A bronchial cast is a solid, branching plug that forms inside the airways and takes on the exact shape of the bronchial tree, like a mold of the inside of your lungs. When coughed up or removed, it looks strikingly like a rubber replica of the airways it was blocking. These casts obstruct airflow and can cause severe breathing difficulty or, in serious cases, complete collapse of a section of lung. The condition they cause is called plastic bronchitis, a rare but potentially life-threatening disorder.
What Bronchial Casts Are Made Of
Unlike ordinary mucus plugs, bronchial casts are dense, rubbery structures that hold their shape outside the body. Their composition depends on what’s driving their formation. In people with underlying heart conditions, particularly children who have had a type of cardiac surgery called the Fontan procedure, the casts are made primarily of fibrin, the same protein the body uses to form blood clots. Researchers at the American Thoracic Society found that fibrin was far more abundant than mucus in these casts, making the old assumption that they form from mucus overproduction unlikely.
These cardiac-related casts also contain immune cells, including neutrophils, eosinophils, and macrophages, along with proteins normally found in blood and lymph fluid. In people whose casts are triggered by lung inflammation or allergic conditions, the casts tend to contain a higher proportion of inflammatory cells, especially a type of white blood cell called eosinophils. Regardless of the trigger, most casts turn out to contain some degree of inflammatory material.
Why They Form
Bronchial casts develop through two broad pathways: inflammatory disease and abnormal lymphatic drainage.
- Inflammatory causes: Asthma, cystic fibrosis, severe respiratory infections, and sickle cell disease can all trigger cast formation. During the 2009 H1N1 influenza pandemic, cases emerged of children forming airway casts large enough to collapse an entire lung. Severe infections drive intense inflammation and mucus or fibrin buildup that solidifies into a cast rather than draining normally.
- Lymphatic causes: The Fontan procedure, a surgery used to treat children born with only one functioning heart ventricle, reroutes blood flow so the large vein from the lower body connects directly to the pulmonary artery. This passive circulation raises pressure in the pulmonary veins, which impairs lymph drainage from the lungs. Lymph fluid leaks into the airways, where its high protein content, especially fibrin, solidifies into casts. About 4% of Fontan patients develop plastic bronchitis, and the problem can worsen in a vicious cycle: as casts block more airway, pulmonary pressure rises further, driving even more cast formation.
In adults, abnormal lymphatic anatomy can produce the same result without prior heart surgery. Cases have been documented in people with HIV-related cancers and in adults with congenital lymphatic malformations that cause lymph to drain directly into a bronchus instead of following its normal route back to the bloodstream.
What It Feels Like
The hallmark symptoms are a persistent cough, thick and sticky sputum, and progressive difficulty breathing. Some people cough up intact casts, which are pale, rubbery, and branching, sometimes several inches long. In one well-known case published in the New England Journal of Medicine, a 52-year-old man with no prior lung disease presented with shortness of breath, a cough producing copious thick sputum, and a visible shadow on his chest X-ray that turned out to be a large cast filling his right lower lung.
When a cast is large enough to block a major airway, the effect can be sudden and dramatic. Oxygen levels drop, breathing becomes labored, and part of the lung may collapse entirely. In children, this can present as episodes of intermittent choking or near-suffocation, sometimes called “intermittent asphyxia syndrome,” if a cast shifts position in the airway.
How Casts Are Removed
In an acute episode, the immediate priority is clearing the blocked airway. Bronchoscopy, a procedure where a thin, flexible camera is guided into the airways, allows doctors to see and physically extract the cast. Because the casts are solid and branching, pulling them out in one piece can be challenging, and fragments left behind may seed new cast growth.
Medications can help break down casts or prevent new ones. A clot-dissolving drug called tPA (tissue plasminogen activator), typically used for strokes and heart attacks, has been used successfully in plastic bronchitis by spraying it directly onto casts during bronchoscopy or delivering it through a nebulizer. In one reported case, a 6-year-old boy with recurring casts that had not responded to steroids or other inhaled treatments saw significant improvement after tPA was instilled directly into his airways. Another medication, dornase alfa, which thins mucus by breaking down DNA strands in it, is sometimes tried but appears less effective on its own against fibrin-heavy casts.
Long-Term Treatment for Recurring Casts
For people whose casts keep coming back, especially those with lymphatic causes, addressing the root problem matters more than removing individual casts. A newer, minimally invasive procedure called thoracic duct embolization has shown promising results. In this approach, interventional radiologists thread a catheter into the lymphatic system and use a glue-like substance to seal off the abnormal lymphatic channels that are leaking fluid into the airways.
In a recently published case, an adult with plastic bronchitis caused by an abnormal lymphatic vessel draining directly into the left lower lobe bronchus underwent this procedure. After embolization, the patient had complete resolution of symptoms, with no further coughing or cast production. The procedure is attractive because it is minimally invasive, requires a short hospital stay, and targets the underlying plumbing problem rather than just managing symptoms.
For Fontan patients, the outlook remains more serious. The five-year mortality rate for Fontan patients who develop plastic bronchitis has been reported as high as 50%, reflecting both the severity of the underlying heart condition and the difficulty of controlling recurring cast formation. Some of these patients ultimately require heart transplantation if their lymphatic complications cannot be managed.
Who Is Most at Risk
Plastic bronchitis is rare in the general population. The largest identifiable group is children with single-ventricle heart defects who have undergone the Fontan procedure, where the incidence sits around 4%. Beyond this group, cases pop up across a wide range of conditions: severe asthma, sickle cell disease with acute chest syndrome, cystic fibrosis, and serious respiratory infections including influenza. It can occur at any age, from early childhood to adulthood, and occasionally appears in people with no known underlying disease, presumably from undiagnosed lymphatic abnormalities.
Because the condition is rare and its symptoms overlap with more common respiratory problems like pneumonia or asthma attacks, it is often diagnosed late, sometimes only after a patient coughs up a recognizable cast or a bronchoscopy reveals one. If you or a child has recurrent episodes of unexplained breathing difficulty with thick, rubbery material being coughed up, plastic bronchitis is worth raising with a pulmonologist.