Small vessel vasculitis.
BACKGROUND
Vasculitis is a primary inflammatory process of large, medium-sized, and small blood vessels. There are multiple entities particularly among small vessel vasculitides. Some are life threatening and require treatment with toxic agents. Diagnostic challenges are posed by low prevalence, controversial classification, inconsistency of clinical and pathological features, and the existence of clinical and pathological "look-alikes". Also, patients may present to a variety of medical specialists, and the diagnosis is often unexpected. The cause often is unknown.


REVIEW
Classification is by vessel size and then on an immunopathologic basis. Generalised and "organ-limited" forms occur. The principal clinical and pathological features are outlined for each of antibasement membrane disease, immune complex disorders including Henoch-Schönlein purpura and cryoglobulinemic vasculitis and the pauci-immune group, which is often associated with antineutrophil cytoplasmic antibody (ANCA), comprising microscopic polyangiitis, Wegener's granulomatosis, and Churg-Strauss syndrome. A brief account is given of "look-alikes" including microthrombotic conditions, which can confound the diagnosis of small vessel vasculitis.


CONCLUSION
Requirements for diagnosis include full disclosure of the past and present medical history with review of laboratory results, especially diagnostic immunology. Histology should be of targeted biopsies of recent active lesions in preference to blind biopsies. Sampling should be extensive using high-quality thin sections. Systematic microscopic evaluation of architectural features and cellular detail is necessary. Tissue immunofluorescence is a useful adjunct. The final opinion must take all available information into account but may ultimately depend on a critical judgement by the pathologist.