Hi Ebru: You are definitely a responsible cavalier owner and lover to make sure you know about this condition.
Even though the number of cavaliers that are affected over their lifetime is high -- probably around 70% according to studies -- most of those will not have serious symptoms and some will not have any symptoms. But almost all cavaliers have a skull too small for the brain and that is obviously not normal -- so the goal must be to reduce the risk of this widespread breed problem, which breeders can do by working with researchers, using the MRI databases, and testing and following breeding protocols.
Unfortunately no one can know what to expect with their dog over its life which makes owning and loving this breed difficult, as there are always some worries in the background. The best anyone can do is be very careful in selecting a breeder and choose one who properly MRIs and follows the recommended breeding protocols. But often we already have our cavalier or have a rescue dog and do not know the parents' SM status as the breeder didn't MRI or the breeder isn't even known. In that case, the best we can do is be smart and be aware and informed about SM.
Surgery is not the only option and many cavaliers live many years and even a full lifespan on medications, but there are no clear answers as to which works best and neurologists each have their own opinion as to what option -- surgery or medications -- is best. Some do push strongly for surgery. Surgery generally halts progression. Medications won't stop progression. But dogs progress at different rates. I have a 9 year old who had symptoms from around 2, who has always been treated with medications. He has a quite good quality of life but even at his age I always wonder if I made the right decision. For now I feel that I did. But these questions sit in the minds of all owners of SM cavaliers I think.
The best resource for information on the condition is Dr Clare Rusbridge's web materials:
http://www.veterinary-neurologist.co.uk/Syringomyelia/
She is easily the leading researcher into this condition and has been treating cavaliers with it for over a decade now. She leans towards using medications until they are no longer adequate to manage pain, before considering surgery, but I know there are some dogs where she would suggest surgery as the best approach depending on its symptoms, age and MRI result. I like her more conservative approach but other people might be more comfortable with a more aggressive approach towards going right away to surgery.
Unfortunately there are no definitive answers. But there is research, including by Dr Rusbridge, that suggests surgery gives a bit better longer term outcome for more dogs than medications, but the study sample was small and also relied on owner opinions which can be very subjective. That could work either way -- it could be that people using medications report that they see less pain than the dog would actually be experiencing -- meaning even more dogs really did better with surgery compared to medications -- or vice versa for people whose dogs had surgery, meaning owners thought their dogs with surgery were doing better than they really were. More dogs were put to sleep that had medications alone, though.