Features of microscopic polyangitis

Cited text from Nadeau's

Clinical Neurology chapter from 2000:
This recently recognized variant of PAN52 is defined by inflammation that is largely confined to smaller vessels than in PAN - arterioles, capillaries and postcapillary venules. Angiograms are correspondingly normal. Major organ infarction is rare, glomerulonephritis is universal, and there is a high frequency of pulmonary hemorrhage. MPA is considerably more common than classic PAN and it is a considerably more common cause of a pulmonary-renal syndrome than Goodpasture's syndrome. A forme-fruste of MPA is characterized exclusively by glomerulonephritis. Evidence of hepatitis B infection is usually absent. In contrast to classic PAN as well as such small vessel vasculitides as cryoglobulinemic vasculitis and Henoch-Schönlein purpura, there is little or no evidence of immune deposits. Fifty to 80% of patients with MPA have circulating ANCA - usually myeloperoxidase (perinuclear or p-ANCA)(see WG), a rare phenomenon in classic PAN. Some have antibodies to proteinase-3 (central or c-ANCA) as in WG. Clinical distinction from WG may be difficult. Polyneuropathy is present in only 10-20% of patients, in contrast to the 50% or greater frequency in classic PAN. Patients with MPA have a high early case-fatality rate due to pulmonary and renal failure and they are considerably more prone to relapse after treatment.