Clinical meaning
Microscopic polyangiitis (MPA) is an ANCA-associated small-vessel vasculitis characterized by necrotizing vasculitis of small vessels (capillaries, venules, arterioles) without granuloma formation, distinguishing it from granulomatosis with polyangiitis. Perinuclear ANCA (p-ANCA) directed against myeloperoxidase (MPO) activates neutrophils, causing them to adhere to endothelial cells and release reactive oxygen species and lytic enzymes that destroy vessel walls. The kidneys are the most commonly affected organ, with rapidly progressive glomerulonephritis (pauci-immune crescentic glomerulonephritis) causing hematuria, proteinuria, and rapidly rising creatinine. Pulmonary involvement presents as diffuse alveolar hemorrhage (hemoptysis, bilateral infiltrates, dropping hemoglobin) from capillaritis. The nurse monitors renal function (creatinine, urinalysis for red cell casts), assesses respiratory status and monitors for hemoptysis, monitors p-ANCA/MPO levels, administers remission induction therapy (rituximab or cyclophosphamide with corticosteroids), monitors for immunosuppression complications, manages blood pressure to slow renal progression, and recognizes the overlap with Goodpasture syndrome (concurrent anti-GBM and ANCA antibodies occur in some patients).