Syringomyelia is caused by congenital cranial deformity or acquired injury. The former is caused by a narrow posterior cranial fossa, causing the cerebellar tonsils to herniate into the subspinal canal, resulting in poor circulation of cerebrospinal fluid and the formation of syringomyelia.
The latter are caused by trauma, inflammation, tumors, and degenerative neuropathy.
Therefore, the spinal fluid that originally protected the spinal cord flows into the spinal cord. Since the spinal cord space is occupied by spinal fluid, produces syringomyelia.