Synopsis
Recognise microscopic polyangiitis as a non-granulomatous small-vessel vasculitis dominated by glomerulonephritis, pulmonary capillaritis, skin and nerve disease, confirm mechanism with biopsy, and deliver safe remission induction and maintenance.
- MPA commonly presents with rapidly progressive glomerulonephritis, constitutional inflammation, pulmonary capillaritis, palpable purpura or mononeuritis multiplex without granulomatous ENT destruction.
- Check urine and creatinine urgently in any suspected systemic vasculitis; renal disease can be clinically silent until substantial filtration is lost.
- MPO-ANCA supports the diagnosis in the correct phenotype but is neither required nor sufficient, and persistent titre alone does not prove relapse.
- Kidney biopsy is the reference tissue test when active sediment is present, confirming pauci-immune necrotising crescents and quantifying reversible activity versus scar.
- Send anti-GBM antibodies immediately in pulmonary-renal disease because double-positive disease occurs and changes plasma-exchange decisions.
- Use rituximab or cyclophosphamide with reduced-dose glucocorticoid for organ-threatening disease; avacopan can replace much oral steroid in eligible severe active MPA.
- Consider plasma exchange selectively when creatinine is very high or dialysis is required, or for hypoxaemic alveolar haemorrhage, after balancing serious-infection risk.
- For non-organ-threatening systemic disease, specialist immunosuppression is still needed; steroid monotherapy creates relapse and toxicity.
- Maintenance uses rituximab or azathioprine after remission, with duration tailored to relapse risk, kidney recovery, age, infection and MPO phenotype.
- Before calling respiratory or renal deterioration relapse, assess infection, pulmonary oedema, thrombosis, chronic scar, drug toxicity and adherence.
Key red flags
Haemoptysis is not required for alveolar haemorrhage; new hypoxia, diffuse opacity and unexplained anaemia is sufficient for emergency assessment.
Oliguria, potassium rise, acidosis, fluid overload or rapidly changing creatinine requires urgent nephrology and dialysis assessment.
Painful asymmetric weakness or foot drop indicates nerve infarction and needs prompt induction before deficit becomes fixed.
Mesenteric pain, myocardial injury, scleritis or severe skin necrosis indicates life- or organ-threatening vasculitis.
Infection can mimic relapse and MPO-ANCA may remain positive in remission, so treatment must follow direct organ evidence.
New dysmorphic haematuria, casts, proteinuria, pressure rise and creatinine progression is the dominant and sometimes silent MPA manifestation.
Falling haemoglobin, hypoxia and diffuse bilateral opacity indicates pulmonary capillary bleeding even when haemoptysis is absent.
Investigation priorities
Detect capillary injury and define AKI severity and dialysis complications.
Management branches
Active urine, renal decline or alveolar haemorrhage raises MPA or another pulmonary-renal syndrome.
- Admit according to physiology, treat oxygen, potassium, fluid and sepsis, and send cultures before antimicrobial therapy when safe.
- Obtain MPO or PR3, anti-GBM, complement and immune-complex tests and arrange urgent kidney biopsy; sample lungs when infection or haemorrhage remains uncertain.
- Begin organ-saving induction before final pathology only when delay is more dangerous than diagnostic uncertainty.