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Microscopic polyangiitis (MPA) is a necrotizing vasculitis that affects small vessels (capillaries, venules, arterioles), often involving the kidneys and lungs, but lacking granulomatous inflammation.

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It is one of the major ANCA-associated vasculitides (AAV), typically associated with p-ANCA (MPO-ANCA).

https://www.youtube.com/watch?v=xGFX2ULGfho

Epidemiology

Peak age: 50–60 years
No strong sex predilection
Incidence: ~3–24 cases per million/year
More common in Caucasian populations

Pathophysiology

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Unlike GPA, MPA does not form granulomas

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Immune-mediated small-vessel injury
Associated with myeloperoxidase (MPO)-ANCA, detected as p-ANCA on immunofluorescence
Inflammation causes:
- Fibrinoid necrosis
- Capillaritis (especially in lungs)
- Pauci-immune crescentic glomerulonephritis

Clinical Features

System	Manifestations
Renal	Rapidly progressive glomerulonephritis (RPGN)
– hematuria, proteinuria, renal failure
Pulmonary	Pulmonary capillaritis, diffuse alveolar hemorrhage (DAH): cough, dyspnea, hemoptysis
Constitutional	Fever, weight loss, fatigue
Skin	Palpable purpura, ulcers (especially on lower extremities)
Peripheral nerves	Mononeuritis multiplex
GI/CNS involvement	Less common than in PAN or GPA

Epidemiology

Pathophysiology

Clinical Features

Laboratory Findings