Microscopic Polyangiitis (MPA) By Alexandra Villa-Forte, MD, MPH, Cleveland Clinic Reviewed / Revised Jun 2022 | Modified Sep 2022 View Patient Education Symptoms and Signs Diagnosis Treatment Key Points Topic Resources Alveolar Hemorrhage Microscopic polyangiitis is a systemic necrotizing vasculitis without immune globulin deposition (pauci-immune) that affects mainly small vessels.