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

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.