Microscopic Polyangiitis

Microscopic polyangiitis (MPA) is a pauci-immune necrotizing small vessel vasculitis in which necrotizing crescentic glomerulonephritis (GN) is very common and pulmonary capillaritis often occurs. It belongs to the group of anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis and can be differentiated from granulomatosis with polyangiitis (GPA) for the absence of granulomatous inflammation. It is a rare disease and affects patients usually >50 years old with no clear gender prevalence. Renal involvement, characterized clinically by rapidly progressive GN and histologically by pauci-immune necrotizing crescentic GN, is present in more than 90% of patients. Other features include, with decreasing frequency, constitutional symptoms, pulmonary involvement with diffuse alveolar hemorrhage or interstitial lung disease, peripheral neuropathy, skin vasculitis, gastrointestinal and heart disease. Anti-neutrophil cytoplasmic antibodies, usually P-ANCA/MPO-ANCA, are detected in up to 80–90% of patients with active MPA. Untreated MPA has an unfavorable course and is associated with increased mortality and end-stage renal disease (ESRD). Factors independently related to death are the presence and severity of renal involvement, cardiac symptoms, gastrointestinal involvement, and age over 65 years. Corticosteroids and cyclophosphamide or rituximab (with the addition of plasma-exchange in the most severe cases) are the cornerstone of induction treatment of ANCA-associated renal vasculitis, followed by azathioprine (or methotrexate, mycophenolate, rituximab) for maintenance. Despite significant improvement in patient outcomes over the past decades, MPA still results in ESRD and death in a significant percentage of patients.