Immune Checkpoint Inhibitor-Induced Myositis Causing Severe Ptosis: A Case Report.

Pembrolizumab (Keytruda), an immune checkpoint inhibitor, has been increasingly utilized in the treatment of metastatic urothelial carcinoma. While offering a favorable safety profile compared to traditional chemotherapy, it presents unique risks, including immune-related adverse events (irAEs). This case report describes a 77-year-old woman with a history of invasive bladder cancer treated with pembrolizumab who developed severe bilateral ptosis, myositis, and myocarditis. Initially considered to have 3M syndrome, further evaluation indicated a myasthenia gravis mimic. This report emphasizes the critical need for vigilant monitoring for early signs of irAEs, such as ptosis, myocarditis, and myositis, in patients receiving pembrolizumab. The patient presented with severe ptosis and respiratory difficulties and was successfully treated with high-dose steroids and plasma exchange therapy, leading to overall improvement. This case highlights the diagnostic challenges of differentiating myositis from myasthenia gravis in patients with ICI-induced complications while advocating for an aggressive treatment approach.