The purpose of this study is to establish the mechanism of development and progression of syringomyelia. Although syringomyelia usually accompanies anatomic abnormalities at the craniocervical junction, the pathophysiology that relates these anatomic abnormalities to syringomyelia development and progression is controversial. We have been testing the hypothesis that progression of syringomyelia associated with the Chiari I malformation is produced by the cerebellar tonsils partially occluding the subarachnoid space at the foramen magnum and acting as a piston on the partially enclosed spinal subarachnoid space, creating enlarged cervical subarachnoid pressure waves which compress the spinal cord from without, not from within, and propagate syrinx fluid caudally with each heartbeat, which leads to syrinx progression. We are also testing the hypothesis that development of syringomyelia results from increased transit of CSF through the spinal cord parenchyma and into the syrinx. Patients are treated with posterior fossa craniectomy, upper cervical laminectomy, and duroplasty. We evaluate cerebrospinal fluid flow and pressure, syrinx size, neurologic function, and the rate of entrance of CSF into the syrinx before and after surgery. These studies and the intraoperative evaluation of the movement of the cerebellar tonsils and the walls of the syrinx are providing data which elucidate the hydrodynamic mechanism of development and progression of syringomyelia.