Interstitial lung diseases are a heterogeneous group of disorders characterized by alveolar septal thickening, fibroblast proliferation, collagen deposition, and, if the process remains unchecked, pulmonary fibrosis. Interstitial lung diseases can be classified using various criteria (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 most systemic rheumatic disorders and occupational lung exposures, many medications, and some illicit drugs (see table Causes of Interstitial Lung Disease). A number of interstitial lung diseases of unknown etiology have characteristic histology, clinical features, or presentation and thus are considered unique diseases, including the following:
Pulmonary Langerhans cell histiocytosis (granulocytosis)
In up to 30% of patients who have interstitial lung diseases that have no clear cause, the disorders are distinguished primarily by characteristic histopathologic features; these disorders are termed the idiopathic interstitial pneumonias.
