Interstitial lung diseases: Imaging evaluation

Republished with permission from Applied Radiology, September 2000

Interstitial lung diseases (ILDs) are a heterogeneous group of conditions that involve the lung parenchyma. These entities are characteristically diffuse and infiltrative in nature, involving the peribronchovascular interstitium of the lung, although alveolar involvement may occur as well. ILDs are not malignant or infectious in etiology, although they may cause significant morbidity and mortality. The precise causes of most ILDs are unknown. They are grouped together because they share similar clinical, radiographic, physiologic, and/or pathologic manifestations.1 Various approaches have been used to classify this heterogeneous group of disorders, and none are very satisfying.

In the following classification scheme, certain disorders are grouped together because of a common pathologic finding or a shared radiographic abnormality. Because numerous ILDs exist, it is inevitable that no classification scheme will satisfactorily categorize them with complete accuracy. For purposes of organization, in broad terms, they may be grouped as follows:

1. interstitial pneumonias (IPs):
   a) usual interstitial pneumonia,
   b) non-specific interstitial pneumonitis,
   c) acute interstitial pneumonia,
   d) alveolar macrophage pneumonia (formerly called desquamative IP), and
   e) bronchiolitis obliterans organizing pneumonia;
   
   
2. diffuse infiltrative disease characterized by granulomas:
   a) sarcoidosis
   b) hypersensitivity pneumonitis;
   
   
3. lymphocytic interstitial pneumonia (LIP);
   
   
4. pneumoconioses;
   
   
5. ILD characterized by cysts (Langerhans' cell histiocytosis, lymphangiomyomatosis);
   
   
6. disorders characterized by interlobular septal thickening:
   a) pulmonary lymphangitic carcinomatosis,
   b) pulmonary edema, and
   c) pulmonary alveolar proteinosis;
   
   
7. eosinophilic syndromes;
   
   
8. pulmonary hemorrhage syndromes;
   
   
9. vasculitis.

The diagnosis of these entities rests on integration of clinical, physiologic, and radiologic information. Frequently, a tissue diagnosis is required for definitive diagnosis. For some of the conditions, flexible fiberoptic bronchoscopy with bronchoalveolar lavage and transbronchial biopsy may suffice, but surgical lung biopsy is often required. A complete description of the above classification scheme of diffuse lung disease is beyond the scope of this article. We will instead focus on the IPs and several other diffuse lung diseases that may have characteristic high resolution computed tomography (HRCT) appearances: hypersensitivity pneumonitis, Langerhans cell histiocytosis, and lymphangiomyomatosis. Several authoritative texts and excellent review articles are available for a more complete discussion the imaging of diffuse lung diseases.^1-5

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