Vasculitis: Microscopic Polyangiitis

Microscopic polyangiitis (MPA) is a rare disease of unknown etiology. It is characterized by necrotizing inflammation of small blood vessels (arterioles, capillaries, and venules) in the absence of immune deposits and granulomatous inflammation. Antibodies against the content of azurophilic granules in neutrophilic granulocytes (antineutrophil cytoplasmic antibodies (ANCA)), most commonly targeting myeloperoxidase (MPO-ANCA), are present in the majority of patients. Although the etiology of MPA remains unknown, clinical observations and animal models suggest a causal association with MPO-ANCA. Clinically, MPA commonly presents with systemic symptoms, palpable purpura, renal dysfunction, pulmonary hemorrhage, and mononeuritis multiplex. If larger arteries are involved, patients may display disease manifestations historically attributed to panarteritis nodosa. Rapidly progressive glomerulonephritis and diffuse alveolar hemorrhage represent the most frequent life-threatening complications. The combination of glucocorticoids and cyclophosphamide is typically used for remission induction therapy. Despite aggressive therapy, relapses occur commonly and treatment-associated morbidity and mortality are frequently encountered.