Microscopic polyangiitis (also known as microscopic polyarteritism, 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.linical features may include constitutional symptoms like fever, loss of appetite, weight loss, fatigue, and kidney failure.