What is interstitial lung disease (ILD)?

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Last updated: September 20, 2026 • View editorial policy

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What is Interstitial Lung Disease (ILD)?

Interstitial lung disease (ILD) is a diverse group of over 200 disorders characterized by inflammation and/or fibrosis of the lung parenchyma (the interstitium, alveolar walls, and distal airways), resulting in impaired gas exchange, progressive dyspnea, and declining lung function. 1, 2

Core Characteristics

ILD encompasses heterogeneous conditions affecting the lung interstitium with:

  • Prevalence: 72.1 to 164.2 cases per 100,000 Europeans 1
  • U.S. burden: Approximately 650,000 affected individuals with 25,000-30,000 deaths annually 2
  • Clinical presentation: Exertional dyspnea (primary symptom), with approximately 30% reporting cough 2
  • Pathophysiology: Varying degrees of tissue inflammation, immune cell accumulation, abnormal extracellular matrix deposition, and progressive fibrosis 3

Major Categories of ILD

Idiopathic Interstitial Pneumonias (IIPs)

The most clinically significant category includes 4, 5:

  • Idiopathic Pulmonary Fibrosis (IPF): The most common and aggressive form, accounting for 55% of IIPs and approximately one-third of all ILDs 4, 2. Characterized by the usual interstitial pneumonia (UIP) pattern with 5-year survival worse than several cancers 4
  • Nonspecific Interstitial Pneumonia (NSIP): 25% of IIPs, with variable prognosis 4, 5
  • Respiratory Bronchiolitis-ILD (RB-ILD): 10-15% of IIPs, smoking-related 4, 5
  • Cryptogenic Organizing Pneumonia (COP): 5% of IIPs 4
  • Desquamative Interstitial Pneumonia (DIP), Acute Interstitial Pneumonia (AIP), and Lymphoid Interstitial Pneumonia (LIP): Each <2% 4, 5

Connective Tissue Disease-Associated ILD (CTD-ILD)

Accounts for approximately 20-25% of all ILDs 1, 2:

  • Systemic sclerosis-ILD (SSc-ILD): 31% of CTD-ILDs, affects nearly 50% of SSc patients 1
  • Rheumatoid arthritis-ILD (RA-ILD): 39% of CTD-ILDs 1
  • Idiopathic inflammatory myopathy-ILD (IIM-ILD): Including dermatomyositis, polymyositis, antisynthetase syndrome 1
  • Sjögren's syndrome, systemic lupus erythematosus (2-10% prevalence), mixed connective tissue disease 1, 6

Exposure-Related ILD

  • Hypersensitivity pneumonitis (HP): 15% of all ILDs 2
  • Occupational exposures: Asbestosis, silicosis, coal miner's pneumoconiosis, berylliosis 7, 3
  • Drug-induced ILD 3

Other ILDs

  • Sarcoidosis: A major granulomatous ILD 4, 7
  • Lymphangioleiomyomatosis (LAM), Pulmonary alveolar proteinosis (PAP), Langerhans cell histiocytosis (LCH) 4, 7

Clinical Course and Prognosis

ILD behavior ranges from:

  • Stable disease: Some patients remain stable or improve with treatment

  • Progressive Pulmonary Fibrosis (PPF): Defined by ≥2 of the following within one year 7:

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

  • Acute presentations: Including acute exacerbations of chronic ILD, acute eosinophilic pneumonia, acute hypersensitivity pneumonitis 3

Critical Diagnostic Considerations

High-resolution computed tomography (HRCT) is central to diagnosis, with 91% sensitivity and 71% specificity for ILD subtypes 2. In IPF with appropriate clinical context, a UIP pattern on HRCT is sufficient for definitive diagnosis without surgical lung biopsy 4.

Key Pitfall

Early ILD can be asymptomatic with irreversible lung function loss already occurring 1. Nonspecific symptoms (fatigue, cough, dyspnea on exertion) may be attributed to other conditions, causing diagnostic delays 1.

Prognostic Markers

  • FVC decline ≥5% over 12 months: Associated with approximately 2-fold increased mortality 2
  • Traction bronchiectasis progression: Strong independent predictor of mortality in IPF 7
  • Up to 85% of end-stage fibrotic ILD patients develop pulmonary hypertension 2

Clinical Significance

ILD represents a major cause of morbidity and mortality, particularly in CTD patients where PPF is a primary cause of death alongside cardiovascular disease 1. The heterogeneous nature demands multidisciplinary evaluation involving pulmonologists, radiologists, pathologists, and when appropriate, rheumatologists 4, 1.

References

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