Spinal intradural arachnoid cysts are cerebrospinal fluid -filled sacs that are located between the spinal cord and the arachnoid membrane (one of the three membranes that cover the brain and spinal cord). The signs and symptoms of the condition vary based on the size and location of the cysts. Some affected people may have no suspicious symptoms while others experience progressive back and leg pain; tingling or numbness in the hands or feet; weakness of the legs; and involuntary muscle spasms (spasticity) that result in slow, stiff movements of the legs. When present, symptoms usually occur when the cysts compress the spinal cord or other nearby nerves. Spinal intradural arachnoid cysts are often present at birth and arecaused by developmental abnormalities in the spinal cord that occur during the pregnancy. They can also result from a previous infection or injury and develop later in life. Although there is disagreement in the medical community regarding when to treat spinal intradural arachnoid cysts, the need for treatment generally depends on the size and location of the cyst and whether or not it is causing symptoms. When indicated, the cysts are typically treated with surgery.