A device that can be placed within or near the cranial vault that fluctuates in size in response to changes in CSF pressure is disclosed. The device diminishes in size when CSF pressures rise and increases in size as CSF pressures diminish. The device has the effect of reducing the flow of CSF occurring in the foramen magnum and provides an alternative to craniovertebral decompression in Chiari I patients. The device may have applications in other neurologic illnesses associated with abnormal CSF flow, such as Idiopathic Syringomyelia, Normal Pressure Hydrocephalus, and CSF dural leaks.