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

Abstract

Context.— Chronic hypersensitivity pneumonitis (CHP) has emerged from obscurity during the past 15 years and is now recognized as a very common form of fibrosing interstitial pneumonia but one that is frequently misdiagnosed both clinically and on surgical lung biopsy as usual interstitial pneumonia/idiopathic pulmonary fibrosis (UIP/IPF) or fibrotic nonspecific interstitial pneumonia. Objective.— To review the pathologic features of CHP. Data Sources.— Clinical, pathology, and radiology literature were used. Conclusions.— Upper lobe–predominant fibrosis and/or air-trapping on computed tomography scan are features of CHP but not UIP/IPF; however, radiologic separation is possible in only about 50% of cases. Morphologically, CHP sometimes mimics UIP/IPF, but CHP often shows isolated foci of peribronchiolar (centrilobular) fibrosis, frequently associated with fibroblast foci, and in CHP, fibrosis may bridge from the centrilobular region to another bronchiole, an interlobular septum, or the pleura (“bridging fibrosis”). This set of findings is uncommon in UIP/IPF. In addition, CHP may produce a picture of fibrotic nonspecific interstitial pneumonia. Although giant cells/granulomas are usually present in subacute hypersensitivity pneumonitis, they are much less frequently found in CHP, and their absence does not contradict the diagnosis. This diagnostic separation is clinically important because CHP is treated differently than UIP/IPF is (immunosuppressive agents versus antifibrotic agents); further, there are some data to suggest that removing the patient from antigen exposure improves outcome, and there is evidence that patients with CHP have a much better survival prognosis after lung transplantation than do patients with UIP/IPF. In most cases, accurate diagnosis of CHP requires consultation among clinicians, radiologists, and pathologists.

Grade 4 asbestosis does not extend directly from the respiratory bronchiole to the peripheral lung

“…Chronic interstitial pneumonias, including those with the UIP pattern, have been seen in various inhalational diseases, including pneumoconiosis and chronic hypersensitivity pneumonia . A secondary UIP pattern related to pneumoconiosis is located in the subpleural regions of the lower lobes and affects the peripheral lobules.…”

“…Chronic interstitial pneumonias, including those with the UIP pattern, have been seen in various inhalational diseases, including pneumoconiosis [32][33][34] and chronic hypersensitivity pneumonia. 35,36 A secondary UIP pattern related to pneumoconiosis is located in the subpleural regions of the lower lobes and affects the peripheral lobules. Typical manifestations of AI are upper-lobe-predominant fibrosis 37 and pleuroparenchymal fibroelastosis, 38 which shows clear subpleural or peripheral lobular distribution, mainly in the upper lobe.…”

Histopathological spectrum of hypersensitivity pneumonitis with clinico‐radiologic correlation

“…There was also no significant difference in the clinical characteristics between the subjects with subacute and chronic HP. There was no significant difference in the mean duration of symptoms between the subjects histopathologically diagnosed as subacute HP (median [IQR], 6 months) and chronic HP (median [IQR], 18 [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] months) (p = 0.30). Six subjects (n = 6/13; 46.1%) with histopathologically diagnosed subacute HP had symptoms of more than 6 months' duration.…”

Chronic interstitial lung disease in children

“…Hypersensitivity pneumonitis Hypersensitivity pneumonitis, also called exogenous allergic alveolitis in Europe, represents ∼50% of chILD [1,29] and is thus an important entity, as it may lead to chronic and severe lung disease if not recognised [30]. SISMAN et al [31] performed a cross-sectional follow-up study in patients previously diagnosed with biopsy and high-resolution CT-verified hypersensitivity pneumonitis during childhood.…”