What are the 5 most common causes of Interstitial Lung Disease (ILD) and their incidence rates?

The 5 Most Common Causes of Interstitial Lung Disease and Their Incidence Rates

The five most common causes of interstitial lung disease (ILD) are idiopathic pulmonary fibrosis (55% of idiopathic interstitial pneumonias), connective tissue disease-associated ILD (25% of all ILD cases), hypersensitivity pneumonitis (15% of all ILD cases), sarcoidosis, and pneumoconioses. 1, 2

1. Idiopathic Pulmonary Fibrosis (IPF)

• Incidence rate: 31.5 per 100,000 in men and 26.1 per 100,000 in women 3
• Accounts for approximately one-third of all ILD cases 2
• Accounts for 55% of all idiopathic interstitial pneumonias 1
• Characterized by usual interstitial pneumonia (UIP) pattern on HRCT or lung biopsy 3
• Predominantly affects older adults with average age of 60 ± 11 years 4
• Male to female ratio is approximately equal 4

2. Connective Tissue Disease-Associated ILD (CTD-ILD)

• Incidence rate: Accounts for 25% of all ILD cases 2
• Average age of onset: 46 ± 17 years 4
• Equal gender distribution 4
• Rheumatoid arthritis is the most common underlying CTD causing ILD 4
• CTD-ILDs represent a substantial subset accounting for almost 20% of all ILDs 3
• Systemic sclerosis-related ILD (SSc-ILD) and rheumatoid arthritis-related ILD (RA-ILD) account for approximately 31% and 39% of CTD-ILDs, respectively 3

3. Hypersensitivity Pneumonitis (HP)

• Incidence rate: Accounts for 15% of all ILD cases 2
• Average age of onset: 53 ± 8.1 years 4
• Equal gender distribution 4
• Associated with exposure to organic antigens 3
• Can present in acute, subacute, or chronic forms 3
• Characterized by lymphocytic cellular pattern (>50% lymphocytes) on bronchoalveolar lavage 3

4. Sarcoidosis

• Incidence rate: Specific incidence not provided in the evidence, but recognized as one of the most common ILDs 3
• Female predominance 4
• Often presents with normal pulmonary function tests 4
• Characterized by non-caseating granulomas 3
• CD4+/CD8+ ratio >4 on BAL is highly specific for sarcoidosis 3

5. Pneumoconioses

• Incidence rate: Specific incidence not provided in the evidence, but recognized as one of the common ILDs 3
• Male predominance 4
• Often presents with normal pulmonary function tests 4
• Associated with occupational exposure to inorganic dusts 3
• Includes conditions such as silicosis, asbestosis, and coal worker's pneumoconiosis 3

Clinical Considerations

• The overall prevalence of clinically significant ILD ranges from 25 to 74 per 100,000 population 3
• In the US, ILD affects approximately 650,000 people and causes approximately 25,000 to 30,000 deaths per year 2
• The overall mortality rate for ILD is approximately 12.1% 4
• Diagnosis often requires a multidisciplinary approach involving HRCT, pulmonary function tests, bronchoalveolar lavage, and sometimes surgical lung biopsy 1
• Treatment approaches vary significantly based on ILD classification, with predominantly inflammatory ILDs treated with immunosuppressive therapy and predominantly fibrotic ILDs treated with antifibrotic drugs 1

Diagnostic Pitfalls

• Approximately 10-25% of ILD cases remain unclassifiable even after multidisciplinary discussion 1
• Up to 10% of patients with ILD may have normal chest radiograph findings, so a normal CXR cannot rule out clinically significant disease 3
• Multiple ILD pathologies may coexist in the same patient when multiple regions of the lung are biopsied 3
• Certain lung pathology patterns occur in multiple ILD subtypes and thus are not disease-specific 3

Understanding the relative frequency of these conditions helps guide diagnostic approaches and treatment decisions for patients presenting with interstitial lung abnormalities.