Abstract: Background: Anti-N-methyl-D-aspartate receptor (anti-NMDA-R) and glial fibrillary acidic protein (anti-GFAP) encephalitis are autoimmune conditions that can occur concurrently, often presenting with severe neurological symptoms. Coexistence of both antibodies has been associated with greater resistance to immunotherapy and a higher risk of underlying neoplasms, particularly teratomas, which are commonly linked to anti-NMDA-R encephalitis. Detecting such tumors is crucial, though standard imaging may fail to identify them, especially in atypical locations. Case presentation: We report the case of a 32-year-old woman presenting with severe overlapping of anti-NMDA-R and anti-GFAP encephalitis. She required long-term admission to the intensive care unit and showed no improvement after receiving both first- and second-line immunotherapies. Despite unremarkable imaging studies, the lack of therapeutic response prompted further investigation for a potential hidden neoplasm. An exploratory laparoscopy was performed and revealed a fallopian tube teratoma. Following surgical removal of the tumor, the patient experienced marked clinical improvement and returned to normal daily activities within six months. Conclusions: This case underscores the importance of investigating occult neoplasms in patients with overlapping anti-NMDA-R and anti-GFAP encephalitis who do not respond to immunotherapy. The coexistence of these antibodies increases the likelihood of an underlying teratoma, which may not be detectable through conventional imaging, especially when located in atypical sites such as the fallopian tube. In such cases, additional imaging studies and early consideration of invasive diagnostic procedures, including laparoscopy, is warranted. Prompt tumor removal can significantly improve clinical outcomes, even when initial imaging is normal.

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