[A case of myasthenia gravis developed during pembrolizumab administration, suggesting an excitation-contraction connection disorder].

The patient was a 50-year-old woman. Pembrolizumab was started for bladder cancer recurrence. From the day after the second administration, ptosis, diplopia, restriction of eye movement, muscle weakness, fatigue resistance, increase in serum creatine kinase (CK) level, and muscle pain were observed. Tests for anti-acetylcholine receptor (AChR) antibody and anti-muscle specific kinase (MuSK) antibody were negative. Electrophysiological examination of the neuromuscular junction showed negative results, and electromyography revealed no myogenic changes. We considered that the immune checkpoint inhibitor caused neuromuscular damage. The patient's symptoms were gradually improved by immunotherapy, such as steroid and plasma exchange. In this case, tests for the anti-titin antibody, an anti-striational antibody, were positive. We considered that myasthenia gravis-like symptoms and serum CK level elevation might have been caused by impairment of excitation-contraction coupling, and not the neuromuscular junction.