Microscopic angiitis
Microscopic polyangiitis (MPA) is a systemic vasculitis involving small vessels. Its clinical manifestations are similar to those of PAN, but it is different from PAN in that it is characterized by rapidly progressive glomerulonephritis (RPGN) and pulmonary involvement. Classic PAN mainly invades small and medium-sized arteries. If the lesions involve smaller arteries, veins and capillaries, classic PAN is excluded and MPA is included. Its clinical features are pulmonary-renal vasculitis and serum anti-MPO is positive.