Definition:

Microscopic polyangiitis (MPA) is an idiopathic autoimmune disease characterized by systemic vasculitis. MPA predominantly affects small-calibre blood vessels and is associated with the presence of antineutrophil cytoplasmic autoantibodies (ANCA). The association with ANCAs originally defined the group of ANCA-associated vasculitides, comprising granulomatosis with polyangitis (GPA, formerly known as Wegener granulomatosis) [DS:H01655], microscopic polyangiitis (MPA), and eosinophilic granulomatosis with polyangiitis (EGPA, formerly known as Churg-Strauss syndrome) [DS:H01468], which have different frequencies of ANCA-positivity. ANCA in MPA are predominantly directed against myeloperoxidase (MPO-ANCA) but may, in a minority of patients, be directed against proteinase 3 (PR3-ANCA). Not all patients, however, have ANCA. MPA is clinically characterized by small-vessel vasculitis primarily affecting the kidneys and the lungs but other organs may be involved as well. Renal involvement, which can be the only manifestation, is clinically apparent as rapidly progressive glomerulonephritis and histopathologically as pauci-immune necrotizing and crescentic glomerulonephritis. Diagnosis is mainly established by clinical manifestations, computed tomography (TC), ANCA antibody detection, and renal and pulmonary biopsy. The introduction of aggressive immunosuppressive treatment has substantially improved the prognosis. The standardized therapeutic regimen is based on cyclophosphamide and corticosteroids. Using this regimen, remission can be achieved in most of the patients. Rituximab may represent an important alternative to cyclophosphamide for patients who may not respond adequately to antimetabolite therapies.