Microscopic Polyangiitis
ANCA-associated small-vessel vasculitis characterised by necrotising vasculitis without granulomata, predominantly affecting the kidneys and lungs. Strongly associated with p-ANCA/MPO antibodies. Distinguished from GPA by the absence of granulomatous inflammation and upper airway involvement.
Key Facts
MPA is characterised by pauci-immune crescentic GN and pulmonary haemorrhage WITHOUT granulomata or upper airway involvement p-ANCA (anti-MPO): positive in ~70%; c-ANCA (anti-PR3) in ~20% Renal involvement is the most common organ manifestation (>80%); MPA is the most common cause of pulmonary-renal syndrome after GPA No upper airway involvement (unlike GPA); no asthma (unlike EGPA) Treatment: same as GPA – rituximab or cyclophosphamide induction; rituximab or azathioprine maintenance Lower relapse rate than GPA (MPO-ANCA associated with fewer relapses than PR3-ANCA) Higher mortality than GPA: more severe renal disease at presentation Skin: palpable purpura; nervous system: mononeuritis multiplex
Overview
Key Facts
MPA is often considered the 'renal-dominant' ANCA vasculitis. It presents with severe renal disease and/or pulmonary haemorrhage.
Epidemiology
- Incidence: ~5-10 per million/year
- Peak age: 60-70; M=F or slight male predominance
- More common in Japanese populations
Pathophysiology
- Anti-MPO antibodies activate neutrophils → necrotising small-vessel vasculitis
- NO granuloma formation (distinguishing feature from GPA and EGPA)
- Pauci-immune GN: minimal immunoglobulin deposition on biopsy
- Pulmonary capillaritis → diffuse alveolar haemorrhage
Clinical Presentation
Renal (>80%)
- Rapidly progressive GN: rising creatinine, haematuria, proteinuria
- May present as RPGN
- Often the predominant organ involved
Pulmonary
- Diffuse alveolar haemorrhage: haemoptysis, dyspnoea, ground-glass on CT
- Pulmonary-renal syndrome
- ILD (rarely fibrotic)
Other
- Constitutional: fever, weight loss, malaise
- Skin: palpable purpura, livedo reticularis
- Nervous system: mononeuritis multiplex (30%)
- GI: mesenteric vasculitis
- Eyes: scleritis (less common than GPA)
Red Flags
- Pulmonary haemorrhage → life-threatening; consider plasma exchange
- Rapidly rising creatinine → urgent renal biopsy and treatment
Differential Diagnosis
| Diagnosis | Key Features | Investigation |
|---|---|---|
| GPA | Upper airway involvement, granulomata, PR3-ANCA | ANCA, biopsy |
| Anti-GBM disease | Linear IgG, anti-GBM positive | Anti-GBM, biopsy |
| SLE | ANA/dsDNA positive, low complement | Immunology |
| IgA vasculitis | Purpura, IgA deposits | Biopsy |
| Polyarteritis nodosa | Medium vessels, aneurysms, no GN | Angiography |
Diagnosis / Investigation
Bloods
- p-ANCA (anti-MPO): positive in ~70%
- c-ANCA (anti-PR3): positive in ~20%
- U&Es: often significantly impaired
- Urinalysis: haematuria, red cell casts
- CRP/ESR: elevated
- Anti-GBM: exclude dual positivity
Imaging
- CT chest: ground-glass (haemorrhage)
- Renal USS: normal-sized kidneys
Biopsy
- Renal biopsy: focal segmental necrotising GN with crescents; pauci-immune (no/minimal Ig on IF)
- No granulomata (distinguishes from GPA)
Management
Induction
- Rituximab 375mg/m² × 4 or IV cyclophosphamide + prednisolone 1mg/kg tapering
- Plasma exchange: consider if severe renal disease (creatinine >500) or pulmonary haemorrhage
Maintenance
- Rituximab 500mg every 6 months (MAINRITSAN) or azathioprine for ≥24 months
Supportive
- PCP prophylaxis (co-trimoxazole)
- Bone protection
- BP control with ACEi/ARB
Referral
- All MPA → nephrology + rheumatology
- Pulmonary haemorrhage → ICU
Prognosis
- 5-year survival: 70-75% (slightly lower than GPA)
- Relapse rate: 20-30% (lower than GPA; MPO < PR3 for relapse)
- Renal prognosis: depends on creatinine at presentation; dialysis-dependent at presentation = 30% recover renal function
- Leading cause of death: infections (immunosuppression), renal failure
- More severe renal disease at presentation compared to GPA
Other Relevant Information
MPA vs GPA
| Feature | MPA | GPA |
|---|---|---|
| ANCA | p-ANCA/MPO (70%) | c-ANCA/PR3 (90%) |
| Upper airway | No | Yes (sinusitis, saddle nose) |
| Granulomata | No | Yes |
| Renal | +++ (predominant) | ++ |
| Pulmonary | DAH | Cavitating nodules, DAH |
| Relapse rate | Lower | Higher |