Vasculitides and occluding vasculopathies, challenges in recognizing histopathological patterns, and their solutions.

In this review, we propose a classification of vasculitides and occluding vasculopathies using the clinicopathological correlation as the basic process. We use an algorithmic approach with pattern analysis, which allows reliable reporting of microscopic findings. We first differentiate between small and medium vessel vasculitis. Second, we differentiate the subtypes of small- and medium-sized vessels. Finally, we differentiate vasculitides according to the predominant cell type into leukocytoclastic and/or granulomatous vasculitis. Regarding leukocytoclastic vasculitis as a central reaction pattern of cutaneous small/medium vessel vasculitides, its relation or variations may be arranged in a wheel-like order. With respect to occluding vasculopathies, the first two steps are identical to the algorithm of vasculitides, and we finally differentiate according to the time point of the coagulation/reorganization process and the involved inflammatory cells/stromal features. By visualizing the criteria in the style of bar codes, clinical and histological overlaps and differences may become more transparent.