Interstitial lung disease unmasked as myeloperoxidase-antineutrophil cytoplasmic antibody-associated vasculitis following acute kidney injury

Small-vessel vasculitis driven by antineutrophil cytoplasmic antibodies (ANCA) is a severe, systemic autoimmune condition whose clinical behavior can be notoriously unpredictable. When the offending antibody targets myeloperoxidase (MPO), the disease most commonly manifests as microscopic polyangiitis (MPA), a necrotizing vasculitis that, unlike its granulomatous counterparts, destroys vessel walls without forming granulomas. We report a 49-year-old woman who had been living with what appeared to be idiopathic interstitial lung disease (ILD) for a full year before presenting to our emergency department in florid acute respiratory failure. She required emergent intubation for severe metabolic acidosis and, within 48 hours of admission, her kidneys, previously unaffected, collapsed into acute kidney injury (AKI) necessitating hemodialysis (HD). Serological testing came back strongly positive for MPO-ANCA, and renal biopsy showed crescentic glomerulonephritis (GN) in a pauci-immune pattern, confirming the diagnosis. We initiated plasmapheresis, high-dose pulse methylprednisolone, and cyclophosphamide, with rituximab planned as sequential therapy. Despite a brief biochemical response at three weeks, her clinical trajectory proved irreversible and she died. A high index of suspicion for underlying vasculitis is essential in any patient with established pulmonary fibrosis who develops sudden, unexplained renal deterioration.