The pneumonia that wasn't: evolving pulmonary-renal syndrome in microscopic polyangiitis

Microscopic polyangiitis (MPA) is an anti-neutrophil cytoplasmic antibodies (ANCA)-associated necrotising small-vessel vasculitis that characteristically involves the kidneys and lungs but may also include cutaneous, musculoskeletal and gastrointestinal symptoms. Atypical symptoms may delay diagnosis. In our case, a woman in her late 40s developed migratory abdominal and flank pain followed by cough and presumed pneumonia. Within 2 weeks she developed haematuria and rising creatinine. Imaging revealed diffuse pulmonary infiltrates; laboratory evaluation showed anti-myeloperoxidase p-ANCA positivity and negative anti-glomerular basement membrane and anti-nuclear antibody serologies. Renal biopsy demonstrated pauci-immune necrotising crescentic glomerulonephritis. She was diagnosed with MPA causing pulmonary-renal syndrome. Aggressive immunosuppressive therapy with intravenous methylprednisolone and rituximab induced remission, and renal function improved substantially. This case highlights the variable manifestations of MPA and the potential for misdiagnosis. Prompt ANCA testing and tissue biopsy are critical for diagnosis; early recognition and treatment of MPA can prevent end-organ damage.