Microscopic Polyangiitis Mimicking Colitis: A Fatal Case of Fulminant Multi-organ Vasculitis

Microscopic polyangiitis (MPA) is a systemic necrotising small vessel vasculitis that predominantly affects the kidneys and lungs.Gastrointestinal (GI) manifestations are less common and may mimic primary enteric disease, delaying recognition of the underlying systemic condition.A 72-year-old woman presented with a one-week history of bloody diarrhoea, abdominal pain, vomiting, lower limb oedema, and purpura following recent ibuprofen use.Initial investigations demonstrated severe metabolic acidosis, stage 3 acute kidney injury (AKI), with an estimated glomerular filtration rate (eGFR) of 4 mL/min/1.73m², active urinary sediment, and computed tomography (CT) evidence of extensive colitis with small bowel enteritis.Faecal testing for multiple enteric pathogens, including Shiga toxin-producing Escherichia coli (STEC), and testing for Clostridioides difficile were negative.Strongly positive myeloperoxidase anti-neutrophil cytoplasmic antibodies (MPO-ANCA; >221.9IU/mL) supported a diagnosis of ANCA-associated vasculitis (AAV), while anti-glomerular basement membrane (anti-GBM) antibodies were negative.Despite prompt treatment with intravenous (IV) methylprednisolone, cyclophosphamide, therapeutic plasma exchange (PLEX), and intermittent haemodialysis, the patient developed fulminant diffuse alveolar haemorrhage (DAH), refractory hypoxaemia, consumptive coagulopathy, and severe acidaemia.She died on day 8 of admission.Renal biopsy histopathology, reported after death, demonstrated necrotising crescentic pauci-immune glomerulonephritis.This case highlights the diagnostic challenges of MPO-ANCA-associated vasculitis presenting with severe gastrointestinal inflammation and bleeding alongside rapidly progressive renal disease.Although the renal findings confirmed ANCA-associated vasculitis, the underlying cause of the gastrointestinal abnormalities remained uncertain, with intestinal vasculitis and ischaemic injury secondary to hypoperfusion among the possible explanations.Recognition of systemic features in patients presenting with gastrointestinal illness and unexplained renal dysfunction is essential to facilitate timely investigation and treatment.