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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, caution must be employed when making treatment decisions based on ANCA testing.

General Considerations

Microscopic polyangiitis (MPA) is a form of systemic vasculitis that may affect many major organs with crippling or fatal effects. The great majority 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; formerly the Churg-Strauss syndrome). 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.

MPA has been recognized formally as a disease entity since the first Chapel Hill Consensus Conference on the nomenclature of systemic vasculitides in 1994. Many cases before then were considered to be forms of polyarteritis nodosa (PAN), a disease with which MPA shares substantial overlap. Table 29–1 compares the features of MPA with those of GPA and PAN.

Table 29–1.Comparison of the features of MPA, GPA, and PAN.

The term “polyangiitis” is preferred to “polyarteritis” for MPA because the disease can involve veins as well as arteries. MPA is defined 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 ...

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