Pulmonary fibrosis is a condition in which excessive scar tissue develops within the lung interstitium, the supporting tissue surrounding alveoli and other pulmonary structures. Fibrosis causes the lungs to become thicker, stiffer and less capable of expanding normally. It belongs to the broader group of interstitial lung diseases, which involve abnormalities of the lung interstitium and sometimes the alveolar epithelium, pulmonary vasculature and other compartments. Pulmonary fibrosis is not a single disease: it is a descriptive term encompassing several disorders with different causes, patterns, rates of progression and treatments. Idiopathic pulmonary fibrosis is one specific form in which the cause is not identified.
Normal gas exchange requires oxygen and carbon dioxide to diffuse across an extremely thin alveolar-capillary interface. Fibrosis deposits excessive extracellular matrix, including collagen, within the interstitial tissue, increasing the distance and altering the architecture through which gases must diffuse. As disease progresses, the normal arrangement of alveoli can become distorted or replaced by scarred tissue. Oxygen transfer may therefore become particularly impaired during exercise, when the body's oxygen requirements increase and pulmonary capillary transit time decreases. Carbon dioxide generally diffuses more readily than oxygen, so significant carbon-dioxide retention tends to occur later and is not the defining physiological abnormality of pulmonary fibrosis.
Fibrotic lungs are also mechanically stiff, producing a restrictive ventilatory defect. A person may be unable to inhale as much air as expected because total lung capacity and other lung volumes decrease. Unlike COPD, where airflow obstruction and air trapping predominate, pulmonary fibrosis generally causes restriction and reduced lung compliance. Spirometry may show a reduced forced vital capacity, while full pulmonary function testing can demonstrate reduced total lung capacity and a reduced diffusing capacity for carbon monoxide. The pattern helps distinguish interstitial disease from obstructive conditions, although additional testing is required to establish the specific diagnosis.
Breathlessness and persistent dry cough are typical symptoms. Breathlessness often first becomes apparent during exertion and may gradually progress as fibrosis increases. Fatigue and reduced exercise tolerance can develop because breathing requires greater effort and oxygen transfer becomes less efficient. Some people develop a characteristic fine inspiratory crackle on chest examination, sometimes described as resembling the sound of separating hook-and-loop fastener, although the physical finding is not specific to one disease. Finger clubbing occurs in some forms of pulmonary fibrosis but is not universal.
High-resolution computed tomography is central to evaluating suspected pulmonary fibrosis because it can reveal patterns of interstitial abnormality and help distinguish different diseases. Certain forms produce characteristic combinations of reticulation, traction bronchiectasis and honeycombing, while other patterns may suggest alternative diagnoses such as non-specific interstitial pneumonia, hypersensitivity pneumonitis or connective-tissue-disease-associated interstitial lung disease. The radiological pattern is interpreted alongside exposure history, medications, autoimmune features and clinical findings. In selected cases, multidisciplinary discussion among respiratory physicians, radiologists, pathologists and other specialists is important because classification can determine treatment.
Pulmonary fibrosis can have many causes. Some forms are associated with connective-tissue diseases such as systemic sclerosis, rheumatoid arthritis and certain inflammatory autoimmune disorders. Occupational or environmental exposures can cause interstitial lung injury, including exposure to mineral dusts, metal particles, agricultural materials and mould or other biological antigens. Certain medicines and medical treatments, including some chemotherapy drugs, amiodarone and radiotherapy involving the chest, can produce drug- or treatment-associated lung injury and fibrosis. Previous infections, autoimmune processes and other interstitial lung diseases can also leave fibrotic changes.
The course varies substantially between diseases and individuals. Some forms remain stable for long periods, some improve when the underlying inflammatory or exposure-related cause is removed and treated, and others progress despite treatment. Progressive fibrosis can lead to chronic respiratory failure, pulmonary hypertension, right-sided cardiac strain and increasing functional limitation. Acute exacerbations can occasionally cause sudden deterioration with new widespread lung injury superimposed on established fibrosis. Prognosis therefore cannot be inferred accurately from the word “fibrosis” alone; the specific underlying disease, radiological pattern, physiological impairment, rate of change and response to treatment are all relevant.
Treatment depends on the specific cause and disease behaviour. Removing an offending exposure, treating an associated autoimmune disease, discontinuing a causative medicine when medically appropriate and using disease-specific antifibrotic or immunomodulatory therapy can be relevant in selected conditions. Antifibrotic medicines such as nintedanib and pirfenidone are used in particular fibrotic lung diseases under specialist guidance, while immunosuppressive treatment is appropriate for some inflammatory connective-tissue-related disorders but can be harmful or inappropriate in other fibrotic conditions. Supportive care can include pulmonary rehabilitation, oxygen for documented hypoxaemia, vaccination and management of complications. Lung transplantation may be considered for selected patients with advanced disease.
