Facts and controversies in the classification of idiopathic interstitial pneumonias.

Idiopathic interstitial pneumonias are defined from the pathological point of view as non granulomatous intralobular inflammatory and fibrotic processes involving the alveolar walls. More than thirty years ago Liebow and Carrington pioneered the notion that morphological characteristics could be used with benefit in separating the different entities found in this group, which present with typical, but not pathognomonic clinical features. In the mid-1980s some entities, including giant cell interstitial pneumonia (GIP) and lymphocytic interstitial pneumonia (LIP), were removed from this group and considered as peculiar forms. In the early 90s the concept of cellular or nonspecific interstitial pneumonia was reconsidered, leading to an in depth revision of various types of interstitial pneumonia of unknown etiology. The histological pattern observed in patients with idiopathic pulmonary fibrosis is now referred to as usual interstitial pneumonia (UIP). Other entities that have been revised during the last ten years are desquamative interstitial pneumonia/alveolar macrophage pneumonia (DIP/AMP), respiratory bronchiolitis-interstitial lung disease (RB-ILD), acute interstitial pneumonia (AIP), cryptogenic organizing pneumonia (COP) and nonspecific interstitial pneumonia (NSIP). This paper provides a detailed description of pulmonary disorders which have been included in the new classification systems of idiopathic interstitial pneumonias. In the second part of the paper we will discuss several doubts and controversies that this new classification schemes leave unresolved.