Brain cysts usually refer to arachnoid cysts. Arachnoid cysts are cerebrospinal fluid-filled sacs that are located between the brain or spinal cord and the arachnoid membrane. Arachnoid cysts can be classified as primary (congenital) and secondary (acquired). Primary arachnoid cysts are present at birth and are the result of developmental abnormalities in the brain and spinal cord. Secondary arachnoid cysts are not as common as primary cysts and develop as a result of meningitis, head injury, or as a complication of brain surgery.