Infantile arachnoid cyst compressing the sacral nerve root associated with spina bifida and lipoma--case report.

A 2-year-old boy presented with a rare sacral arachnoid cyst manifesting as gait disturbance. Neuroimaging revealed an intradural cyst in the sacral nerve root sheath associated with spina bifida occulta and a lipoma at the same level. At surgery, the conus medullaris was situated at the L-1 level and not tethered. The highly pressurized arachnoid cyst had exposed the dural sheath of the left S-2 nerve root and compressed the adjacent nerves. An S-2 nerve root pierced through the cyst. There was no communication between the cyst and spinal arachnoid space. We thought the one-way valve mechanism had contributed to the cyst enlargement and the nerve compression. Radical resection of the cyst was not attempted. A cyst-subarachnoid shunt was placed to release the intracystic pressure. Postoperatively, his gait disturbance improved and no deterioration occurred during the 4-year follow up. Both tethered cord syndrome and sacral arachnoid cyst in the nerve root sheath should be considered in pediatric progressive gait disturbance. Cyst-subarachnoid shunt is an alternative method to cyst resection or fenestration to achieve neurological improvement.