OBJECTIVETo study the pathologic characteristics of chronic hypersensitivity pneumonitis, especially the pattern of pulmonary interstitial fibrosis; and to compare the histologic features with those of idiopathic interstitial pneumonitis.

METHODSThe HE-stained paraffin sections of 10 cases of chronic hypersensitivity pneumonitis encountered during the period from 2000 to 2008 were retrospectively analyzed.

RESULTSThere were altogether 6 males and 4 females, with age of patients ranging from 23 to 59 years (mean=47.2 years). Clinically, the patients presented with chronic cough and shortness of breath for 4 months to 6 years. Histologically, 7 cases showed usual interstitial pneumonitis (UIP)-like fibrosis. Patchy fibrosis was observed under the pleura, adjacent to interlobular septa and around bronchioles. In all of the 7 cases, foci of fibroblastic proliferation, as well as bronchiolar metaplasia of peribronchiolar alveoli and mild bronchiolitis, were noted. Three cases presented with mild honeycomb changes of lung and 3 cases showed non-specific interstitial pneumonitis (NSIP)-like fibrosis, in which the alveolar septa were expanded by fibrous tissue and collagen, with relative preservation of alveolar architecture. Bronchiolitis and lymphocytic infiltrates in alveolar septa were seen. Schaumann bodies were identified in 1 case. In general, patients with chronic hypersensitivity pneumonitis were younger than patients with idiopathic UIP. Computed tomography often showed upper and middle lobar involvement and mosaic attenuation. Compared with idiopathic UIP, the UIP-like fibrosis of chronic hypersensitivity pneumonitis often occurred not only under the pleura and adjacent to interlobular septa, but also around bronchioles and was accompanied by bronchiolar metaplasia.

CONCLUSIONSChronic hypersensitivity pneumonitis can mimic other types of lung conditions with interstitial fibrosis, especially UIP and NSIP. As a result, some cases of chronic hypersensitivity pneumonitis may be misdiagnosed as such.