air leakage is often uncertain. Fluorescein-enhanced autofluorescence thoracoscopy (FEAT) is a novel method to examine the site of air leak in PSP. FEAT-positive lesions can be detected that appear normal when viewed under normal white-light thoracoscopy. [3]

In normal respiration, the pleural space has a negative pressure. As the chest wall expands outward, the surface tension between the parietal and visceral pleura expands the lung outward. The lung tissue intrinsically has an elastic recoil, tending to collapse inwards. If the pleural space is invaded by gas from a ruptured bleb, the lung collapses until equilibrium is achieved or the rupture is sealed. As the pneumothorax enlarges, the lung becomes smaller. The main physiologic consequence of this process is a decrease in vital capacity and partial pressure of oxygen.

Lung inflammation and oxidative stress are hypothesized to be important to the pathogenesis of PSP. [4] Current smokers, at increased risk for PSP, have increased numbers of inflammatory cells in the small airways. Bronchoalveolar lavage (BAL) studies in patients with PSP reveal that the degree of inflammation correlates with the extent of emphysematouslike changes (ELCs). One hypothesis is that ELCs result from degradation of lung tissue due to imbalances of enzymes and antioxidants released by innate immune cells. [5] In one study, erythrocyte superoxide dismutase activity was significantly lower and plasma malondialdehyde levels higher in patients with PSP than in normal control subjects. [4]

A growing body of evidence suggests that genetic factors may be important in the pathogenesis of many cases of PSP. Familial clustering of this condition has been reported. Genetic disorders that have been linked to PSP include Marfan syndrome, homocystinuria, and Birt-Hogg-Dube (BHD) syndrome.

Birt-Hogg-Dube syndrome is an autosomal dominant disorder that is characterized by benign skin tumors (hair follicle hamartomas), renal and colon cancer, and spontaneous pneumothorax. Spontaneous pneumothorax occurs in about 22% of patients with this syndrome. The gene responsible for this syndrome is a tumor suppressor gene located on band 17p11.2. The gene encoding folliculin (FLCN) is thought to be the etiology of Birt-Hogg-Dube syndrome. Multiple mutations have been found, and phenotypic variation is recognized. In one study, eight patients without skin or renal involvement had lung cysts and spontaneous pneumothorax. [6] A germ-line mutation to this gene has been found in five patients, and genetic testing is now available.


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