Anti-<i>N</i>-methyl-d-aspartate Receptor Encephalitis Caused by a Teratoma Arising From the Retroperitoneum

Anti-N-methyl-d-aspartate receptor (NMDAR) encephalitis is an autoimmune encephalitis, and 46% of women with anti-NMDAR encephalitis have tumors, of which 94% are ovarian teratomas. Patients with anti-NMDAR encephalitis rarely have extra-ovarian teratomas, particularly extra-ovarian mature cystic teratomas arising from the retroperitoneum. We herein report a 27-year-old woman who was diagnosed with anti-NMDAR encephalitis caused by a teratoma arising from the retroperitoneum. In patients with anti-NMDAR encephalitis, it is important to perform a systemic search, including for extra-ovarian teratomas. Plasma exchange significantly improved the patient's clinical symptoms.