The term can refer to dilatation of the persistent central canal of the spinal cord which communicates with the fourth ventricle (cavity wall is lined by ependyma).
It is similar to but quite distinct from syringomyelia where there is dissection through the ependymal lining of the central canal.
It is very difficult to distinguish hydromyelia from syringomyelia and hence the collective terms hydrosyringomyelia or simply "syrinx" are sometimes used to refer to a fluid collection within the cord.
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