What are the meanings of hydromyelia and syringomyelia?

Spinal cord fluid. Hydromelia means an abnormal collection of fluid within the spinal cord & connected to 4th ventricle within the brain and usually associated with hydrocephalus (xs fluid within the brain). Syringomyelia is also a collection of fluid, like an elongated cyst, in the center of the spinal cord itself, but does not communicate with any other fluid filled spaces, & can also lead to loss of nerve functio.
Spinal cord cavity. Hydromyelia refers to excess spinal fluid in the central canal of the spinal cord. Syringomyelia is when the spinal fluid dissects into the spinal cord causing a cavity (syrinx). These conditions coexist in the majority of cases- syringohydromyelia. All of these terms are often used interchangeably.