Cerebrospinal fluid
A number of medical conditions can cause an obstruction in the normal flow of cerebrospinal fluid, redirecting it into the spinal cord itself. For reasons that are only now becoming clear, this results in syrinx formation. Cerebrospinal fluid fills the syrinx. Pressure differences along the spine cause the fluid to move within the cyst. Physicians believe that it is this continual movement of fluid that results in cyst growth and further damage to the spinal cord.