What is the classification of Interstitial Lung Disease (ILD)?

Classification of Interstitial Lung Disease (ILD)

Interstitial lung disease (ILD) is classified into four main categories: known causes, idiopathic interstitial pneumonias (IIPs), granulomatous lung diseases, and unique entities, with IIPs further subdivided into specific pathological patterns including idiopathic pulmonary fibrosis which accounts for 55% of IIPs. 1

Primary Classification Framework

The American Thoracic Society (ATS) and European Respiratory Society (ERS) classification system provides the most comprehensive framework for categorizing ILDs. According to this classification, ILDs are divided into:

1. Known Causes

   • Drugs
   • Collagen vascular diseases
   • Environmental/occupational exposures
   • Genetic disorders

2. Idiopathic Interstitial Pneumonias (IIPs) - accounting for a significant proportion of ILDs

   • Idiopathic pulmonary fibrosis (IPF) - 55% of IIPs
   • Nonspecific interstitial pneumonia (NSIP) - 25% of IIPs
   • Respiratory bronchiolitis-associated ILD (RB-ILD) - 10-15% of IIPs
   • Desquamative interstitial pneumonia (DIP) - <5%
   • Cryptogenic organizing pneumonia (COP) - 5%
   • Acute interstitial pneumonia (AIP) - <2%
   • Lymphocytic interstitial pneumonia (LIP) - <1%

3. Granulomatous Lung Diseases

   • Sarcoidosis
   • Fungal infections
   • Mycobacterial infections

4. Unique Entities

   • Pulmonary alveolar proteinosis (PAP)
   • Eosinophilic granulomatosis and pneumonia (EG and EP)
   • Lymphangioleiomyomatosis (LAM) 1

Clinical Behavior Classification

Beyond the etiological classification, ILDs can also be categorized based on their clinical behavior patterns, which helps guide treatment approaches:

1. Reversible and self-limited (e.g., many cases of RB-ILD)

   • Treatment goal: Remove possible cause
   • Monitoring: Short-term (3-6 month) observation

2. Reversible disease with risk of progression (e.g., cellular NSIP, some fibrotic NSIP, DIP, COP)

   • Treatment goal: Initially achieve response, then rationalize longer-term therapy
   • Monitoring: Short-term to confirm response, long-term to ensure preservation of gains

3. Stable with residual disease (e.g., some fibrotic NSIP)

   • Treatment goal: Maintain status
   • Monitoring: Long-term observation

4. Progressive, irreversible disease with potential for stabilization (e.g., some fibrotic NSIP)

   • Treatment goal: Stabilize
   • Monitoring: Long-term observation

5. Progressive, irreversible disease despite therapy (e.g., IPF, some fibrotic NSIP)

   • Treatment goal: Slow progression
   • Monitoring: Long-term observation to assess for transplant need or palliation 1

Unclassifiable ILD

Despite comprehensive classification systems, approximately 10-25% of ILD cases remain unclassifiable after multidisciplinary discussion. This may occur due to:

1. Absence of lung biopsy with insufficient clinical/radiological data
2. Discrepant or overlapping histopathological findings
3. Discrepancies between clinical, radiological, and pathological findings
4. Significant overlapping clinical or radiological features 1

Pediatric ILD Classification

Childhood ILD (chILD) requires a separate classification system as the entities affecting children, especially infants under 2 years of age, are distinct from those affecting adults. The chILD Research Network classification divides these into:

1. Disorders More Prevalent in Infancy
2. Disorders Not Specific to Infancy 1

Diagnostic Approach

Accurate classification of ILD requires a multidisciplinary approach involving:

• High-resolution computed tomography (HRCT) - 91% sensitive and 71% specific for diagnosing subtypes like IPF
• Pulmonary function tests - A 5% decline in FVC over 12 months indicates approximately 2-fold increase in mortality
• Bronchoalveolar lavage - To exclude infection or identify specific cellular patterns
• Surgical lung biopsy - When other diagnostic investigations haven't identified the precise ILD 2

Treatment Considerations Based on Classification

Treatment approaches vary significantly based on ILD classification:

• Predominantly inflammatory ILDs: Immunosuppressive therapy (e.g., mycophenolate mofetil for CTD-ILD)
• Predominantly fibrotic ILDs: Antifibrotic drugs (nintedanib or pirfenidone) which slow annual FVC decline by 44-57% in IPF and progressive pulmonary fibrosis 2

Common Pitfalls in ILD Classification

1. HRCT misdiagnosis - Particularly by less experienced radiologists
2. Lack of management recommendations for "probable" or "possible" ILD cases
3. Confusion regarding diagnostic role of bronchoalveolar lavage
4. Insufficient integration of clinical data in determining diagnostic likelihood 1

Understanding the classification of ILD is essential for appropriate management decisions and prognostication, as different ILD subtypes have markedly different clinical courses and treatment responses.