Syrinxes usually result from lesions that partially obstruct CSF flow. At least ½ of syrinxes occur in patients with congenital abnormalities of the craniocervical junction (e.g. herniation of cerebellar tissue into the spinal canal, called Chiari malformation), brain (e.g. encephalocele), or spinal cord (e.g. myelomeningocele—see Congenital Neurologic Anomalies: Brain Anomalies). For unknown reasons, these congenital abnormalities often expand during the teen or young adult years. A syrinx can also develop in patients who have a spinal cord tumor, scarring due to previous spinal trauma, or no known predisposing factors. About 30% of people with a spinal cord tumor eventually develop a syrinx.