Microscopic polyangiitis

Definition

Microscopic polyangiitis (MPA) is a systemic necrotizing vasculitis mainly of small vessels without immunoglobulin deposition (pauci-immune). It may begin as a pulmonary-renal syndrome with rapidly progressive glomerulonephritis and alveolar hemorrhage; the pattern depends on the organs affected. Unlike GPA, granulomas are absent.

Classification

In the 2012 Chapel Hill nomenclature, MPA belongs to the ANCA-associated small-vessel vasculitides. Among the features of the 2022 ACR/EULAR classification criteria, MPO or p-ANCA and pauci-immune glomerulonephritis are the most frequent in MPA; pulmonary fibrosis or interstitial lung disease are also included.

Aetiopathogenesis

It is an autoimmune disease; at least 70 % of patients have ANCA, usually perinuclear ANCA (p-ANCA) with antibodies against myeloperoxidase (MPO). Vascular inflammation occurs without relevant immune complex deposition.

Clinical features

A prodromal illness with fever, weight loss, myalgia, and arthralgia usually precedes.

• Kidney (in up to 90 %): hematuria, proteinuria (sometimes above 3 g/24 h), red cell casts; without prompt diagnosis, renal failure may follow rapidly.
• Skin: purpuric rash in about one-third at diagnosis; nail bed infarcts, splinter hemorrhages.
• Lung: alveolar hemorrhage with rapid-onset dyspnea and anemia, with or without hemoptysis; later pulmonary fibrosis; sometimes interstitial lung disease as the first manifestation. Mild rhinitis or sinusitis possible.
• Nervous system: frequently mononeuritis multiplex of peripheral or cranial nerves; rarely cerebral vasculitis.
• Gastrointestinal: abdominal pain, nausea, vomiting, diarrhea, bloody stools.
• Rarely the heart; eyes usually as episcleritis.

Diagnosis

• Suspicion with an unexplained combination of fever, weight loss, arthralgias, abdominal pain, alveolar hemorrhage, new-onset nephritic syndrome, or mononeuritis multiplex.
• Laboratory tests: ESR, CRP, white cell and platelet counts elevated; anemia of chronic disease; an acute drop in hematocrit indicates alveolar or gastrointestinal hemorrhage. Urinalysis with sediment (hematuria, proteinuria, casts) and creatinine.
• ANCA by immunofluorescence and ELISA: usually p-ANCA/MPO.
• Biopsy of the most accessible involved tissue; in the kidney, focal segmental pauci-immune necrotizing glomerulonephritis with fibrinoid necrosis and cellular crescents.
• Chest imaging: bilateral patchy infiltrates suggest alveolar hemorrhage; CT is much more sensitive than radiographs. Bronchoscopy shows blood from all bronchi and later hemosiderin-laden macrophages and excludes infection.

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Further reading (open access)

1. MSD Manual Professional: Microscopic Polyangiitis (MPA)
2. MSD Manual Professional: Overview of Vasculitis
3. Arch Rheumatol 2024: Performance of the 2022 ACR/EULAR Classification Criteria for ANCA-associated vasculitis (PMC-Volltext)
4. Ther Adv Musculoskelet Dis 2022: A glance into the future of ANCA-associated vasculitis (PMC-Volltext, Tabelle ACR/EULAR 2022)

Cross-references