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Interstitial lung disease is a group of disorders that involve pulmonary parenchyma. The exact pathogenesis of these disorders is not fully understood. There are multiple initiating factors that cause pulmonary injury. However, immunopathogenic responses of lung tissue are quite similar. There are two major histopathologic patterns in response to lung injury which include inflammation, fibrosis and granulomatous response. Interstitial lung disease may be classified into several subtypes based on the lung response to tissue injury and their causes. The exact underlying cause of interstitial lung disease might be well known such as toxic environmental or occupational exposure, cigarette smoking, and radiation. The other causes of interstitial lung disease are idiopathic.

Pulmonary alveolar proteinosis

Pulmonary alveolar proteinosis (PAP) is a rare lung disease that is characterized by the intra-alveolar accumulation of surfactant phospholipid and apoproteins. 

• The main pathogenesis of pulmonary alveolar proteinosis is a reduction in granulocyte-macrophage colony-stimulating factor (GM-CSF) levels or function and/or impaired alveolar macrophage function.
•  Terminal bronchioles and alveoli are filled with a lipoproteinaceous material that will be periodic acid-Schiff (PAS) stain positive.
• The exact etiology of pulmonary alveolar proteinosis is unknown.
• If left untreated, patients with pulmonary alveolar proteinosis may progress to develop pulmonary fibrosis or cor pulmonale.

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Lymphocytic infiltrative disorders

Lymphocytic infiltrative disorders might cause interstitial lung disease in mostly HIV positive children.

• There are two main manifestation of lymphocytic infiltrative disorders that include:

• • Lymphocytic interstitial pneumonitis
  • Pulmonary lymphomatoid granulomatosis
• The etiology of lymphocytic infiltrative disorders is unknown. However, there is an evidence of infectious cause such as EBV in HIV positive patients.

For more information about lymphocytic interstitial pneumonitis, click here.

Pulmonary lymphangioleiomyomatosis

For more information about pulmonary lymphangioleiomyomatosis, click here.

Pulmonary hemorrhage syndromes

• Goodpasture syndrome
• Idiopathic pulmonary hemosiderosis
• Isolated pulmonary capillaritis

Granulomatous lung response

• Hypersensitivity pneumonitis 
• Sarcoidosis
• Granulomatous vasculitides
  • Granulomatosis with polyangiitis (Wegener)
  • Eosinophilic granulomatosis with polyangiitis (ChurgStrauss)
• Bronchocentric granulomatosis

For more information about hypersensitivity pneumonitis, click here.

Interstitial lung disease associated with systemic diseases

Interstitial lung disease associated with connective tissue diseases

• Systemic lupus erythematosus
• Rheumatoid arthritis
• Ankylosing spondylitis
• Systemic sclerosis
• Sjögren syndrome
• Polymyositis/dermatomyositis
• Granulomatosis with polyangiitis (Wegener)
• Eosinophilic granulomatosis with polyangiitis (Churg Strauss)

Interstitial lung disease associated with inherited diseases

• Tuberous sclerosis
• Neurofibromatosis
• Niemann−Pick disease
• Gaucher disease
• Hermansky−Pudlak syndrome

Interstitial lung disease associated with gastrointestinal or liver diseases

• Crohn disease
• Primary biliary cirrhosis
• Chronic active hepatitis
• Ulcerative colitis

Interstitial lung disease associated with graft−versus−host disease

• Bone marrow transplantation
• Solid organ transplantation