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.
Key red flags
Haemoptysis is not required for alveolar haemorrhage; new hypoxia, diffuse opacity and unexplained anaemia is sufficient for emergency assessment.
New dysmorphic haematuria, casts, proteinuria, pressure rise and creatinine progression is the dominant and sometimes silent MPA manifestation.
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.
Key medicines
RituximabFor licensed adult induction, give 375 mg/m² intravenously on four consecutive weekly occasions. The two-dose 1,000 mg schedule on days 1 and 15 is a common specialist off-label alternative. Licensed maintenance is 500 mg twice, fourteen days apart, followed by 500 mg six-monthly for at least two years; after rituximab induction, the first maintenance course is no earlier than week 16.
Cyclophosphamide intravenous pulsesAdminister 15 mg/kg intravenously at induction, two weeks later and again at week four; subsequent pulses are three-weekly for the planned course, with reductions for age, renal impairment or leukopenia.