Case Report: Pembrolizumab–induced myasthenia gravis with isolated anti-titin antibody positivity after thymectomy for thymoma

Background Immune checkpoint inhibitors (ICIs), including pembrolizumab, are increasingly used in cancer treatment but can cause immune-related adverse events such as myasthenia gravis (MG). We report a case of pembrolizumab-induced myasthenia gravis characterized by isolated anti-titin antibody positivity in a patient with a history of thymoma and prior thymectomy. Case presentation A 65-year-old man presented with progressively worsening dysarthria, dysphagia, and dyspnea that developed 4 weeks after initiation of pembrolizumab. Neurological examination revealed external ophthalmoplegia and bilateral ptosis without significant limb weakness. Anti-acetylcholine receptor (AChR) antibodies and anti-muscle-specific kinase (MuSK) antibodies were negative, whereas anti-titin antibody was positive. Despite high-dose corticosteroid therapy, symptoms did not improve, but intravenous immunoglobulin treatment led to clinical improvement. Conclusion This case is notable for isolated anti-titin antibody positivity in the absence of anti-AChR and anti-MuSK antibodies, without evidence of myositis or myocarditis, a phenotype rarely reported in ICI-related MG. The onset of MG 12 years after thymectomy may reflect persistent autoimmune susceptibility unmasked by immune checkpoint inhibition. This case expands the immunological spectrum of ICI-related MG and highlights the importance of neurological vigilance and consideration of anti-striational antibody testing, including anti-titin antibody, in patients with a history of thymoma receiving ICIs.