A case of severe immune-related adverse events, myocarditis with myositis, and myasthenia gravis overlap syndrome following adjuvant nivolumab administration for muscle-invasive bladder cancer.

Herein, we present a case of severe immune-related adverse events (irAEs), myocarditis with myositis, and myasthenia gravis overlap syndrome (IM3OS) in a patient receiving an immune checkpoint inhibitor (ICI), as adjuvant therapy after surgery for muscle-invasive bladder cancer. An 80-year-old woman who had undergone a total cystectomy for bladder cancer presented with ptosis, diplopia, and paralysis 18 days after receiving nivolumab, an anti-programmed cell death-1 (PD-1) monoclonal antibody, as adjuvant therapy for the first time. Initial testing revealed positive findings on the ice pack test; elevated troponin, creatine kinase, and aldolase levels; and an abnormal electrocardiogram, suggesting that the patient had developed ICI-related myocarditis, myositis, and myasthenia gravis. Despite treatment with intravenous immunoglobulin (IVIG) and high-dose corticosteroids, her condition worsened, leading to a complete atrioventricular block. After cardiac pacemaker insertion and intensive treatment with repeated high-dose corticosteroids, IVIG, plasma exchange, and tacrolimus, left ventricular function and myositis symptoms improved. However, the patient developed a respiratory infection and renal failure, leading to death on day 99. Although ICIs are considered relatively safe with few side effects, they can cause serious complications and lead to death. In particular, when severe irAEs occur in multiple organs, such as IM3OS, the prognosis is poor. Although IM3OS has no specific diagnostic biomarker, making early detection difficult, clinicians should always pay attention to patient symptoms when using ICI and evaluate other pathologies with IM3OS when conditions such as myositis or myocarditis are suspected. Further research is needed to elucidate the pathophysiology and risk factors of IM3OS.