Overview of Interstitial Lung Disease

Interstitial lung diseases are a heterogeneous group of disorders characterized by primary pathologic processes usually affecting the interstitium of the lungs with eventual spread to involve other lung tissues (eg, alveoli, bronchioles, bronchi). These pathologic processes include diffuse alveolar septal thickening, fibroblast proliferation, collagen deposition, and, if the process remains unchecked, pulmonary fibrosis. Interstitial lung diseases can be classified using various distinctions (eg, acute vs chronic, granulomatous vs nongranulomatous, known cause vs unknown cause, primary lung disease vs secondary to systemic disease, history of smoking vs no smoking history).

Among the numerous possible causes are systemic rheumatic disorders, occupational lung exposures, medications, and some illicit drugs (see table Causes of Interstitial Lung Disease). A number of interstitial lung diseases of unknown etiology have characteristic histologies, clinical features, or presentations and are thus considered unique diseases, including the following:

• Eosinophilic pulmonary diseases

• Lymphangioleiomyomatosis

• Pulmonary alveolar proteinosis

• Pulmonary Langerhans cell histiocytosis (granulocytosis)

• Sarcoidosis

The estimated prevalence of interstitial lung disease is approximately 75 per 100,000 people in Europe and the United States (1). There is a slight male preponderance. Older age, especially over 70 years, is a risk factor (2). In up to 30% of patients who have interstitial lung diseases that have no clear etiology, the disorders are distinguished primarily by characteristic histopathologic features; these disorders are termed the idiopathic interstitial pneumonias.