ANCA-associated vasculitis with muscle involvement mimicking polymyalgia rheumatica

We report a case of a woman in her early 80s with antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis presented as myalgia mimicking polymyalgia rheumatica (PMR). She had positive results for the Neer and Hawkins-Kennedy impingement tests, and a normal serum creatine kinase (CK) concentration. At first, we suspected PMR; however, the patient did not strictly meet the classification criteria. Electromyography revealed an abnormal myogenic pattern, and muscle MRI revealed intramuscular and fascial hyperintensity. Moreover, chest CT revealed interstitial lung disease, and test results for ANCAs were positive. We diagnosed the patient with ANCA-associated vasculitis based on the criteria and treated her with corticosteroids and rituximab. Thus, ANCA-associated vasculitis can cause muscle involvement without elevation of the CK concentration and mimic PMR.