Hydromyelia refers to an abnormal widening of the central canal of the spinal cord that creates a cavity in which cerebrospinal fluid (CSF) (commonly known as spinal fluid) can accumulate. As spinal fluid builds up, it may put abnormal pressure on the spinal cord and damage nerve cells and their connections. The term "hydromyelia" is sometimes used interchangeably with "syringomyelia," the name for a condition that also involves cavitation in the spinal cord. In hydromyelia, the cavity that forms is connected to the fourth ventricle in the brain and is almost always associated with infants and children who have hydrocephalus or birth defects such as Chiari malformation II and Dandy-Walker malformation. Syringomyelia, however, features a closed cavity and occurs primarily in adults, the majority of whom have Chiari malformation II or have experienced spinal cord trauma. Symptoms, which may occur over time, include weakness of the hands, arms, and legs and sensory loss in the neck and arms. Some individuals have severe pain in the neck and arms. Diagnosis is made by magnetic resonance imaging (MRI), which reveals abnormalities in the anatomy of the spinal cord, and electromyography, which measures the response of muscle to nerve stimulation.
This answer is based on source information from the National Institute of Neurological Disorders and Stroke.