Syringomyelia, commonly known as SM, can be confirmed by MRI. The current issue is that neurologists have not yet come to a clear definition of the term 'clear' assigned to certain cavaliers, given that most cavaliers have some degree of skull malformation. According to The Healthy Cavalier website, "The late onset of clinical signs and the number of asymptomatic dogs adds to the complexity of the condition. Not enough is known about long term progression to ascertain the optimum age young dogs should be screened for the disease."
Research is not static when it comes to SM. The condition, as well as the veterinary medical health trials, demand evolution with a goal of achieving accurate and consistent results that will eventually lead to a universal testing protocol for breeding. As of today, there are different protocols recommended in the UK, Switzerland, the Netherlands and more.
Recent studies suggests that in the vast majority of cases the syrinx starts in the upper cervical spinal cord so if this is included then scanning of the entire cord (more expensive) may not ultimately be necessary. Any 'normal' dog without the occipital malformation which makes the skull small has a genetic advantage and should be used for breeding. The following breeding recommendations are made using current information and in response to breeder requests for guidelines. It has yet to be proven if this guide is appropriate. The aim of these recommendations is to reduce the incidence of symptomatic syringomyelia in the breed not to create litters of puppies guaranteed not to have SM as the chance of producing an affected dog cannot be predicted without knowing the inheritance. It is recommended that the offspring of any mating is also MRI screened before breeding. As the incidence of syringomyelia is so high in the breed there will be severe depletion of the gene pool if only clear dogs are used (i.e. other problems will develop). Therefore until the genetic defect is determined it is recommended that dogs with syringomyelia be used if they are valuable in another genetic sense e.g. good heart. The general principle of these guidelines is that dogs with code A are more desirable to use than B, etc but that dogs with a higher letter code may still be used in some limited circumstance.
For extensive information, you can follow this link to a Cavalier Health website I prefer to access: http://www.cavalierhealth.org/smprotocol.htm#Rusbridge_Syringomyelia