Erythema nodosum leprosum: nature and extent of the cutaneous microvascular alterations.

Skin biopsy specimens from four patients with erythema nodosum leprosum, when examined as Epon-embedded, 1-micron sections, exhibited a necrotizing vasculitis involving capillaries, venules, and small-to-medium arteries and veins. In the superficial dermis, affected venules and capillaries showed endothelial cell enlargement and focal necrosis associated with perivascular infiltrates of lymphocytes. In the deep dermis and subcutaneous tissue, affected venules, arterioles, and arteries exhibited endothelial cell necrosis and matted fibrin in the vessel walls associated with perivascular infiltrates of neutrophils. Throughout the dermis, mononuclear phagocytes with vacuoles containing numerous fragmented organisms were observed. By electron microscopy, electron-dense material resembling immune complexes was observed in the walls of these vessels. These observations support the concept that erythema nodosum leprosum is an immune complex-mediated necrotizing vasculitis involving capillaries, arterioles, arteries, venules, and veins.