Isolated Diffuse Alveolar Hemorrhage as the Initial Presentation of Microscopic Polyangiitis: A Case Report

Microscopic polyangiitis, a necrotizing small-vessel vasculitis, typically presents with renal involvement and pulmonary-renal syndrome, while diffuse alveolar hemorrhage (DAH) as the sole initial presentation without concurrent glomerulonephritis is exceedingly rare and diagnostically challenging.We present here the case of a 68-year-old female with massive hemoptysis, hypoxemia, and radiologic evidence of bilateral ground-glass opacities, in whom bronchoalveolar lavage confirmed DAH.Coagulation parameters were within normal limits and infectious and cardiogenic causes were excluded.Despite initially preserved renal function, the presence of MPO-ANCA positivity established the diagnosis of microscopic polyangiitis.The patient exhibited rapid clinical improvement following high-dose corticosteroids and cyclophosphamide, although renal dysfunction developed during follow-up.This case underscores the need to consider ANCA-associated vasculitis in patients with isolated pulmonary hemorrhage.Early immunologic evaluation and bronchoscopic confirmation are critical for timely diagnosis and therapeutic intervention, even in the absence of extrapulmonary findings.