Microscopic polyangiitis (MPA) is an antineutrophil cytoplasmic antibody (ANCA)-associated disease resulting in multisystem necrotizing inflammation of small and medium-sized vessels without granuloma formation. It is associated with elevated perinuclear ANCA (p-ANCA) titers and a positive myeloperoxidase (MPO) antibody. The most common sites of involvement are the kidneys, with focal and segmental necrotizing crescentic glomerulonephritis, and the lungs, with pulmonary capillaritis and alveolar hemorrhage. The onset may be insidious or acute. Severe pulmonary or renal symptoms are considered a medical emergency, and immediate treatment is required.

MPA most often occurs in patients between ages 50 and 60 years, but cases have been reported in children and older adults as well. There is a slight male predominance. MPA is most common in patients of Northern European descent.

Complications of MPA include diffuse alveolar hemorrhage, interstitial lung disease, pleural effusion, pulmonary edema, pleuritis, interstitial fibrosis, rapidly progressive focal segmental necrotizing glomerulonephritis, gastrointestinal bleeding, colonic ulcerations, intestinal ischemia, bowel perforation, mononeuritis multiplex, and polyneuropathy.