Microscopic Polyangiitis With Non-fibrotic Hypersensitivity Pneumonitis Patterns on Chest Computed Tomography: A Case Report

In microscopic polyangiitis (MPA)-associated interstitial lung disease (ILD), the usual interstitial pneumonia (UIP) is the most common pattern. We report a rare case of a 71-year-old woman with MPA initially suspected to have non-fibrotic hypersensitivity pneumonitis (HP) based on chest computed tomography (CT) findings of diffuse ground-glass opacities, mosaic attenuation, and air trapping. HP was excluded due to the lack of exposure to inhaled antigens, lack of response to inpatient antigen isolation, and absence of lymphocytosis in bronchoalveolar lavage fluid. The diagnosis of MPA was established based on the presence of ILD and renal impairment in combination with elevated myeloperoxidase-antineutrophil cytoplasmic antibody levels. This case highlights the rarity of a non-fibrotic HP pattern in MPA and the importance of integrating clinical, radiological, and serological data in evaluating MPA with atypical chest CT imaging.