Bronchiectasis

With bronchiectasis, “bronchi” refers to bronchi and bronchioles, which are small airways in the lungs, while “-ectasis” means dilation. So, in bronchiectasis, chronic inflammation in the lungs damages the smooth muscle cells and elastic tissue that support these small airways, resulting in their permanent dilation.

When you take a breath, your diaphragm moves down, creating negative pressure within your chest. This gentle vacuum in the chest pulls air in through your nose or mouth down the trachea. From here, the air travels through the primary bronchi, into the secondary bronchi, then tertiary bronchi, and finally into the bronchioles, which lead directly to the alveoli, where the gas exchange occurs.

Now, the walls of our airways consist of smooth muscle cells and elastic tissue that help the airways open and return to their shape as we breathe. These walls are lined with epithelial cells, which have tiny brush-like projections called cilia, and goblet cells, which produce sticky mucus. The mucus traps dust, bacteria, and other unwanted particles, and the cilia move together in coordinated waves, pushing the mucus and trapped particles toward the throat so we can clear them out of the lungs. This system is called the mucociliary escalator. This way, we can either swallow or cough out foreign particles that end up in our airways.

Now, bronchiectasis can develop due to chronic respiratory infections or lung obstructions. These are closely related and tend to trigger each other, setting off a cycle that slowly damages the airways over time.

First, let’s look at how chronic, recurrent respiratory infections can lead to bronchiectasis. When a pathogen reaches the small airways in the lungs, epithelial cells respond by releasing pro-inflammatory cytokines, which signal neutrophils to join the action. As neutrophils fight the pathogen, they use powerful weapons, like elastase and reactive oxygen species. These substances are great at killing pathogens, but they also cause collateral damage, breaking down the surrounding structures, including epithelial cilia. As a result, epithelial cilia cannot propel the mucus and pathogens out of the lungs, causing mucus to accumulate, which creates a perfect environment for bacteria to thrive. With recurrent infections, chronic inflammation can destroy the surrounding smooth muscle cells and elastic tissue, resulting in permanent dilation of bronchi and bronchioles. Sometimes, airways can dilate up to four times their normal size.

This kind of damage is common in necrotizing pneumonia caused by Staphylococcus aureus, Klebsiella species, and Mycobacterium tuberculosis.

Another setting where bronchiectasis can develop includes immunodeficiencies, particularly the ones with low immunoglobulin levels. It might seem surprising, but when the immune system can’t effectively fight off pathogens, they keep coming back, causing repeated episodes of pneumonia. Over time, chronic inflammation destroys the small airways, eventually resulting in bronchiectasis.

Now, moving on to lung obstruction. In this case, the cilia of epithelial cells cannot move mucus and debris out of the lungs. This might happen when something is physically blocking the airway, such as tumors or foreign objects, like a peanut. It can also occur if the cilia themselves are not functioning properly, as in primary ciliary dyskinesia, or if the mucus is too thick to move, like in cystic fibrosis. In all cases, mucus accumulates and becomes a breeding ground for infection and chronic inflammation. Over time, chronic inflammation leads to bronchiectasis.

In both recurrent inflammation and lung obstruction, if the inflammation continues for too long, fibroblasts step in to repair the damage by depositing collagen. But, instead of healing, this results in fibrosis and scarring of the bronchial and bronchiolar walls. In more severe cases, the fibrosis can extend to the tissues surrounding the bronchioles, a process called peribronchiolar fibrosis.