Clinical and radiological features of spinal extradural arachnoid cysts: Valve-like mechanism involving the nerve root fiber as a possible cause of cyst expansion.

BACKGROUND: Although a valve-like mechanism has been proposed for expansion of spinal extradural arachnoid cysts (SEACs), the detailed mechanism remains unclear. Moreover, closure of the communication site is essential during surgery, but the method to identify the communication site remains unclear. The aim of this study was to determine the detailed mechanism of expanding SEACs through retrospective analysis of SEAC cases undergoing surgery and to elucidate the characteristics of the communication sites. METHODS: The authors retrospectively evaluated 12 patients with SEACs who underwent surgery between 2000 and 2014 and analyzed their perioperative findings. RESULTS: Dural defects were detected in 11 out of 12 patients, and a valve-like mechanism was observed in 7 patients, wherein a nerve root fiber moved back and forth through the dural defect along with the flow of cerebrospinal fluid (CSF) between the intradural space and the extradural arachnoid cysts. The dural defect was located at the thoracolumbar junction in 7 patients, below the distal end of the bridging ossification in 2, at the level of vertebral wedge deformity in 2, and at the level of disc herniation in 1. CONCLUSIONS: A valve-like mechanism was observed in 7 of the 12 patients, which suggests that it could serve as a mechanism of SEAC formation. The communication sites were variously located at the end of ossification in patients with diffuse idiopathic skeletal hyperostosis (DISH), wedge deformity of the vertebral body, or disc herniation, indicating the contribution of mechanical stress to SEAC formation.