Immune checkpoint inhibitors (ICIs) are increasingly encountered in primary care, and rheumatologic immune-related adverse events (irAEs) can rarely mimic polymyalgia rheumatica (PMR). We describe an 83-year-old male with metastatic melanoma treated with Nivolumab who developed debilitating proximal muscle weakness over the course of 6 months. Repeated testing of ESR and CRP yielded normal results, which is a finding at odds with standard PMR criteria. Further evaluation did not support differential diagnoses of subclinical adrenal insufficiency, atypical myositis or atypical cardiomyopathy. Given the temporal relationship to ICI therapy and pattern of symptoms, an ICI associated PMR-like irAE was favored. The patient completed a one-month prednisone 15 mg taper and physical therapy. At follow-up, marked improvement of proximal muscle weakness and mobility led to increased quality of life. This case demonstrates that normal inflammatory marker results do not rule out PMR-like disease in the setting of checkpoint inhibition. For primary care, prompt recognition of PMR-like irAE and timely initiation of corticosteroid treatment with physical rehabilitation may shorten time to treatment and preserve function. It also decreases the risk of unintended sequelae such as giant cell arteritis.
