Pembrolizumab-Induced Myositis Presenting with Ocular Symptom and Head Drop in a Patient with Positive Acetylcholine Receptor Binding Antibody

A 55-year-old man with malignant thymoma and pre-existing acetylcholine receptor antibodies developed ptosis and head drop after pembrolizumab treatment. Although recurrence of myasthenia gravis was initially suspected, markedly elevated creatine kinase levels and myopathic findings on electromyography supported immune checkpoint inhibitor (ICI)-induced myositis. This case highlights that ocular and respiratory symptom after ICI therapy should raise suspicion for ICI-induced myositis, even in acetylcholine receptor antibody-positive patients.