Microscopic polyangiitis (MPA) is an ANCA-associated small-vessel vasculitis. Typical clinical features include pulmonary vasculitis, pauci-immune glomerulonephritis, and palpable purpura. ANCAs (most commonly MPO-ANCAs) are present in up to 75% of patients with MPA. CT chest is indicated in all patients with pulmonary symptoms and often reveals ground-glass opacifications or nodular lesions. Biopsy is required to confirm the diagnosis; histopathology findings include fibrinoid necrosis without granulomas. Management involves immunosuppressive agents (e.g., glucocorticoids plus a glucocorticoid-sparing agent such as rituximab or methotrexate). Plasmapheresis may be considered for patients with severe disease.
- Constitutional symptoms
- Renal (∼ 90%): pauci-immune glomerulonephritis; with hypertension 
- Lungs (∼ 50%): pulmonary vasculitis; → diffuse alveolar hemorrhage → hemoptysis
- Skin (∼ 40%): palpable purpura, nodules, necrosis
- Gastrointestinal: abdominal pain (∼ 50%), GI bleeding (∼ 25%)
- Neurological (∼ 70%): mononeuritis multiplex, distal symmetric polyneuropathy
Diagnosis is primarily clinical and confirmed by biopsy. Chest imaging is required for all patients with lower respiratory symptoms.
- Laboratory studies 
- Indication: patients with lower respiratory symptoms
- Findings: (> 90% of patients), nodular lesions
- Biopsy of involved organ: confirms the diagnosis ; 
Microscopic PolyAngiitis has MyeloPeroxidase Antibodies (i.e., pANCA).
General principles 
- Consult rheumatology and other specialties (e.g., nephrology, pulmonology) as required.
- Goal of pharmacotherapy (e.g., glucocorticoids PLUS cyclophosphamide) is induction of remission
- Plasmapheresis is only considered for patients with severe end-organ damage.
Induction of remission: for patients with active disease
- Nonsevere disease
- Severe disease
- Maintenance of remission: indicated for patients who presented with severe disease
- Indications 
- Monitor for .
- Consider .