Intradural cyst: intramedullary or extramedullary? Illustrative case

BACKGROUND: Intradural spinal cord cysts are uncommon and generally benign. It can be difficult to determine whether the cyst is intramedullary or extramedullary on preoperative imaging, and the location of the cyst may be determined intraoperatively. This patient presented with intractable back pain associated with imbalance and was found to have a cystic lesion of the ventriculus terminalis (VT). OBSERVATIONS: The patient was found to have a cystic lesion of the VT that was intramedullary rather than extramedullary, as initially expected based on preoperative imaging. LESSONS: VT is a rare cystic expansion of the conus medullaris that can appear extramedullary on preoperative imaging. Intraoperatively, this lesion was found to be intramedullary and was successfully treated with fenestration.