Anti-N-methyl-D-Aspartate Receptor Autoimmune Encephalitis Due to Ovarian Teratoma in a 17-Year-Old Young Woman: Case Report and Review of the Literature.

Many cases of encephalitis are caused by an autoimmune response, and some are associated with underlying malignancies. Here we present the case of a 17-year-old female patient who was admitted to the pediatric ward with acute gastroenteritis, seizures, and behavioral changes. A brain computed tomography (CT) scan ruled out acute infarction, haemorrhage, mass lesions, and other intracranial injuries, while the initial EEG showed no epileptic discharges. In the absence of a definitive diagnosis, midazolam was administered to treat tonic-clonic seizures, while ceftriaxone and acyclovir were started for suspected infection. However, her condition deteriorated further and she was transferred to the ICU, where she was intubated. A Cell-Based Assay (CBA) revealed the presence of anti-NMDAR antibodies and a contrast-enhanced brain MRI showed leptomeningeal enhancement without limbic involvement. She was subsequently treated with intravenous immunoglobulins and high-dose corticosteroids. A subsequent transabdominal ultrasound scan revealed an oval, fluid-filled ovarian cyst. Based on these findings, her right ovary was removed, resulting in a gradual improvement in her condition. This case highlights that paraneoplastic autoimmune encephalitis may be misclassified due to persistent psychiatric symptoms with negative EEG and CSF findings. When clinical suspicion remains high despite negative initial instrumental and laboratory findings, it is essential to repeat these investigations. In such cases, neuroantibody testing may play a crucial role in establishing the diagnosis.