The back half of the cavalier King Charles spaniel's skull typically may be too small to accommodate all of the brain's cerebellum, which may also be too large, and so it squeezes through the foramen magnum - the hole at the back of the skull - partially blocking the flow of cerebrospinal fluid (CSF) down the spinal cord. The variable pressure created by the abnormal flow of CSF is believed to create the SM cavities - called syrinx - in the spinal cord.

SM is rare in most breeds but has become very widespread in cavalier King Charles spaniels. The number of diagnosed cases in cavaliers has increased dramatically since 2000. Researchers estimate that up to 95% of CKCSs have Chiari-like malformation (CM or CLM) - also known as caudal occipital malformation syndrome (COMS) or occipital hypoplasia (OH), the skull bone malformation present in all cases and believed to be at least part of the cause of syringomyelia - and that more than 50% of cavaliers have SM. The severity and extent of syringomyelia also appear to get worse in each succeeding generation of cavaliers. It is worldwide in scope and not limited to any country, breeding line, or kennel, and experts report that it is believed to be inherited in the cavalier.