Elevated Creatine Kinase (CK) and Myositis Triggered by Immune Checkpoint Inhibitors: Muscle Mysteries Unveiled.

Immune checkpoint inhibitors (ICIs), including programmed cell death protein 1 (PD-1) inhibitors, such as nivolumab, and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) inhibitors, such as ipilimumab, have revolutionized cancer treatment, particularly in metastatic melanoma. However, these therapies can cause immune-related adverse events (irAEs), including myositis, a rare but significant complication characterized by elevated creatine kinase (CK) and muscle weakness. We present the case of a 79-year-old male with a history of metastatic melanoma to the brain, previously treated with nivolumab and ipilimumab, who was admitted after an unwitnessed fall. Initial laboratory tests revealed significantly elevated CK levels (5277 U/L) and mildly elevated liver function tests (LFTs). Despite the absence of muscle pain or weakness upon presentation, the patient developed proximal muscle weakness and myalgias during hospitalization. Extensive workup, including negative autoimmune panels and imaging, raised suspicion of immune-related myositis given the recent ICI therapy. The patient was treated with prednisone, which resulted in a rapid decrease in CK levels and improvement of symptoms, supporting the diagnosis of ICI-related myositis.