The natural history of vasculitis. What the histology tells us about pathogenesis.

While histopathologic analysis may offer some clues as to the pathogenesis of vasculitis, observations must be interpreted with caution, as there is considerable overlap in the histologic pattern. In most cases, a predominantly neutrophilic vasculitis affecting small dermal venules suggests a relatively acute, immune complex-mediated reaction. Less commonly, this histologic pattern may be seen in non-immunologically mediated processes, such as in the presence of bacterial toxins or malignant hypertension, or in more chronic disease states, such as granuloma faciale or erythema elevatum diutinum. A predominantly lymphocytic vasculitis may represent several pathogenetic mechanisms. In lesions more than 24 to 48 hours old, a lymphocytic vasculitis may represent a resolving phase of an immune complex-mediated neutrophilic vasculitis. Alternatively, this histologic pattern may be seen de novo in conditions with a presumed cell-mediated immunologic pathogenesis. Lymphocytic vasculitis may also be seen in rickettsial infections such as Rocky Mountain spotted fever. The pathogenesis of granulomatous vasculitis remains poorly understood and is thought to be induced by a combination of circulating immune complexes and a cell-mediated immune response.