What is the definition of interstitial lung disease?

Definition of Interstitial Lung Disease

Interstitial lung disease (ILD) is a diverse group of disorders characterized by diffuse inflammation and/or fibrosis affecting the lung parenchyma, including structures beyond the interstitium such as alveolar spaces, small airways, vessels, and sometimes the pleura, resulting in progressive dyspnea and impaired gas exchange. 1

Anatomical Considerations

While the term "interstitial" suggests involvement limited to the microscopic anatomic space between epithelial and endothelial basement membranes, ILDs typically involve multiple lung compartments:

• The true pulmonary interstitium is confined to the space bounded by basement membranes of epithelial and endothelial cells
• However, ILDs regularly affect structures beyond this space, including:
  • Alveolar spaces
  • Small airways
  • Pulmonary vasculature
  • Pleura 1

Classification of Interstitial Lung Diseases

ILDs can be broadly categorized into three main groups:

1. ILDs associated with underlying systemic disorders:

   • Connective tissue diseases (e.g., rheumatoid arthritis, systemic sclerosis)
   • Autoimmune diseases 1

2. ILDs associated with antigenic or toxic exposures:

   • Drug-induced lung disease
   • Occupational exposures
   • Environmental exposures
   • Avocational exposures 1

3. Idiopathic ILDs:

   • Idiopathic pulmonary fibrosis (IPF)
   • Nonspecific interstitial pneumonia (NSIP)
   • Cryptogenic organizing pneumonia (COP)
   • Acute interstitial pneumonia (AIP)
   • Desquamative interstitial pneumonia (DIP)
   • Respiratory bronchiolitis interstitial lung disease (RBILD)
   • Lymphoid interstitial pneumonia (LIP) 1

Epidemiology

ILD affects approximately 650,000 people in the United States and causes about 25,000-30,000 deaths annually 2. The prevalence of clinically significant ILD ranges from 25 to 74 per 100,000 population, with an incidence of 31.5 per 100,000 in men and 26.1 per 100,000 in women 1.

The most common forms include:

• Idiopathic pulmonary fibrosis (IPF) - approximately one-third of all ILD cases
• Hypersensitivity pneumonitis - about 15% of cases
• Connective tissue disease-associated ILD - about 25% of cases 2

Pathological Features

ILDs are characterized by:

• Variable amounts of inflammation (lymphocytic, neutrophilic, eosinophilic, or granulomatous)
• Fibrosis
• Architectural distortion of lung parenchyma 1

In some cases, ILDs may present with a progressive fibrosing phenotype that influences treatment decisions 3.

Clinical Importance

Understanding the precise definition and classification of ILD is crucial because:

• Different ILD subtypes have distinct prognoses and treatment approaches
• Early and accurate diagnosis is essential for appropriate management
• Some ILDs progress to irreversible lung damage and respiratory failure
• Multidisciplinary evaluation is necessary for proper classification 1

Diagnostic Considerations

The diagnosis of specific ILD types requires:

• High-resolution computed tomography (HRCT) - approximately 91% sensitive and 71% specific for diagnosing subtypes like IPF 2
• Pulmonary function tests showing restrictive pattern
• In some cases, lung biopsy for definitive diagnosis
• Multidisciplinary discussion integrating clinical, radiological, and pathological findings 1

Understanding this comprehensive definition of ILD is essential for proper diagnosis, classification, and management of these complex disorders that can significantly impact morbidity, mortality, and quality of life.