[A case of interstitial pneumonia preceding microscopic polyangiitis].

We encountered a case of interstitial pneumonia preceding microscopic polyangiitis (MPA). A 64-year-old asymptomatic woman was found to have interstitial pneumonia on a chest radiograph taken during a work-up before an operation for cataract. Six months later she presented with non-productive cough, low grade fever and renal dysfunction. Interstitial pneumonia worsened with new lesions. The test for MPO-ANCA was positive, though it had been negative six months previously. The histopathology of the lung by video-assisted lung biopsy showed the usual interstitial pneumonitis pattern and small vessel pulmonary vasculitis. Renal biopsy showed necrotizing glomerulonephritis. She was given a diagnosis of MPA and was immediately treated with methylprednisolone pulse therapy and a combination of prednisolone and cyclophosphamide. She has remained stable for two years. This positive inversion of MPO-ANCA, while interstitial pneumonia advanced, was of interest concerning the etiology of MPA. We discuss the important topic of the mechanism of the development of MPA.