Synovial Cyst Mimicking an Intraspinal Sacral Mass

A 68-year-old female had a three-week history of severe low back pain radiating down the posterior left buttocks and left leg exacerbated by standing and walking. Lumbar spine MRI revealed cystic mass with similar intensity to cerebrospinal fluid located on dorsolateral left side of the sacral spinal canal inferior to the S1 pedicle. There was compression of left exiting S1 and traversing S2 nerve roots. Neurosurgery consult was requested to evaluate the cystic mass in the sacral spinal canal. After clinical evaluation, an unusually located synovial cyst was thought possible. Cyst contents were heterogeneous, suggestive of small hemorrhage and acute clinical history seemed reasonable. Left S1 and partial left S2 hemilaminectomy was performed and an epidural, partially hemorrhagic cyst was removed. There was no obvious connection to the ipsilateral L5-S1 facet joint. Pathology revealed synovial cyst, and the patient’s leg pain was improved postoperatively. This synovial cyst was unusual as it had no connection with the facet joint intraoperatively and its location in the sacral canal was uncommon. 1. Case Presentation 1.1. Presentation and History A 68-year-old female with a long history of intermittent back pain had a relatively acute onset of severe low back pain radiating down the posterior left buttocks and left leg for three weeks. She denied any trauma or other possible inciting factors. The pain had been getting progressively worse, and she also had developed numbness in the left S1 and S2 dermatomal distributions. An epidural steroid injection performed at a local facility provided partial relief and lasted for approximately 24 hours. The pain was described as intense, electrical pain different from any pain she had ever felt in terms of distribution and severity. Due to the exacerbating factors including standing and walking, thus, she presented to clinic in a wheelchair and preferred to lay in the fetal position. She had presented to the emergency department two days prior to neurosurgery consultation due to continued worsening of pain and numbness. 1.2. Physical Examination A thorough neurological examination was negative except for a positive straight leg raise on the left. 1.3. Initial Diagnosis and Radiography Lumbar spine X-ray did not reveal any abnormal alignment or transitional anatomy (Figure 1). Lumbar spine MRI (Figures 2 and 3) revealed a cystic mass with similar intensity to cerebrospinal fluid (T2 hyperintense/T1 hypointense) located on the dorsolateral left side of the sacral spinal canal inferior to the left S1 pedicle. It was