Interstitial pneumonias

Liebow⁶ initially classified IPs into five types on the basis of their histologic appearance:

1. usual interstitial pneumonia (UIP);
2. desquamative interstitial pneumonia (DIP);
3. giant cell interstitial pneumonia (GIP);
4. lymphocytic interstitial pneumonia (LIP); and
5. bronchiolitis obliterans interstitial pneumonia (BIP).

These entities do not represent diseases per se; rather, they represent fundamental responses of the lungs to injury or a pathologic condition. Each of the IPs may be seen in association with several different diseases; conversely, several IPs may be seen in the same patient.

Recently, IPs have been reclassified.^7-10 The current pathologic classification of the IPs includes:

1. UIP;
2. nonspecific interstitial pneumonitis (NSIP);
3. acute interstitial pneumonia (AIP);
4. alveolar macrophage pneumonia (AMP, which was previously known as DIP); and
5. bronchiolitis obliterans organizing pneumonia (BOOP).

LIP is no longer believed to represent an interstitial pneumonia. It is now thought to be a potentially malignant lymphoproliferative pulmonary disorder. GIP is now believed to be a hard metal pneumoconiosis, and is no longer considered among the IPs. The above classification scheme is commonly employed in the radiographic (chest radiograph, HRCT) approach to patients with IPs.

Usual interstitial pneumonia
Histologic findings--UIP is the most common IP.^{1, 2, 10} In the United Kingdom, UIP is known as cryptogenic fibrosing alveolitis. Pathologically, at low power magnification, UIP is characterized by areas of normal lung alternating with regions of interstitial inflammation, fibrosis, and honeycombing.^{1, 2, 10} Findings are predominantly subpleural. At higher power, the interstitial inflammation is seen to consist of lymphocytic and plasma cell infiltration within alveolar septa. Proliferating fibroblasts (fibroblastic foci) and dense collagen deposition are seen, as is hyperplasia of type 2 pneumocytes. These changes represent a response of the lung to injury, and are not specific to UIP. The simultaneous presence of multifocal areas of active inflammation, fibroblastic proliferation, and fibrosis interspersed with normal lung parenchyma ("temporal variegation") is the histologic hallmark of UIP.

The above pathologic pattern of UIP refers to a histologic abnormality. The clinical syndrome that is most commonly associated with this histologic pattern is idiopathic pulmonary fibrosis (IPF). Other conditions in which a UIP-like histologic pattern may be encountered include connective tissue diseases (such as rheumatoid arthritis and scleroderma), pneumoconiosis (particularly asbestosis), radiation injury, chronic aspiration, end-stage hypersensitivity pneumonitis, and drug reactions (e.g., nitrofurantoin). Because there may be other characteristic histopathologic features present in these other syndromes, the term UIP is best reserved for cases in which UIP histology is present but not associated with another condition; i.e., for patients with IPF.

Next page: Idiopathic pulmonary fibrosis

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