Microscopic polyangiitis

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.

• Necrotizing vasculitis of small vessels, typically with pulmonary, renal, and skin involvement ^[1]
• Manifestations are similar to granulomatosis with polyangiitis.
• The nasopharynx is usually not affected.

• Constitutional symptoms
• Renal (∼ 90%): pauci-immune glomerulonephritis; with hypertension ^[2][3]
• 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

Clinical features are very similar to those of granulomatosis with polyangiitis (GPA), but MPA spares the vessels in the upper respiratory tract (no sinusitis or rhinitis). ^[3]

Diagnosis is primarily clinical and confirmed by biopsy. Chest imaging is required for all patients with lower respiratory symptoms.

• Laboratory studies ^[3][4]
  • Inflammatory markers: ↑ ESR, ↑ CRP
  • CBC: leukocytosis, thrombocytosis, anemia
  • Serology: ANCA (positive in 50–75% of patients) ^[3][4]
    • MPO-ANCA in > 50% of patients
    • PR3-ANCA in ∼ 25% of patients
  • Urinalysis: proteinuria, microscopic hematuria, erythrocyte casts
• CT chest
  • Indication: patients with lower respiratory symptoms
  • Findings: ground-glass opacifications (> 90% of patients), nodular lesions
• Biopsy of involved organ: confirms the diagnosis ; ^[3][5]
  • Absence of granulomas
  • Fibrinoid necrosis with neutrophil infiltration

Microscopic PolyAngiitis has MyeloPeroxidase Antibodies (i.e., pANCA).

The absence of granulomas on histopathology plus pANCA positivity helps differentiate MPA from GPA, as they have very similar clinical features. ^[3][4]

General principles ^[3][5][6]

• 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.

Pharmacotherapy ^[6]

• Induction of remission: for patients with active disease
  • Nonsevere disease
    • Oral glucocorticoids: e.g., prednisone
    • PLUS a glucocorticoid-sparing agent: e.g., methotrexate (preferred), cyclophosphamide, rituximab, OR azathioprine
  • Severe disease
    • High-dose glucocorticoids: e.g., methylprednisolone pulses OR prednisone
    • PLUS a glucocorticoid-sparing agent: e.g., rituximab (preferred) OR cyclophosphamide
• Maintenance of remission: indicated for patients who presented with severe disease
  • Slowly taper glucocorticoids to the minimum effective dose. ^[6]
  • Glucocorticoid-sparing agents: e.g., rituximab (preferred), methotrexate, OR azathioprine
  • Duration of treatment: typically ≥ 18 months

Plasmapheresis ^[6]

• Indications ^[6]
  • Rapidly progressive glomerulonephritis at risk of end-stage renal disease
  • Concomitant anti-GBM disease
  • Severe diffuse alveolar hemorrhage refractory to other therapies

Supportive care

• Prevent complications of glucocorticoid therapy.
• Monitor for adverse effects of immunosuppressants.
• Consider pneumocystis pneumonia prophylaxis.