[Pathology of vasculitis: differential diagnosis and selected disorders]

The morphological diagnosis of vasculitis is a challenge for the pathologist for many reasons. Primary systemic vasculitides are rare diseases (ann. incidence 4-5/100,000 according to the Vaskulitis-Register Schleswig-Holstein), pathognomonic histopathological findings are hard to obtain in many patients even with multiple biopsies, and the nomenclature of vasculitis has long been confusing and unequivocal. The Chapel-Hill Consensus Conference (1992) is credited with the establishment of clinico-pathological definitions for primary systemic vasculitides. For the discrimination of primary systemic vasculitis from secondary vasculitis as well as pseudovasculitic syndromes, correlation with clinical data is essential; it has been facilitated by the American College of Rheumatology (ACR) classification criteria (1990). Differential diagnostic schemes for vasculitis are based on the predominant type and size of the blood vessel involved, as well as the histopathological type of inflammation and the presence or absence of immune complex deposits. However, differential diagnostic problems remain: even with the clarified nomenclature of Chapel-Hill, considerable overlap still occurs. A clinical diagnosis of vasculitis is now often established in an earlier disease stadium, in part of the cases facilitated by ANCA testing; the pathologist may therefore at first be confronted with a more subtle or abortive morphology, while in the course of disease, histopathology is modulated by immunosuppressive therapy. A number of immunopathogenic mechanisms prevailing in vasculitis can be verified with immunohistochemical and molecular biological methods, although it is frequently not known how they are initiated and perpetuated. ANCA-associated vasculitides serve as a good example to demonstrate the central role of morphology, its potentials but also its limitations in the differential diagnosis of vasculitis.