LAUSR

© BMJ Publishing Group Limited 2022. No commercial reuse. See rights and permissions. Published by BMJ. DESCRIPTION A woman in her late 30 s complained of leg paraesthesias more prominent on the left, mild gait imbalance and bilateral leg spasms for the last 2 years. The symptoms aggravated during the prior month, and she was consulted at the emergency department. She denied urinary or faecal incontinence. The physical examination revealed numbness in the left leg and mild gait imbalance but no leg weakness. A thoracolumbar spine MRI with contrast showed a large nonenhancing cyst at the level of T11 with local compression of the conus medullaris (figure 1). Because of the progressive symptoms, the patient was taken to the operating room to drain the cyst. Under intraoperative neuromonitoring, a T11 bilateral laminectomy with a midline durotomy was completed with exposure of a superodorsally displaced conus medullaris due to a tense intramedullary cyst (figure 2). The cyst was fenestrated with a small midline myelotomy achieving adequate cyst drainage, decompression of the conus medullaris, and improved craniocaudal cerebrospinal fluid flow. A cystosubarachnoid shunt using a 4 cm segment of an MRI compatible epidural catheter was introduced into the cyst cavity to minimise the probability of cyst recurrence. It was sutured to the overlying arachnoid layer with a prolene 6–0 suture. No changes in neuromonitoring were observed during the procedure. No cyst wall tissue was obtained to avoid an additional neurological deficit. In the early postoperative period, she showed improvement in leg spasms with fewer paraesthesias. Three months later, she continued improving her gait with fewer leg spasms. Although the MRI suggested that the cyst could be intramedullary, it was not possible to reach a specific diagnosis between an intramedullary arachnoid cyst (AC) versus cystic dilation of the ventriculus terminalis (VT). These two lesions are rare cystic lesions encountered in the conus medullaris region. 2 The VT and intramedullary AC appear identically in the MRI, showing a hypointense signal in T1weighted sequences and hyperintense in T2weighted sequences with fluid characteristics similar to cerebrospinal fluid without cyst wall enhancement. A histopathological examination is the only way to differentiate them. The cyst wall of the AC is composed of arachnoid cells, and the cyst wall of the VT is composed of ependymal cells. Although imaging does not provide a specific preoperative diagnosis, other characteristics may support one diagnosis over another. Studies have reported a more eccentrical location for intramedullary AC than a midline location for VT. 8 The management