Excerpt from Microscopic Polyangiitis

Synonyms, Key Words, and Related Terms: microscopic polyangiitis, MPA, small vessel vasculitis, microscopic polyarteritis nodosa, microscopic PAN, small vessel vasculitides, systemic vasculitis, Wegener granulomatosis, Wegener's granulomatosis, Churg-Strauss syndrome, polyarteritis nodosa

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Background

Microscopic polyangiitis (MPA) is vasculitis of small vessels. It was initially considered as a microscopic form of polyarteritis nodosa (PAN). In 1990, the American College of Rheumatology developed classification criteria for several types of systemic vasculitis but did not distinguish between polyarteritis nodosa and microscopic polyarteritis nodosa. In 1994, a group of experts held an international consensus conference in Chapel Hill, North Carolina to attempt to redefine the classification of small vessel vasculitides.

The vasculitis in small vessels, including arterioles, capillaries, and venules, that is characteristic of MPA is absent in polyarteritis nodosa, making this the proposed distinguishing feature of the two conditions. MPA, Wegener granulomatosis, and Churg-Strauss syndrome comprise a category of small-vessel vasculitis related to antineutrophil cytoplasmic antibodies (ANCAs) and are characterized by a paucity of immune deposits. MPA and Wegener granulomatosis seem to be part of a clinical spectrum. However, the absence of granuloma formation and sparing of the upper respiratory tract are features of MPA. These features help to distinguish MPA from Wegener granulomatosis, although the two conditions are occasionally difficult to distinguish.

Pathophysiology

Vasculitis is inflammation of the vessel walls. MPA is characterized by pauci-immune, necrotizing, small-vessel vasculitis without clinical or pathological evidence of necrotizing granulomatous inflammation.

Frequency

United States

The annual incidence is 3.6 cases per million persons. Prevalence is 1-3 cases per 100,000 population.

International

Incidence is approximately 2 cases per 100,000 persons in the United Kingdom and approximately 1 case in 100,000 persons in Sweden.

Mortality/Morbidity

Renal failure and pulmonary involvement are the major causes of morbidity and mortality.
With treatment, Falk and Guillevin reported 2- and 5-year survival rates of 75% and 74% respectively.

Race

In the United States, the disease is more frequent among white persons than black persons.

Sex

Males are affected slightly more frequently than females.

Age

The age of onset is approximately 50 years.

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