Description

• Interstitial lung diseases (ILDs) represent a diverse group of chronic progressive lung diseases associated with alveolar inflammation and/or potentially irreversible pulmonary fibrosis.
• >200 individual diseases may present with similar characteristics, making ILD difficult to classify.
• A classification scheme proposed by the American Thoracic Society and European Respiratory Society includes these subtypes:
  • Known causes (environmental, occupational, or drug-associated disease)
  • Systemic disorders (e.g., sarcoidosis, Wegener granulomatosis, collagen vascular disease)
  • Rare lung diseases (e.g., pulmonary histiocytosis, lymphangioleiomyomatosis)
  • Idiopathic interstitial pneumonias (IIPs)
• Based on clinical, radiologic, and histologic features, IIPs are further subclassified into the following diagnoses:
  • Major IIPs, including idiopathic pulmonary fibrosis (IPF), nonspecific interstitial pneumonia (NSIP), respiratory bronchiolitis-associated ILD (cryptogenic organizing pneumonia [COP], etc.)
  • Rare IIPs
  • Unclassifiable IIPs
• Classification of IIPs and relationships between the subtypes are difficult to classify due to mixed patterns of injury.

Pediatric Considerations
ILD in infants and children represents a heterogeneous group of respiratory disorders. Diseases result from a variety of processes involving genetic factors and inflammatory or fibrotic responses, and processes are distinct from those that cause ILD in adults. Some diseases result from developmental disorders and growth abnormalities in infancy.

Epidemiology

Incidence
Exact incidence and prevalence are difficult to determine because of differences in case definitions and procedures used in diagnosis. U.S. incidence: 16.3 to 17.4

Prevalence
Prevalence in United States: 42.7 to 63 cases/100,000 in the general population and pediatric ILD of 3.6/1 million

Etiology and Pathophysiology

• Alveolar inflammation may progress into irreversible fibrosis.
• Varying degrees of ventilatory dysfunction occur among the ILD subtypes.
• ILD associated with collagen vascular disease and systemic connective disorders can manifest involvement of skin, joints, muscular, and ocular systems.
• Some types of ILD are associated with specific exposures:
  • Medications (amiodarone, antibiotics [especially nitrofurantoin], chemotherapy agents, gold, illicit drugs)
  • Inorganic dusts (silicates, asbestos, talc, mica, coal dust, graphite)
  • Organic dusts (moldy hay, inhalation of fungi, bacteria, animal proteins)
  • Metals (tin, aluminum, cobalt, iron, barium)
  • Gases, fumes, vapors, aerosols

Genetics
Role of genetic factors is unknown.

Risk Factors

• Environmental or occupational exposure to inorganic or organic dusts
• 66–75% of patients with ILD have a history of smoking.
• Due to diversity of diseases, age is not a reliable predictor of pathology:
  • Most patients with connective tissue disease–related pathology and inherited subtypes present between ages 20 and 40 years
  • Median age of patients with IPF is 66 years. Studies of clinical predictors of survival including age, ethnicity, and smoking status have been inconsistent.

Commonly Associated Conditions

Many systemic disorders and primary diseases are associated with ILD. A partial list includes the following:

• Collagen vascular disease
• Sarcoidosis
• Amyloidosis
• Goodpasture syndrome
• Churg-Strauss syndrome
• Wegener granulomatosis