Microscopic Polyangiitis (MPA)
MPA is defined as a systemic necrotizing vasculitis that clinically and histologically affects small vessels (i.e., capillaries, venules, or arterioles) with few or no immune deposits. MPA is frequently associated with focal segmental necrotizing glomerulonephritis and pulmonary capillaritis.
Patients typically present with acute onset of rapidly progressive glomerulonephritis (100%) and up to 50% have pulmonary infiltrates and/or effusions. Up to 30% of cases have diffuse alveolar hemorrhage with hemoptysis. Other manifestations include fever (50% to 70%), arthralgias (30% to 65%), gastrointestinal tract involvement (50%), purpura (40%), ears/nose/throat involvement (30%), and peripheral or CNS disease (25% to 30%).
MPA is differentiated from GPA by the absence of granulomas.
MPA diagnosis is made on the basis of a characteristic clinical presentation and a renal biopsy showing necrotizing glomerulonephritis without immune deposits. p-ANCA directed against MPO is found in up to 60% of patients and is supportive of the diagnosis. Notably, some patients (15% to 30%) may have c-ANCA (directed against PR3) but MPA patients are less likely to have upper respiratory tract involvement (i.e., sinusitis), which distinguishes MPA from GPA.
Clinical Features of Polyarteritis Nodosa (PAN), Microscopic Polyangiitis (MPA), and Granulomatosis With Polyangiitis (GPA)
Renal vasculitis with infarcts and microaneurysms
Rapidly progressive glomerulonephritis with crescents
Hepatitis B virus infection
Perinculear antineutrophil cytoplasmic antibody
Abnormal angiogram with microaneurysms
Necrotizing vasculitis (no granulomas)