CASPR2 antibody-positive glioblastoma-associated pseudoparaneoplastic syndrome: a case report.

Paraneoplastic neurological syndromes (PNS) are rare immune-mediated disorders typically associated with systemic malignancies, while their occurrence in primary brain tumors is exceptionally uncommon. We report a 59-year-old man who presented with acute-onset epileptic seizures and transient serum CASPR2 antibody positivity, initially suggestive of autoimmune encephalitis. Early magnetic resonance imaging (MRI) showed T2-FLAIR hyperintensity in the right temporoparietal region without mass effect; however, follow-up imaging revealed a space-occupying lesion consistent with glioblastoma (GBM). The patient experienced transient improvement following immunoglobulin and corticosteroid therapy but achieved definitive recovery only after surgical tumor resection. Histopathology confirmed IDH1 wild-type GBM (CNS WHO Grade IV), and postoperative antibody seroconversion supported a diagnosis of pseudoparaneoplastic syndrome rather than true PNS. This case underscores that primary intracranial tumors such as GBM can transiently mimic autoimmune encephalitis through tumor-associated immune activation without direct antibody pathogenicity. Clinicians should maintain a high index of suspicion for tumor-related processes in patients with neuronal antibody positivity-particularly when antibodies are low-titer, serum-restricted, and clinically discordant-and prioritize dynamic imaging surveillance alongside serial antibody monitoring to avoid diagnostic delay. Further research is warranted to elucidate the mechanisms of GBM-induced immune dysregulation and its implications for neuroimmunological disease.