Early development of pembrolizumab‐induced fulminant myositis and cardiotoxicity in a patient with metastatic thymoma

Herein, we report the case of a 48‐year‐old woman with metastatic thymoma who developed fulminant myositis with cardiotoxicity after one cycle of pembrolizumab treatment. She presented with severe muscle weakness and dyspnea, and her laboratory test results revealed increased muscle and cardiac enzyme levels. Despite an urgent initiation of systemic steroids, her muscle weakness and hypercapnia worsened, for which intravenous immunoglobulin G was initiated. However, hypercapnia did not improve, but the patient recovered completely after plasma exchange. Patients with thymic neoplasms could be susceptible to fulminant forms of immune‐related adverse effects because they lack normal thymic physiology. Clinicians must not hesitate to consider immunoglobulin G administration and plasma exchange therapy as the next treatment steps for steroid‐refractory patients.