Interstitial Lung Disease (ILD) and Pulmonary Fibrosis (PF) are serious, related conditions affecting the lungs, but they are not the same. ILD is a broad category of disorders. Pulmonary Fibrosis describes a specific, damaging outcome—scarring—that can occur within that category. Understanding the connection requires clear terminology, as this distinction is crucial for grasping the diagnosis and treatment of these progressive pulmonary conditions.
Defining the Relationship Between ILD and PF
Interstitial Lung Disease (ILD) is an umbrella term encompassing over 200 chronic disorders that cause inflammation or scarring in the lungs’ supportive framework, known as the interstitium. Pulmonary Fibrosis (PF) is a descriptive term referring specifically to the presence of scar tissue in the lungs. The term “pulmonary” means lung, and “fibrosis” means scarring.
ILD is the overall category of diseases affecting the interstitium, while PF is the specific pathological consequence—irreversible scarring—that occurs in many, but not all, types of ILD. All cases of Pulmonary Fibrosis are considered a type of Interstitial Lung Disease, but not all Interstitial Lung Diseases result in Pulmonary Fibrosis.
The Spectrum of Interstitial Lung Diseases
The interstitium is a delicate, lace-like network of tissue surrounding the tiny air sacs (alveoli) and small blood vessels in the lungs. This space is where the exchange of oxygen into the bloodstream and carbon dioxide out of it takes place. When ILD develops, this tissue becomes inflamed or damaged, causing it to thicken and stiffen.
This damage makes it increasingly difficult for gases to cross the alveolar-capillary membrane, slowing the movement of oxygen into the blood. ILD can be caused by an array of factors, often grouped into categories based on their origin.
Causes of ILD
One major grouping involves environmental or occupational exposures, such as inhaling inorganic dusts like asbestos or silica, leading to conditions like asbestosis or silicosis. Another group is associated with autoimmune or connective tissue diseases, where the immune system mistakenly attacks the lung tissue. Conditions such as Rheumatoid Arthritis, Scleroderma, and Lupus can cause a secondary ILD. Drug-induced ILD also results from adverse reactions to certain medications, including some chemotherapy agents or heart medicines. When the cause of the interstitial lung damage cannot be determined, the condition is classified as an idiopathic ILD.
When Scarring Occurs: Pulmonary Fibrosis
Pulmonary Fibrosis represents the physical consequence of the lung’s abnormal attempt to repair itself after injury. When the cells lining the lung’s air sacs are repeatedly injured, the body initiates a wound-healing response. In a fibrotic response, this process is dysregulated and excessive, leading to the deposition of excessive extracellular matrix proteins, primarily collagen, which forms scar tissue.
This scarring causes the lung tissue to become progressively thicker and stiffer, reducing the lung’s overall capacity and compliance. The deposition of scar tissue destroys the normal lung architecture, sometimes creating a characteristic “honeycomb” appearance on imaging. This loss of functional tissue severely impairs the transfer of oxygen into the bloodstream, often quantified by a reduced diffusion capacity.
The most common form of Pulmonary Fibrosis is Idiopathic Pulmonary Fibrosis (IPF), an ILD where the cause remains unknown. IPF is characterized by a specific scarring pattern called Usual Interstitial Pneumonia (UIP). The faulty repair mechanism involves repeated micro-injuries to the alveolar cells, leading to the accumulation of myofibroblasts. This results in the persistent production of a pathologic scar, making IPF an aggressive and progressive disease.
Diagnosis and Treatment Approaches
Diagnosing ILD and the presence of Pulmonary Fibrosis requires integrating patient history, physical examination, and specific diagnostic tests. High-resolution computed tomography (HRCT) scans are a standard and often definitive test, providing detailed images of the lung structure that reveal the extent and pattern of scarring or inflammation. Pulmonary function tests (PFTs) measure lung capacity and how efficiently oxygen is transferred into the blood, often showing a restrictive ventilatory defect and a reduced diffusion capacity.
In some cases, doctors may need to obtain a lung tissue sample through a bronchoscopy or surgical lung biopsy to determine the exact type of ILD. Treatment depends heavily on the specific ILD diagnosis and whether significant fibrosis is present. For ILDs driven by inflammation, such as those related to autoimmune diseases, immunosuppressive therapies like corticosteroids may be used to slow the disease.
For progressive fibrosing conditions, including IPF, two anti-fibrotic medications, pirfenidone and nintedanib, have been approved to slow the rate of lung function decline by targeting the scarring process itself. Supportive care is also a major component of management, including supplemental oxygen therapy and pulmonary rehabilitation to improve exercise tolerance and quality of life. Since fibrosis scarring is irreversible, the only option for patients with advanced, end-stage disease is lung transplantation.