Demonstration of Ball Valve Mechanism of Spinal Arachnoid Cyst Expansion Using Intraoperative Ultrasonography: 2-Dimensional Operative Video

Intradural spinal arachnoid cysts are an uncommon cause of spinal cord compression. The most common location of these cysts is within the dorsal thoracic region, but they are found throughout the spine.(1)Patients most commonly present with the symptoms of progressive myelopathy. The pathogenesis of these cysts has not been determined, but some authors theorize that the cysts arise within diverticula of the septum posticum, which is an arachnoid septum between the posterior dura and the dorsal spinal cord.(2)The predominant theory as to the cause of cyst expansion overtime is through a ball valve mechanism at a communication between the cyst and the subarachnoid space. This mechanism allows for progressive fluid entry into the cyst while also preventing fluid outflow.3However, adequate visual documentation of this phenomenon is lacking in the literature. In addition, intraoperative techniques for determining the exact location of this communication are not well described. We present a case of a 68-year-old woman with progressive myelopathy due to a thoracic intradural spinal arachnoidcyst. Intraoperative ultrasonography was used to clearly demonstrate a ball valve mechanism of pulsatile flow within the subarachnoid space, exhibiting the combined dynamic and static compressive components of these lesions. Post-resection ultrasound was also obtained, which demonstrates physiologic flow of spinal fluid. Operative technique for cyst removal is also highlighted. The patient consented to the procedure.