Hydromyelia refers to an abnormal widening of the central canal of the spinal cord that creates a cavity in which cerebrospinal fluid (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.  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 in infants and children with hydrocephalus or birth defects such as Chiari Malformation II and Dandy-Walker syndrome.  Syringomyelia, however, features a closed cavity and occurs primarily in adults, the majority of whom have Chiari Malformation type 1 or have experienced spinal cord trauma.   Symptoms, which may occur over time, include weakness of the hands and arms, stiffness in the 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..