ABSTRACT
Approximately 50 years ago, in an attempt to differentiate vasculitis into clinical and pathological categories, Zeek et al. (1,2) described a form of small-vessel vasculitis that they called hypersensitivity angiitis. This form of vasculitis differed from periarteritis nodosa in that it involved vessels other than arteries and frequently affected capillaries in the lungs and kidneys. In 1954, Godman and Churg (3) confirmed the distinctive clinical and pathological characteristics of this form of vasculitis, but preferred the term microscopic periarteritis because they did not find evidence for an allergic response in their patients with this category of vasculitis. The 1993 Chapel Hill Consensus Conference on the Nomenclature of Systemic Vasculitis adopted the name microscopic polyangiitis for Zeek's hypersensitivity angiitis and Godman and Churg's microscopic periarteritis (4). This designation was preferred over microscopic polyarteritis because, in some patients, microscopic polyangiitis affects capillaries (e.g., alveolar and glomerular capillaries) and venules (e.g., dermal venules) with no identifiable involvement of arteries. The Chapel Hill Nomenclature System also requires that microscopic polyangiitis have few or no immunoglobulin deposits in vessel walls, which distinguishes microscopic polyangiitis from a number of other small-vessel vasculitides that appear to be mediated by immune complexes localized in vessel walls (e.g., cryoglobulinemic vasculitis and Henoch-Schonlein purpura) or by antibodies directed against constituents of vessel walls (e.g., Goodpasture's syndrome). This paucity of immunoglobulin is the basis for the designation pauci-immune small-vessel vasculitis, which refers to a category of necrotizing small-vessel vasculitis with little or no immunoglobulin localization that includes not only microscopic polyangiitis but also Wegener's granulomatosis and ChurgStrauss syndrome (4,5). The pathological similarity among these three categories of small-vessel vasculitis suggested to Godman and Churg that they share the same pathogenic mechanism (3), and this is supported further by their association with antineutrophil cytoplasmic autoantibodies (4-8).
