Thymoma-associated anti-AMPAR encephalitis with myasthenia gravis: a case report

Background: Thymoma can trigger a spectrum of paraneoplastic neurological disorders, including autoimmune encephalitis (AE) and myasthenia gravis (MG). However, the sequential development of anti-α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptor (AMPAR) autoimmune encephalitis followed by MG in the same patient is rarely reported. Objective: To describe the clinical presentation, diagnostic work-up, treatment response, and outcome of a patient who developed thymoma-associated anti-AMPAR encephalitis and later presented with MG. Methods: We retrospectively reviewed the medical records, imaging findings, laboratory results, and treatment course of a 49-year-old man who was evaluated at our institution. Serum and cerebrospinal fluid (CSF) autoantibodies were screened using cell-based assays and indirect immunofluorescence. Results: The patient initially presented with subacute cognitive decline, confusion, and behavioral changes. CSF analysis revealed mild lymphocytic pleocytosis. Anti-AMPAR antibodies were positive in both serum and CSF, and contrast-enhanced chest computed tomography (CT) scan showed an anterior mediastinal mass consistent with thymoma. High-dose intravenous methylprednisolone followed by tapering oral prednisone, plasma exchange (PE), and intravenous immunoglobulin (IVIG) and rituximab led to marked neurological improvement. Two months after discharge, he developed left-sided ptosis, diplopia, and dyspnea. Repetitive nerve stimulation showed a decremental response, and serology was positive for acetylcholine receptor (AChR), titin, and ryanodine receptor (RyR) antibodies, establishing the diagnosis of MG. Symptoms improved substantially with prednisone and tacrolimus, and subsequent thymectomy combined with efgartigimod further achieved sustained clinical remission. Conclusions: This case illustrates that patients with anti-AMPAR encephalitis should be closely monitored for the subsequent emergence of MG, particularly when an underlying thymoma is present. Comprehensive antibody profiling is essential for early recognition and targeted immunotherapy. Long-term immunosuppression combined with tumor-directed treatment may be required to prevent relapses of both disorders and achieve optimal clinical outcomes.