Microscopic Polyangiitis After Pulmonary Nontuberculous Mycobacterial Disease: A Case Report and Literature Review

ABSTRACT Nontuberculous mycobacterial pulmonary disease (NTM‐PD) is generally considered an infectious condition. Anti‐neutrophil cytoplasmic antibody (ANCA)‐associated vasculitis is a rare complication that may mimic infectious exacerbation. An 80‐year‐old Japanese woman with a history of NTM‐PD complicated by bronchiectasis and Pseudomonas aeruginosa colonisation presented with fever and a productive cough. Despite treatment with broad‐spectrum antibiotics, the signs of inflammation persisted and urinalysis showed haematuria, proteinuria and casts. The myeloperoxidase (MPO)‐ANCA test was strongly positive (783 U/mL). After intravenous methylprednisolone, followed by oral prednisolone, renal biopsy demonstrated pauci‐immune crescentic glomerulonephritis and nerve conduction studies suggested axonal‐type multiple mononeuropathy. Rituximab was initiated 1 week later. The MPO–ANCA titre decreased from 783 to 160 U/mL, renal function remained stable and systemic inflammation improved, although the neuropathy persisted. Clinicians should consider ANCA‐associated vasculitis in the differential diagnosis when organ‐specific findings emerge during presumed infectious exacerbations of NTM‐PD.