|Classification and external resources|
Microscopic polyangiitis (also known as "Microscopic polyarteritis," "Microscopic polyarteritis nodosa," "MPA") is an ill-defined autoimmune disease characterized by a systemic, pauci-immune, necrotizing, small-vessel vasculitis without clinical or pathological evidence of necrotizing granulomatous inflammation.
Clinical features may include constitutional symptoms like fever, anorexia, weight loss, fatigue, and renal failure. A majority of patients may have hematuria and proteinuria. Rapidly progressive glomerulonephritis may occur. Because many different organ systems may be involved, a wide range of symptoms are possible in MPA.
Laboratory tests may reveal an increased sedimentation rate, elevated CRP, anemia and elevated creatinine due to renal impairment. An important diagnostic test is the presence of perinuclear antineutrophil cytoplasmic antibodies (p-ANCA) with myeloperoxidase specificity (a constituent of neutrophil granules), and protein and red blood cells in the urine.
In patients with neuropathy, an EMG may reveal a sensorimotor peripheral neuropathy.
While the mechanism of disease has yet to be fully elucidated, the leading hypothesis is that the process is begun with an autoimmune process of unknown etiology that triggers production of p-ANCA. These antibodies will circulate at low levels until a pro-inflammatory trigger — such as infection, malignancy, or drug therapy. The trigger upregulates production of p-ANCA. Then, the large number of antibodies make it more likely that they will bind a neutrophil. Once bound, the neutrophil degranulates. The degranulation releases toxins that cause endothelial injury. Most recently, two different groups of investigators have demonstrated that anti-MPO antibodies alone can cause necrotizing and crescentic glomerulonephritis.
Plasmapheresis may also be indicated in the acute setting to remove ANCA antibodies.
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