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Checkpoint inhibitors have revolutionized the current management of solid tumors. However, immunologic side effects are well known. There are increasing case reports of myasthenia gravis induced by checkpoint inhibitors. Here, we present a case of checkpoint inhibitor-induced myositis and myasthenia gravis presenting separately.

An 81-year-old man with bladder cancer on pembrolizumab presented with generalized muscle aches after his second infusion. He had difficulty holding his head up, and a waddling gait. His creatine kinase (CK) level was 4839 (32-200) IU/L, aldolase was 101 (ref<7.7 IU/L) IU/L, ANA (anti-nuclear antibody) and other autoimmune workup was negative. He was diagnosed with inflammatory myositis and started on steroids. He was scheduled for IVIg (intravenous immunoglobulin) infusions. He had clinical improvement after starting steroids and they were tapered over the next three months. Pembrolizumab was held. He presented to the clinic three months later with worsening weakness, ptosis, difficulty swallowing, and choking episodes. His CK this time was 200 IU/L, and serum aldolase was 9.2 IU/L. His NIF (Negative Inspiratory Force) was -60 cm H₂O.

Given his history, with worsening proximal muscle weakness as the day progressed, relatively low CK and aldolase, there was concern for checkpoint induced-myasthenia gravis and he was started on pyridostigmine 60 mg three times a day, 70 mg prednisone, and IVIg 2 gm/kg over 5 days. He perceived definite clinical benefit. His acetylcholine receptor and MuSK (muscle-specific kinase) antibodies were negative. He was followed up in the clinic and EMG (Electromyography) showed a prejunctional neuromuscular defect consistent with myasthenia gravis. He was started on methotrexate and is currently on a prolonged steroid taper.  

With rising interest in immunotherapies for cancer treatment, reporting significant side effects becomes important. This case was interesting as the patient had both myasthenia gravis and myositis induced from pembrolizumab.