Pathological differentiation of chronic hypersensitivity pneumonitis from idiopathic pulmonary fibrosis/usual interstitial pneumonia

Abstract

Centrilobular fibrosis, bridging fibrosis and organizing pneumonia, in addition to bronchiolitis, granulomas and giant cells, are characteristic features of chronic HP with a UIP-like pattern. These features are therefore important in differentiating chronic HP from IPF/UIP, as management strategies differ for the two disorders.

Pathology of Chronic Hypersensitivity Pneumonitis What Is It? What Are the Diagnostic Criteria? Why Do We Care?

“…Lymphoid aggregates are often present in CHP; Takemura et al 26 reported lymphoid aggregates in 17 of 22 CHP cases (72%) with a UIP pattern versus 5 of 13 UIP/IPF cases (38%), and Chiba et al 28 found lymphoid aggregates in 60% of a series of 16 cases (terminology in that article is confusing because the authors appear to be using the term follicles for aggregates.). Myers 29 explicitly noted that true lymphoid follicles can be found in CHP, but in our experience, they are uncommon and raise a question of connective tissue disease-associated interstitial lung disease (ILD) when present.…”

“…Churg et al 25 described subacute HP areas in 7 of 13 CHP cases (54%). Takemura et al 26 reported bronchiolitis in 22 of 22 cases (100%) of CHP with a UIP pattern (it is not clear from that article whether those cases also had peribronchiolar interstitial inflammation). In a later article, Churg et al 27 ; and it has been suggested that this is a useful point of distinction from UIP/ IPF, which uncommonly shows peribronchiolar fibrosis, and when present, the peribronchiolar fibrosis typically does not have associated fibroblast foci.…”

“…Churg et al 27 found giant cells or granulomas in 16 of 18 cases (89%) with a UIPlike pattern and 3 of 3 cases (100%) with only peribronchiolar fibrosis, and Takemura et al 26 found giant cells in 15 of 22 cases (68%) and granulomas in 14 of 22 cases (64%). However, Chiba et al 28 described giant cells or granulomas in only 4 of 16 UIP-like cases (25%).…”

“…Bridging fibrosis is common in CHP ( Table 2), and some authors suggest it is a prototypical lesion that is rarely seen in UIP. 26,30,31 A UIP-like pattern, with patchy, subpleural fibrosis; fibroblast foci; and some amount of microscopic honeycombing, is also common in CHP (Figure 2, C), but often, the pattern in CHP is subtly different from UIP/IPF, with relatively less subpleural fibrosis, little to no honeycombing, and sometimes fibrosis that follows alveolar walls immediately under the pleura, rather than forming the large solid blocks of fibrosis typical of UIP/IPF (Figures 2, A, and 3). As well, the degree of peribronchiolar fibrosis or bridging fibrosis may be as severe as, or worse than, the subpleural fibrosis (Figures 2, A, and 3).…”

“…In a later article, Churg et al 27 ; and it has been suggested that this is a useful point of distinction from UIP/ IPF, which uncommonly shows peribronchiolar fibrosis, and when present, the peribronchiolar fibrosis typically does not have associated fibroblast foci. 26,30 Bridging fibrosis is essentially a more florid form of peribronchiolar fibrosis in which fibrosis spreads in the interstitium between a bronchiole and another bronchiole or, more often, between a bronchiole and an interlobular septum or a bronchiole and an area of subpleural fibrosis (Figures 2, A, and 3). Bridging fibrosis is common in CHP ( Table 2), and some authors suggest it is a prototypical lesion that is rarely seen in UIP.…”