Microscopic Polyangiitis is a form of Anti-Neutrophil Cytoplasmic Antibody (ANCA)-associated small-vessel vasculitis that preferentially involves venules, capillaries and arterioles and may also involve arteries and veins. It is one of the most common primary systemic small-vessel vasculitis. Its clinical presentation is not distinguishable from the Wegener's granulomatosis (WG) and the Churg-Strauss syndrome (CSS). These types of small-vessel vasculitis are histologically similar and can be differentiated by the presence of granulomatous inflammation in WG or asthma in CSS. The case of a 66-year-old man with microscopic polyangiitis presenting with alveolar hemorrhage is reported with a discussion of the differential diagnosis of other types of pulmonary small-vessel vasculitis.

Keywords: Microscopic polyangiitis. Alveolar hemorrhage. Pulmonary vasculitis