465 - Development of lumbar arachnoid cyst with neurologic findings following decompressive suboccipital craniectomy and C1 laminectomy, duraplasty for Chiari I malformation

Case Diagnosis: Lumbar arachnoid cyst following suboccipital craniectomy, C1 laminectomy and duraplasty for decompression of Chiari I malformation

Case Description: 37-year-old female with long-standing history of headaches presented to acute rehabilitation unit following elective decompressive surgery for Chiari I malformation. On initial assessment, patient was noted to have profound 2/5 weakness in right hip and knee flexion, and 3/5 strength in right dorsiflexion/plantarflexion. Non-contrast MRI L-spine performed post-operatively prior to rehab admission demonstrated anterior displacement with draping and bowing of the cauda equina nerve roots from L3-S1 possibly representing an arachnoid cyst. During rehabilitation, patient made significant improvements in strength and mobility, from ambulating 8 feet with moderate assistance utilizing an unsteady gait to ambulating 200 feet independently without assistive device. Six months later, patient was seen in neurosurgery clinic. On manual muscle testing, patient had 5/5 strength in the right lower extremity. Repeat MRI L-spine noted resolution of anterior displacement of cauda equina in the lower lumbar spine.

Discussions: This is the first known instance of lumbosacral arachnoid cyst following decompressive Chiari surgery presenting with right lower extremity weakness. Arachnoid cysts are cerebrospinal fluid collections occurring within protrusions of arachnoid mater. Intradural arachnoid cysts in the lumbosacral region are quite rare. The etiology is not well established, but changes within the arachnoid trabeculae overlying the spinal cord are probably the root cause, which could be congenital or caused by inflammation from previous trauma or surgery. This may be treated surgically, or in this case conservatively with aggressive therapy. The mechanism of spontaneous resolution of the cyst in this case is unclear, but overall this patient improved tremendously with conservative management.

Conclusions: Arachnoid cyst is an extremely rare complication of Chiari decompressive surgery that can present with neurologic symptoms. Although surgery is a primary option for symptomatic cysts, conservative management with therapy can also be beneficial.