Pathophysiology
The development of bronchiectasis requires two factors: an initial injury to the lung (such as from infection, auto-immune destruction of lung tissue, or other destruction of lung tissue (as seen in gastroesophageal reflux disease or aspiration syndromes)) which leads to impaired mucociliary clearance, obstruction, or a defect in host defense. This triggers a host immune response from neutrophils (elastases), reactive oxygen species, and inflammatory cytokines that results in progressive destruction of normal lung architecture. In particular, the elastic fibers of bronchi are affected. The result is permanent abnormal dilation and destruction of the major bronchi and bronchiole walls.Disordered neutrophil function is believed to play a role in the pathogenesis of bronchiectasis. Neutrophil extracellular traps(NETs), which are extracellular fibers secreted by neutrophils that are used to trap and destroy pathogens, are hyperactive in bronchiectasis. Increased NET activity is associated with more severe bronchiectasis. Neutrophil elastase, which is an extracellular protein secreted by neutrophils to destroy pathogens as well as host tissue, is also hyperactive in many cases of bronchiectasis. An increased neutrophil elastase activity is also associated with worse outcomes and more severe disease in bronchiectasis. The initial lung injury in bronchiectasis leads to an impaired mucociliary clearance of the lung airways, which leads to mucous stasis. This mucous stasis leads to bacterial colonization in bronchiectasis which leads to neutrophil activation. This neutrophil activation leads to further further tissue destruction and airway distortion by neutrophils in addition to direct tissue destruction by the pathogenic bacteria.
A history of mycobacterial infections such as tuberculosis can lead to damage of the airways that predisposes to bacterial colonization. Severe viral infections in childhood can also lead to bronchiectasis through a similar mechanism. Nontuberculous mycobacteria infections such as Mycobacterium avium complex are found to be a cause in some patients. Recent studies have also shown Nocardia infections to been implicated in bronchiectasis. Impaired host defenses
Impairments in host defenses that lead to bronchiectasis may be congenital, such as with primary ciliary dyskinesia, or acquired, such as with the prolonged use of immunosuppressive drugs. Additionally, these impairments may be localized to the lungs, or systemic throughout the body. In these states of immunodeficiency, there is a weakened or absent immune system response to severe infections that repeatedly affect the lung and eventually result in bronchial wall injury. HIV/AIDS is an example of an acquired immunodeficiency that can lead to the development of bronchiectasis. Aspergillosis
Allergic bronchopulmonary aspergillosis (ABPA) is an inflammatory disease caused by hypersensitivity to the fungus Aspergillus fumigatus. It is suspected in patients with a long history of asthma and symptoms of bronchiectasis such as a productive, mucopurulent cough. Imaging often shows peripheral and central airway bronchiectasis, which is unusual in patients with bronchiectasis caused by other disorders. Autoimmune diseases
Several autoimmune diseases have been associated with bronchiectasis. Specifically, individuals with rheumatoid arthritis and Sjögren syndrome have increased rates of bronchiectasis. In these diseases, the symptoms of bronchiectasis usually presents later in the disease course.