Pathologic Classification of Vasculitis

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The diagnostic categorization of systemic noninfectious vasculitis rarely can be made definitively based solely on the light microscopic findings but usually requires integration of the pathologic features with immunohistologic, clinical, and/or laboratory data to reach the most clinically useful diagnosis. Nevertheless, knowledgeable pathologic assessment of the histopathology can be critically important in arriving at the appropriate diagnosis. Noninfectious vasculitides can be broadly categorized as large vessel vasculitis, medium-sized vessel vasculitis, and small vessel vasculitis, although it is important to realize that the type of vessel involved is as important as size, and this is only one of many features that must be assessed to reach a diagnosis. Large vessel vasculitides (giant cell arteritis and Takayasu arteritis) have a predilection for the aorta and its major branches, especially those directed to the extremities and the neck and head. Medium-sized vessel vasculitides (polyarteritis nodosa and Kawasaki disease) affect predominantly medium-sized arteries, especially main visceral arteries such as the renal, hepatic, coronary and mesenteric arteries. Small vessel vasculitides (pauci-immune small vessel vasculitis and immune complex small vessel vasculitis) also can affect arteries, but their predominant targets are arterioles, venules, and capillaries. Pauci-immune small vessel vasculitis is associated with circulating antineutrophil cytoplasmic autoantibodies (ANCA) and includes microscopic polyangiitis, Wegener's granulomatosis, and Churg-Strauss syndrome. Precise diagnosis of immune complex small vessel vasculitis often requires immunohistologic characterization of the composition of the immune complexes or serologic detection of pathogenetically related antibodies.