Myeloperoxidase-positive ANCA-associated vasculitis presenting as myalgia, proximal weakness and a normal CK

We describe an unusual presentation of myeloperoxidase positive antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis managed by a multidisciplinary approach. A 75-year-old man gave a 3-week history of proximal lower limb weakness and exertional myalgia. His serum creatine kinase was normal and many of his non-specific symptoms suggested small vessel vasculitis. His investigations for common causes of muscle weakness were normal, and renal biopsy was normal despite haemoproteinuria. CT scan of the chest identified a pulmonary nodule of uncertain significance, not amenable to biopsy. MR scan of the thighs showed muscle oedema, and muscle biopsy confirmed typical features of vasculitis. Following high-dose corticosteroids his exertional myalgia quickly resolved and his normal mobility returned. Early immunosuppression is essential to improving clinical outcomes in ANCA-associated vasculitis, but diagnostic investigations often lack sensitivity.