Interstitial Lung Disease as a Herald of P-ANCA Vasculitis: A Case of Evolving Multisystem Disease.

We present the case of a 72-year-old man with idiopathic pulmonary fibrosis (IPF) who presented with stiffness and pelvic girdle pain and was initially treated for polymyalgia rheumatica (PMR). Despite transient improvement on steroids, persistent symptoms and atypical MRI findings led to muscle and renal biopsies, revealing necrotizing vasculitis and pauci-immune glomerulonephritis. Positive myeloperoxidase and perinuclear antineutrophil cytoplasmic antibody titers confirmed the diagnosis of antineutrophil cytoplasmic antibody-associated vasculitis (AAV). The patient's course was complicated by diffuse alveolar hemorrhage requiring intubation, which improved with pulse steroids and cyclophosphamide. The patient stabilized on methotrexate and methylprednisolone. This case highlights the complexity of diagnosing AAV, especially with overlapping PMR-like symptoms and underlying interstitial lung disease.