• Small-vessel vasculitis of the skin, often accompanied by little or no apparent involvement of other organs.
  
• Known by a variety of other names, including cutaneous leukocytoclastic angiitis.
  
• Precipitants such as medications and infections are often identifiable, but approximately 40% of cases have no definable cause.
  
• Primary forms of vasculitis such as Henoch-Schönlein purpura, microscopic polyangiitis, and granulomatosis with polyangiitis (formerly Wegener granulomatosis) must be excluded. Similarly, well-recognized forms of secondary vasculitis such as mixed cryoglobulinemia caused by hepatitis C must also be eliminated from the differential diagnosis.
  
• Most cases are self-limited if the precipitant can be identified and removed. Glucocorticoids or other medications are required in other cases.

Hypersensitivity vasculitis refers to small-vessel vasculitis that is restricted to the skin and not associated with any other form of primary or secondary vasculitis. Implicit in this definition is that the condition is not associated with medium- or large-vessel disease at other sites, nor with small-vessel disease in other organs (eg, the glomeruli or pulmonary capillaries). In many cases, an identifiable precipitant such as a drug or an accompanying infection is present—hence the term “hypersensitivity.” In up to 40% of cases, however, no specific cause is identified.

The term “hypersensitivity vasculitis” has been associated with much confusion ever since it was incorporated into the first vasculitis classification scheme in the early 1950s. The condition’s name derives from the fact that by the 1950s, both human and animal models of hypersensitivity to foreign antigens had been shown to cause small-vessel vasculitis involving the kidneys, lungs, and other organs besides the skin. Consequently, even microscopic polyangiitis (see Chapter 33), a disorder that commonly affects internal organs as well as the skin and is often associated with antineutrophil cytoplasmic antibodies (ANCAs), was grouped initially under the heading of hypersensitivity vasculitis. Because of the confusion surrounding its name, many clinicians have suggested that hypersensitivity vasculitis be replaced, but no entirely suitable alternative has been found. Terms used synonymously with hypersensitivity vasculitis have included leukocytoclastic vasculitis, cutaneous leukocytoclastic angiitis, and cutaneous small-vessel vasculitis, among others. In evaluating patients with small-vessel vasculitis of the skin, it is critical to remember that skin findings may only herald an underlying disorder involving other organs, as well. Extracutaneous involvement, which mandates reconsideration of the diagnosis, must be excluded with appropriate tests.

In most cases of hypersensitivity vasculitis, the problem is believed to have an immune complex–mediated pathophysiology. Histopathology generally shows a leukocytoclastic vasculitis, with features of necrosis in some cases but not granulomatous inflammation. Biopsies very early in the course of disease may show a lymphocytic predominance.

Table 37–1 outlines the classification criteria for hypersensitivity vasculitis established in 1990 by the American College of Rheumatology.