Interstitial lung disease refers to a group of more than 200 chronic disorders in which lung tissue is damaged, the walls of the air sacs become inflamed, and scarring (fibrosis) begins in the interstitium—the tissue between the air sacs.
Although many of the disorders underlying interstitial lung disease are uncommon, taken together they are a frequent cause of respiratory problems, accounting for 15 percent of all lung disorders evaluated by pulmonologists.
The lung scarring associated with interstitial lung disease leads to stiffness that makes breathing difficult and interferes with the transfer of oxygen in the lungs. The pace of lung deterioration varies from person to person.
Interstitial lung disease includes the following disorders (among others):
• Pulmonary sarcoidosis is an inflammatory disease in which granulomas—small clusters of cells—develop in the lungs and in the lymph nodes, the small groups of immune-system cells adjacent to the lungs. This disease can increase lung stiffness, which reduces lung volume.
• Bronchiolitis obliterans organizing pneumonia (BOOP), also called cryptogenic organizing pneumonia, refers to inflammation with blockage of the bronchioles (the small airways in the lungs that branch from the large ones).
• Asbestosis and silicosis refer to damage caused by exposure to dust from asbestos and silica, respectively.
• Idiopathic pulmonary fibrosis is a condition of unknown cause (“idiopathic”) that involves progressive scarring of the lungs.
• Hypersensitivity pneumonitis is inflammation in the lungs related to an allergic reaction to inhaled dust, fungus, molds, or chemicals. While this can resolve with removal of the offending agent, chronic exposure leads to progressive scarring of the lungs.
Causes of interstitial lung disease
In many cases, the cause of lung injury is unknown. Factors known to be injurious to the air sacs, potentially resulting in interstitial lung disease, include:
• Certain prescription drugs (including chemotherapy
drugs and cardiovascular medications)
• Exposure to environmental toxins
• Infectious agents (viruses, bacteria, or fungi)
• Substances (such as certain proteins in bird droppings) that set off allergic or hypersensitivity reactions in susceptible persons
• Connective tissue diseases, such as scleroderma, rheumatoid arthritis, and lupus, all of which can cause inflammation and scarring of organs, including the lungs
• The spread of cancer to the lungs from other parts of the body
Symptoms of interstitial lung disease
Symptoms of interstitial lung disease vary widely depending on the specific disease, but most people with one of these conditions become short of breath when they exert themselves and have a persistent cough that does not produce phlegm.
Fatigue, loss of appetite, and weight loss are common; wheezing, chest pain, coughing up blood, and fever also are possible. Depending on the underlying cause, symptoms may last only a few weeks. More often, symptoms persist for months or years.
Many types of interstitial lung disease are also characterized by periodic and sometimes frequent exacerbations in which symptoms worsen significantly.
While not fully understood, these flares are thought to be triggered by infections, but may also be related to development of pulmonary blood clots, pneumothorax, heart failure, or exposure to air pollution.
Diagnosis of interstitial lung disease
Abnormalities on a chest X-ray may be the first evidence of interstitial lung disease, but they are seldom specific to a particular disorder.
For example, lung function tests typically show a decrease in total lung capacity—the amount of air in the lungs after a deep inhalation.
Another abnormality is a decrease in diffusing capacity, a measurement of how well gas passes across the membranes in the lungs. Again, these deviations do not distinguish among the various underlying causes of interstitial lung disease.
Results from high-resolution chest CT scans may be helpful in individuals suspected of having interstitial lung disease.
For example, a scan showing changes in which the edge of a severely damaged lung resembles a honeycomb may eliminate the need for a lung biopsy because it is so specific for one form of interstitial lung disease.
In some cases, it is possible to make a definitive diagnosis based on examination of lung fluids obtained using bronchoalveolar lavage (washing out the lungs by passing a saline solution into them through the thin tube of a bronchoscope inserted into the mouth or nose).
In other cases, a lung biopsy may be necessary to make a definitive diagnosis as well as to determine the severity of the disease.
The least invasive biopsy procedure is performed through a bronchoscope.
This procedure can be performed on an outpatient basis; however, the small size of the tissue sample may not be sufficient for certain diagnoses. If a biopsy through a bronchoscope is inconclusive, an open surgical biopsy in a hospital is required.
Prevention of interstitial lung disease
Avoiding potential triggers of interstitial lung disease is crucial. Wearing appropriate masks and monitoring exposure to irritants in certain occupational environments (for example, mining and jobs that involve working with asbestos or metal dusts) are important precautions to take.
In addition, hypersensitivity pneumonitis can occur as a result of exposure to pet birds; removal of the bird from the home is imperative to prevent ongoing lung damage.
Treatment of interstitial lung disease
Several types of interstitial lung disease, most notably sarcoidosis and hypersensitivity reactions, get better on their own, with removal of the offending agent (for hypersensitivity reactions) or in response to treatment with corticosteroids. Avoidance of toxin or allergen exposures also can be beneficial.
Unfortunately, for some other causes of interstitial lung disease there is no specific treatment to slow the progression of the disease or reverse the damage that has already occurred.
Further inflammation and scarring of lung tissue weaken lung function and can lead to pulmonary hypertension and right heart failure, which slows the movement of blood through the heart.
Although corticosteroid treatment can improve pulmonary sarcoidosis, it often fails to relieve symptoms in people who have other forms of interstitial lung disease.
Effective treatments for idiopathic pulmonary fibrosis have been lacking. In 2014, however, the U. S. Food and Drug Administration approved two new IPF treatments. In clinical trials, these medications—pirfenidone (Esbriet) and nintedanib (Ofev)—were shown to significantly slow disease progression.
The National Heart, Lung, and Blood Institute’s Idiopathic Pulmonary Fibrosis Network, a network of research centers dedicated to the study of the disease, continues to investigate therapies for idiopathic pulmonary fibrosis in both its early and advanced stages.