Immune Checkpoint Inhibitor–Induced Seronegative Myasthenia-Like Syndrome Following Pembrolizumab Therapy: A Novel Adverse Drug Reaction

Immune checkpoint inhibitors (ICIs), including pembrolizumab, are increasingly associated with immune-related adverse events (irAEs), though neuromuscular complications remain rare. We report a distinctive case of pembrolizumab-induced seronegative myasthenia-like syndrome (ICI-MLS) in a 63-year-old man receiving treatment for metastatic melanoma. The patient developed acute bulbar symptoms, ptosis, and generalized fatigability two weeks after his third infusion. Serology for acetylcholine receptor and Muscle specific kinase (MuSK) antibodies was negative; however, electrophysiological testing demonstrated a significant decremental response on repetitive nerve stimulation, confirming a postsynaptic transmission defect. Discontinuation of pembrolizumab and initiation of high-dose steroids with intravenous immunoglobulin resulted in rapid clinical improvement. This case highlights the diagnostic challenges of seronegative ICI-MLS and underscores the critical importance of early recognition and prompt immunosuppressive therapy to prevent life-threatening respiratory complications.