Just uttering the word “mucus” is often sufficient to elicit amusement from those within earshot, but to patients with chronic inflammatory airway diseases, mucus is no laughing matter.
, which are then moved by cilia out of the airways for expulsion through coughing.
But in cystic fibrosis (CF), for example, mucus hypersecretion can be deadly. The underlying pathology of CF – a mutation in the CFTR gene, which codes for the protein CF transmembrane conductance regulator – leads to buildup in the lungs of abnormally viscous and sticky mucus, resulting in frequent, severe infections (particularly with Pseudomonas aeruginosa), progressive lung damage, and prior to the development of effective disease management, significantly premature death.
Mucus hypersecretion is also a feature of chronic obstructive pulmonary disease (COPD), noted Victor Kim, MD, from Temple University, Philadelphia, Christopher M. Evans, PhD, from the University of Colorado at Denver, Aurora, and Burton F. Dickey, MD, from the University of Texas MD Anderson Cancer Center, Houston.
In COPD, “mucus dysfunction arises from several mechanisms, including excess production due to inflammation, decreased elimination due to impaired ciliary clearance and reduced cough efficiency, and excessive concentration due to smoke-induced dysfunction of transepithelial anion transport resembling CF,” they wrote in an editorial published in the American Journal of Respiratory and Critical Care Medicine.
In patients with idiopathic pulmonary fibrosis, a polymorphism in the enhancer region of MUC5B, a gene that encodes for mucin glycoproteins, results in a 20-fold overexpression of the gene and prominent mucus production that has been shown to parallel lung inflammation and decline in forced vital capacity (FVC).
In patients with asthma, up-regulation of MUC5AC and stimulated mucus secretion conspire to obstruct airways, which can in extreme cases lead to death.
‘Short shrift’
Yet until recently, the role of mucus hypersecretion in diseases such as COPD has been largely overlooked, or as Dr. Kim and colleagues put it, “airway mucus often receives short shrift from clinicians.”
“It’s a pretty hot topic in pulmonary medicine today because it has been so neglected for so long,” Dr. Dickey said in an interview with CHEST Physician. “As clinicians we haven’t had a way to identify who needs treatment, which is ridiculous, because many of the people who expectorate a lot, like those with chronic bronchitis, don’t actually have small airway obstruction, and conversely, a lot of asthmatics, who have very serious small airway obstruction, don’t expectorate, so you can’t really tell from symptoms.”
What has changed in recent years is the use of chest CT to image muco-obstructive pathology, commonly called “mucus plugging” in the peripheral airways of patients with COPD and asthma.
“In the last decade or so, we’ve seen the emergence in obstructive lung diseases such as asthma and COPD the use of more objective measures on CT scans, including the problem of mucus plugging, which is unfortunately very common,” Dr. Kim said in an interview.
The discovery of the extent and severity of mucus in obstructive lung diseases has led to new strategies to combat mucus overconcentration, such as hydration, mucolytics, and an intriguing investigational approach to decrease calcium-induced hypersecretion with designer peptides.