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  • Microscopic polyangiitis (MPA) is the most common cause of the pulmonary-renal syndrome of alveolar hemorrhage and glomerulonephritis.
  • Usually includes combinations of two or more of the following:
    • Nonspecific constitutional symptoms, including fatigue, myalgias, weight loss, and fevers.
    • Migratory arthralgias or arthritis, either pauciarticular or polyarticular.
    • Palpable purpura, sometimes with skin ulcerations.
    • Sensorimotor mononeuritis multiplex.
    • Alveolar hemorrhage associated with hemoptysis and respiratory compromise.
    • Glomerulonephritis.
  • Antineutrophil cytoplasmic antibodies (ANCAs) are often critical in making the diagnosis, but a significant minority of patients are ANCA-negative.
  • The majority of patients with MPA who are ANCA positive have antibodies directed against myeloperoxidase (MPO-ANCA).
  • ANCA titers are often elevated during disease flares but do not have a consistent temporal relationship with disease activity. Thus, ANCA titers are not reliable predictors of disease flares.

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Microscopic polyangiitis (MPA) is a form of systemic vasculitis that may affect many major organs with crippling or fatal effects. Seventy percent of patients with MPA have antineutrophil cytoplasmic antibodies (ANCAs). MPA is recognized to be related to both granulomatosis with polyangiitis (GPA; formerly Wegener granulomatosis) and eosinophilic granulomatosis with polyangiitis (EGPA; the Churg-Strauss syndrome) (see Chapters 32 and 34). These disorders are sometimes considered together as the ANCA-associated vasculitides, but important differences exist among these three conditions and significant percentages of patients with these diagnoses do not have ANCA.

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MPA has been recognized increasingly since the first Chapel Hill Consensus Conference on the nomenclature of systemic vasculitides, the results of which were published in 1994. Many cases before then were considered to be forms of polyarteritis nodosa (PAN), a disease with which MPA shares substantial overlap. Table 33–1 compares the features of MPA with those of GPA (Wegener) and PAN.

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Table Graphic Jump Location
Table 33–1. Comparison of the Features of MPA, GPA, and PAN.
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The term “polyangiitis” is preferred to “polyarteritis” for MPA because of the disease’s tendency to involve veins as well as arteries. The first Chapel Hill Consensus Conference defined MPA as a process that (1) involves necrotizing vasculitis with few or no immune deposits; (2) affects small blood vessels (capillaries, arterioles, or venules) and possibly medium-sized vessels; and (3) demonstrates a tropism for the kidneys and lungs. With an estimated incidence ...

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