The histological assessment of cutaneous vasculitis.

Vasculitis is defined as inflammation directed at vessels, which compromises or destroys the vessel wall leading to haemorrhagic and/or ischaemic events. Skin biopsy is the gold standard for the diagnosis of cutaneous vasculitis, whose manifestations include urticaria, infiltrative erythema, petechiae, purpura, purpuric papules, haemorrhagic vesicles and bullae, nodules, livedo racemosa, deep (punched out) ulcers and digital gangrene. These varied morphologies are a direct reflection of size of the vessels and extent of the vascular bed affected, ranging from a vasculitis affecting few superficial, small vessels in petechial eruptions to extensive pan-dermal small vessel vasculitis in haemorrhagic bullae to muscular vessel vasculitis in lower extremity nodules with livedo racemosa. Skin biopsy, extending to subcutis and taken from the earliest, most symptomatic, reddish or purpuric lesion is crucial for obtaining a high-yielding diagnostic sample. Based on histology, vasculitis can be classified on the size of vessels affected and the dominant immune cell mediating the inflammation (e.g. neutrophilic, granulomatous, lymphocytic, or eosinophilic). Disruption of small vessels by inflammatory cells, deposition of fibrin within the lumen and/or vessel wall coupled with nuclear debris allows for the confident recognition of small vessel, mostly neutrophilic vasculitis (also known as leukocytoclastic vasculitis). In contrast, muscular vessel vasculitis can be identified solely by infiltration of its wall by inflammatory cells. Extravasation of red blood cells (purpura) and necrosis are supportive, but not diagnostic of vasculitis as they are also seen in haemorrhagic and/or vaso-occlusive disorders (pseudovasculitis). Vasculitic foci associated with extravascular granulomas (palisaded neutrophilic and granulomatous dermatitis), tissue eosinophilia, or tissue neutrophilia signal the risk for, or co-existence of systemic disease. This essential histological information coupled with direct immunofluorescence and anti-neutrophil cytoplasmic data and clinical findings enables more precise and accurate diagnosis of localized and systemic vasculitis syndromes.