A 22-year-old woman with no previous medical history presented with acute anxiety, hallucinations, and a generalized tonic-clonic seizure. Cerebrospinal fluid (CSF) testing confirmed anti-NMDAR antibodies (1:512, Mayo Clinic Laboratories). Brain MRI was unremarkable, and pelvic imaging was interpreted as normal with a simple ovarian cyst. She received intravenous (IV) methylprednisolone, plasmapheresis, IVIg, and was enrolled in the ExTINGUISH clinical trial (Inebilizumab vs. Placebo, NCT04372615).

Her course was complicated by refractory seizures, dyskinesias, and paroxysmal sympathetic hyperactivity requiring tracheostomy and PEG placement despite immunotherapy. Repeat pelvic imaging was performed 5 weeks after presentation which showed a 3.3 cm thick-walled adnexal cystic lesion with a probable focus of fat. Surgical resection revealed a mature cystic teratoma with embedded hypercellular atypical neuroglial tissue. Histopathology showed frequent mitoses and diffuse GFAP positivity, while IDH1/2 and BRAF mutations were notably absent. Pathogenic mutations in TP53 (figure 1F) and ATRX were identified, consistent with WHO grade 4 glioblastoma arising within the teratoma.