Microscopic polyangiitis (MPA) is a small vessel necrotizing vasculitis that falls within a larger spectrum of disorders known as antineutrophil cytoplasmic antibody (ANCA)–associated vasculitides. This group also includes granulomatosis with polyangiitis (GPA), MPA, eosinophilic granulomatosis with polyangiitis (EGPA or Churg-Strauss disease), and renal limited vasculitis. This classification of vasculitides is based on the type of vessels involved and the underlying etiology, as established by the International Chapel Hill Consensus Conference on the Nomenclature of Vasculitides (CHCC 2012).
The term "microscopic polyangiitis" was first described as "microscopic polyarteritis" and introduced in the literature by Davson in 1948 to describe the pattern of glomerulonephritis observed in patients with polyarteritis nodosa. This condition was later described as a pattern of necrotizing vasculitis without immune complex deposition, primarily affecting small vessels such as the capillaries, venules, and arterioles. The disease commonly involves glomerulonephritis, pulmonary capillaritis, and other systemic capillary beds, with considerable overlap with GPA.
The absence of granulomatous inflammation in the upper respiratory tract and the presence of pulmonary capillaritis help differentiate MPA from GPA. MPA is more likely to have renal involvement than GPA. MPA is part of a group of disorders known as pulmonary-renal syndrome, which also includes GPA, Goodpasture disease, and systemic lupus erythematosus. While ANCAs have a central role in the pathogenesis of ANCA-associated vasculitis, they are neither necessary nor sufficient to cause the disease patterns of ANCA-associated vasculitides. Clinical symptoms and biopsy results are equally important for diagnosis. The role of ANCAs and the distinctions between the various ANCA-associated vasculitides are also discussed in this activity.
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