Case Report: Efgartigimod: a beacon of hope for overlapping stiff-person syndrome and myasthenia gravis following thymoma.

The coexistence of stiff person syndrome (SPS) and myasthenia gravis (MG) is exceptionally rare, with fewer than 20 cases reported to date. Thymoma has been identified in a substantial proportion of these patients, suggesting a potential shared autoimmune mechanism. We report a 43-year-old woman who initially presented with progressive limb stiffness, painful muscle spasms, and gait impairment. She was diagnosed with SPS based on positive anti-glutamic acid decarboxylase (GAD) antibodies and characteristic electromyographic findings. Her symptoms improved after immunotherapy but relapsed following corticosteroid withdrawal. Four years later, she developed fluctuating limb weakness and ptosis and was subsequently diagnosed with anti-acetylcholine receptor antibody-positive MG associated with type B2 thymoma. Following thymectomy and postoperative radiotherapy, COVID-19 infection precipitated a myasthenic crisis requiring intensive care support. Conventional treatments, including plasma exchange, intravenous immunoglobulin, corticosteroids, and rituximab, failed to achieve satisfactory clinical improvement. In contrast, treatment with efgartigimod resulted in marked recovery of respiratory and motor function. During follow-up through December 2025, no recurrence of SPS symptoms or MG exacerbation was observed. This case highlights the complex association among SPS, MG, and thymoma and supports the possibility that thymoma may contribute to the development of multiple autoimmune neurological disorders. Furthermore, efgartigimod may represent a promising therapeutic option for refractory thymoma-associated MG in patients with overlapping SPS and MG. Long-term surveillance for thymoma and additional autoimmune manifestations should be considered in patients with SPS.