Unmasking Weakness: A Case of Pembrolizumab-induced Myasthenia Gravis With Concurrent Myositis

Immune check point inhibitors (ICIs) have revolutionized modern oncology by enhancing endogenous antitumor immunity by targeting multiple regulatory pathways. Despite their efficacy in cancer management, ICIs can cause many immune-related adverse events (irAE), including neurologic adverse events (irNAEs) such as myasthenia gravis (MG)-like syndromes or myositis. We describe the case of an 86-year-old patient with renal cell carcinoma newly treated with pembrolizumab who developed generalized weakness, dysphagia, dysphonia, eyelid ptosis, and ophthalmoparesis shortly after receiving his second dose of pembrolizumab. Examination revealed weak palatal elevation, mild symmetric muscle weakness, mild weakness of neck flexion, and a positive ice-pack test. His workup revealed coexistence of ICI-induced MG and myositis. He was started on high dose corticosteroids, intravenous immunoglobulin, and pyridostigmine, resulting in clinical improvement. ICI therapy was permanently discontinued. ICI-induced NAEs carry significant morbidity and mortality, requiring early recognition, multidisciplinary management, and aggressive immunosuppressive therapy to improve clinical outcomes.