Myositis-Myasthenia Overlap Syndrome After Immune Checkpoint Inhibitor Treatment: A Case Report.

Immune checkpoint inhibitors (ICIs) are increasingly used in cancer therapy and are associated with immune-related adverse events (irAEs) that can mimic common emergency department (ED) presentations. These complications may resemble cardiopulmonary or neuromuscular disease, making early recognition challenging for emergency clinicians. We report the case of a 77-year-old man who presented to the ED with chest pressure and exertional dyspnea shortly after initiating immunotherapy. Initial evaluation focused on the treatment of a non-ST-elevation myocardial infarction. During his hospitalization, he developed progressive generalized weakness, ptosis, and worsening dyspnea with increasing oxygen requirements and reduced negative inspiratory force consistent with evolving respiratory compromise. He was ultimately diagnosed with probable ICI-associated myositis-myasthenia gravis overlap syndrome based on clinical findings despite negative serologic and electrodiagnostic testing. The patient was initially treated with high-dose intravenous methylprednisolone with transient improvement, but subsequently deteriorated after transition to oral corticosteroids. Empiric plasmapheresis was initiated, after which his respiratory status and neuromuscular strength gradually improved. He was discharged on a high-dose oral steroid taper with close neurology and oncology follow-up. This case highlights the diverse presentations of ICI-related irAEs and underscores the importance of obtaining a thorough oncology and immunotherapy history in patients presenting to the ED with unexplained cardiopulmonary or neuromuscular symptoms.