""Non-communicating'' syringomyelia: a non-existent entity.

Encroachment by man's overlarge forebrain is responsible for the anatomic substrate of syringomyelia, i.e. a hindbrain hernia developing in fetal life with persisting hydromyelia. Communication between the syrinx and 4th ventricle is readily disclosed at operation, but because of postmortem shrinkage, almost never at autopsy. Syringomyelia developing in 16 of 864 post-traumatic paraplegics has been classified non-communicating. However, the syrinx, its fluid, and the ultimate clinical picture are the same as in the non-traumatic, and surgical exposure has disclosed the communication. The symptoms develop because the traumatic subarachnoid block exaggerates the causative intracranial fluid pulse waves by eliminating the dampling effect of the yielding dural sac below. A non-traumatic spinal block also may result in syringomyelia.