The Cavalier King Charles Spaniel is a small spaniel classed as a toy dog by The Kennel Club and the American Kennel Club, that originated in the United Kingdom. The two breeds share similar history and only diverged from each other about 100 years ago. There are several major differences between the two breeds, with the primary difference being size. This blocks the flow of cerebral spinal fluid (CSF) around the brain and spine and increases the fluid's pressure, creating turbulence which in turn is believed to create fluid pockets, or syrinxes (hence the term syringomyelia), in the spinal cord. Syringomyelia is rare in most breeds but has become widespread in the Cavalier King Charles Spaniel, with international research samples in the past few years consistently showing over 90% of Cavaliers have the malformation, and that between 30–70% have syrinxes. However, most dogs with syrinxes are not symptomatic.