'Benign' ovarian teratoma and N-methyl-D-aspartate receptor (NMDAR) encephalitis in a child.

N-methyl-D-aspartate receptor (NMDAR) encephalitis is a life-threatening paraneoplastic neuropsychiatric encephalitis that predominantly affects young women and has a strong association with ovarian teratomas. Removal of the ovarian teratomas improves the prognosis and decreases the risk of recurrence. We present an 11-year-old girl with NMDAR encephalitis with small bilateral teratomas not initially appreciated on abdominal CT or pelvic MRI. A 12-mm teratoma was identified in the right ovary and a 7-mm teratoma was identified in the left ovary on US follow-up at 5 months. Intraoperative sonography was used to localize the teratomas for excision. In NMDAR encephalitis, the ovarian teratomas can be very small, particularly in children, and easily missed on cross-sectional imaging. Awareness of the association of NMDAR encephalitis and ovarian teratomas will improve the diagnostic accuracy and imaging interpretation. Periodic sonography and MRI might be warranted in children if the initial study is negative.