Occult teratoma in a case of N-methyl-D-aspartate receptor encephalitis.

N-methyl-D-aspartate receptor encephalitis (NMDARe) is one of 13 autoimmune-mediated encephalitides that have been discovered over the last decade. This case report describes the course of a 26-year-old female who presented with new-onset seizures and insomnia, complicated by encephalitis. The initial workup ruled out common causes of encephalitis, while a transvaginal ultrasound (TVUS), and computed tomography (CT) scans of the chest, abdomen, and pelvis did not identify a mass. Based on the suspicion that she may have autoimmune encephalitis, the patient was treated with intravenous immunoglobulins and plasma exchange, but continued to deteriorate. Whole-body positron emission tomography (PET) scan identified a small hypermetabolic pelvic mass. Shortly thereafter serum and cerebral spinal fluid NMDAR antibody titers were reported as positive, prompting repetition of the TVUS, which confirmed the presence of an ovarian teratoma. The patient had a laparoscopic oophorectomy with subsequent resolution of her symptoms, further confirming the diagnosis. Despite the sensitivities of TVUS and CT of up to 94% and 98%, respectively, the teratoma was unusually small, necessitating the addition of a PET scan to identify the lesion. These neoplasms are thought to have low uptake on PET; however, it is possible that focal inflammation may have enhanced the detection. It is unlikely that the teratoma grew during hospitalization as the average growth rate is 1.8 mm per year. Regardless, the lesson that can be learned is that imaging modalities beyond CT and TVUS, such as PET, can be helpful, as identification of a resectable tumor may alter management and ultimately improve outcomes.