What is interstitial lung disease (ILD)?

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What is Interstitial Lung Disease (ILD)?

Interstitial lung disease (ILD) refers to a heterogeneous group of over 200 acute and chronic bilateral parenchymal infiltrative lung disorders characterized by variable degrees of inflammation and fibrosis affecting the distal airways, alveolar walls, and interstitium in immunocompetent hosts without infection or neoplasm. 1

Clinical Presentation

ILD typically manifests with the following features:

  • Exertional dyspnea as the hallmark symptom 1, 2
  • Chronic cough in approximately 30% of patients 1, 2
  • Bilateral pulmonary infiltrates on thoracic imaging 1
  • Abnormal pulmonary physiology with restrictive pattern and reduced gas transfer 1
  • Progressive course evolving over months to years in most cases 1

Pathologic Characteristics

The pathologic hallmark involves:

  • Accumulation of inflammatory and immune effector cells (lymphocytic, neutrophilic, eosinophilic, or granulomatous patterns) 1
  • Abnormal extracellular matrix deposition in the distal airways, alveolar walls, and interstitium 1
  • Variable degrees of fibrosis that can progress to architectural distortion and honeycombing 1

Major Etiologic Categories

ILD can be systematically classified into three primary categories 1, 3, 4:

1. ILD of Known Cause or Association

  • Connective tissue disease-associated ILD (CTD-ILD): Accounts for 20-25% of all ILD cases 3, 4, 2
    • Rheumatoid arthritis (39% of CTD-ILD) 3, 4
    • Systemic sclerosis (31% of CTD-ILD) 3, 5
    • Sjögren's syndrome, systemic lupus erythematosus, inflammatory myopathies 3, 5, 4
  • Occupational/environmental exposures: Pneumoconioses (asbestosis, silicosis, coal worker's pneumoconiosis, berylliosis) 1, 3
  • Hypersensitivity pneumonitis: Immune reactions to inhaled organic antigens; critically, 47% of patients with apparently "idiopathic" ILD actually have hypersensitivity pneumonitis when thoroughly evaluated 3, 4
  • Drug-induced ILD: Immunosuppressive agents (methotrexate, TNF-alpha inhibitors, rituximab), nitrofurantoin, and others carry approximately 1% risk 3, 4

2. Idiopathic Interstitial Pneumonias (IIP)

This encompasses ILD of unknown etiology 1:

  • Idiopathic pulmonary fibrosis (IPF): Represents one-third of all ILD cases, characterized by usual interstitial pneumonia (UIP) pattern 1, 4, 2
  • Nonspecific interstitial pneumonia (NSIP) 1
  • Cryptogenic organizing pneumonia (COP) 1
  • Desquamative interstitial pneumonia (DIP) and respiratory bronchiolitis-ILD (RBILD): Smoking-related ILD 1
  • Acute interstitial pneumonia (AIP) and lymphoid interstitial pneumonia (LIP) 1

3. Other ILD of Unknown Cause

  • Sarcoidosis: Characterized by discrete nonnecrotizing granulomas with lymphatic distribution 1, 4

Epidemiology and Impact

  • Prevalence: Affects approximately 650,000 people in the United States 2
  • Mortality: Causes 25,000-30,000 deaths annually in the US 2
  • Increasing burden: The Global Burden of Disease Study projects that ILD-associated deaths will double over the next 20 years 6

Progressive Pulmonary Fibrosis (PPF)

A subset of ILD patients develop a progressive fibrosing phenotype defined by at least two of the following within one year 1:

  1. Worsening respiratory symptoms
  2. Physiological decline (FVC decline >5% predicted OR DLCO decline >10% predicted)
  3. Radiological progression (increased traction bronchiectasis, new ground-glass opacity with traction bronchiectasis, new/increased reticulation, new/increased honeycombing, increased lobar volume loss)

A 5% decline in FVC over 12 months is associated with approximately 2-fold increase in mortality compared with stable FVC. 2

Critical Diagnostic Pitfall

The most critical error is accepting an "idiopathic" diagnosis without systematically excluding medication history, environmental exposures, connective tissue disease, and occupational exposures, because identification and removal of causative factors may result in improved clinical outcomes. 3, 4 Nearly half of patients initially thought to have idiopathic disease actually have hypersensitivity pneumonitis when properly evaluated with detailed exposure history 3, 4.

Acute Presentations

While most ILD is chronic, some forms present acutely 1:

  • Acute interstitial pneumonia (AIP)
  • Acute eosinophilic pneumonia (AEP)
  • Acute hypersensitivity pneumonitis
  • Diffuse alveolar hemorrhage (DAH)
  • Acute exacerbations of chronic ILD (particularly IPF)
  • Drug reactions

References

1
Guideline

Guideline Directed Topic Overview

Dr.Oracle Medical Advisory Board & Editors, 2025

3
Guideline

Causes of Interstitial Lung Disease

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2025

4
Guideline

Causes of Interstitial Lung Disease in Adults

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

5
Guideline

Interstitial Lung Disease Etiology and Diagnosis

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

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Professional Medical Disclaimer

This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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