Coexisting Sacrococcygeal Teratoma With Mild Encephalitis/Encephalopathy With a Reversible Splenial Lesion: A Case Report.

Mild encephalitis/encephalopathy with a reversible splenial lesion (MERS) is a rare disease characterized by a reversible lesion in the splenium of the corpus callosum (SCC) observed on MRI. The exact etiology of MERS is unknown, although infections and antiepileptic drugs have been reported as potential causes. Herein, we present the case of a 56-year-old male patient who experienced fever and headache for 3 days. He was referred to our hospital after symptomatic treatment by his primary care physician failed to improve his symptoms. The patient had no psychiatric symptoms or significant neurological findings. Head MRI revealed a high signal on SCC on diffusion-weighted imaging, raising the suspicion of MERS. All examinations to determine the cause of MERS were negative. The patient's symptoms improved with antibiotics and B complex vitamins. Upon admission, abdominal CT incidentally revealed a well-defined mass on the dorsal surface of the rectum suspected to be a tailgut cyst, warranting surgical resection. The cranial margin of the tumor was caudal to the third sacrum, and a trans-sacral approach was used for resection. The fifth sacrum and the coccyx were resected, and the tumor was resected without damaging the rectum. A histopathological examination revealed a mature teratoma without any malignancy. A follow-up CT at four months postoperatively showed no evidence of clinical recurrence of MERS. Adult-onset MERS is relatively rare, and no association with tumors has been reported. The association between encephalitis and teratomas includes ovarian teratomas, which cause anti-N-methyl-D-aspartate receptor encephalitis and paraneoplastic limbic encephalitis. Although the cause of MERS was unknown in this case, we report the coexistence of a sacral teratoma and MERS to contribute to the knowledge of the association between them.