Cutaneous and systemic vasculitides in dermatology: a histological perspective.

Cutaneous vasculitides encompass a wide and heterogeneous group of diseases affecting skin blood vessels that are clinically characterized by polymorphic skin lesions, particularly including palpable purpura as well as urticarial and necrotic-ulcerative lesions, with possible, albeit rare, extracutaneous involvement. Cutaneous leukocytoclastic angiitis and urticarial vasculitis, which are the two prototypic and most common variants of this group, are usually idiopathic but may also be induced by different triggers, notably drugs and infections, or may manifest in association with systemic disorders, particularly lupus erythematosus. Vasculitis skin lesions can also occur during the chronic-relapsing course of systemic vasculitides, such as granulomatosis with polyangiitis and eosinophilic granulomatosis with polyangiitis, with which cutaneous vasculitides can share clinical and histological features. In order to make the diagnosis of a specific form of vasculitis, the cutaneous picture and clinical history of the patient have to be correlated with the histological pattern, direct immunofluorescence findings, laboratory data and possible presence of extracutaneous manifestations. Histology is pivotal for the diagnosis of vasculitis and timing of the biopsy is relevant to correctly evaluate the characteristics of the inflammatory infiltrate. In this review, we will focus in particular on the histological features of cutaneous vasculitides in order to differentiate them from the systemic forms and to avoid misdiagnosis when skin involvement is the presenting sign of a multisystem vasculitis.