I am sorry you are having worries about SM. You'll probably have had a lot of info on this by now from your vet and I also keep updated information on my SM Infosite (link is on my signature). Many of us here have SM cavaliers of different degrees of affectedness. Unfortunately this is a quite mysterious condition that is very poorly understood in animals and humans both. Younger onset tends to mean more severe forms of the condition but not always. Sometimes it progresses but sometimes seems to stabilise and sometimes symptoms are fairly mild. But generally, when symptoms show in a dog under three the condition does tend to progress and will need either medical or surgical management. The surgery is the only option to actually get rid of the physical problem causing the pain through even the causes and reasons for pain are not well understood. While the skull malformation is *usually* the main cause of the development of syrinxes and resulting pain, it is also known that the dynamics of the flow of the fluid that circulates the brain and down the spine -- called CSF -- is probably at least and perhaps even more important. That's why some dogs with severe syrinxes and malformation may show NO pain or symptoms, while those with mild or even no syrinxes and mild malformation may have moderate to severe symptoms.
The surgery has about an 80% success rate in halting progression but about a fourth of dogs have recurrence of the problem probably due to scar tissue formation. Because the surgery will not reverse most damage that has already occured many dogs will not be able to revert to having no symptoms and many need to remain on neurological painkillers for life. There are risks with the surgery as well but to date I have not heard of a single dog that was lost due to complications from the surgery.
Neurologists also differ greatly in recommended surgery or when to have it or even what type. I tend to follow Clare Rusbridge's advice which is to consider surgery if the dog is showing significant pain and/or limb weakness which is a sign of more serious potential damage. I treat my SM dog with medications at this time and am not considering the surgery at this time. In the US some neuros recommend surgery as the first choice option and feel the condition almost always will progress.
All that aside, as Clare R. says in her introduction sheet on SM, no one can make decisions about what is best for your own dog but the owner and many will feel surgery is not the option they wish to pursue or that medication is not the option they wish to pursue, or any range of reasons. I am just setting out the current consensus view returning from the neurologists who work directly with affected dogs and the results they are seeing back and which are reported by owners of affected dogs.
Scooting is not necessarily a symptom of anything --I find it is very, very common in the breed. I have a dog that has MRId completely clear of SM AND the malformation, in other words,a totally clear dog, as verified by two neurologists including Clare Rusbridge, and he scoots all the time even when his anal glands have been recently emptied. At the same time, affected dogs do seem to scoot frequently because they feel some pain or irritating sensation in their hindquarters. I note all this just to say that it can be quite difficult to make a diagnosis on clinical symptoms. Your vet is right to want to have you keep a diary, and check for other causes, and if all else is eliminated, then will likely send you to a neurologist for a consultation. Symptoms in a younger dog are usually something else other than SM but SM is always a possibility.
I am sure your vet will also be considering PSOM or glue ear -- this can cause some similar symptoms. Both my Leo and Jaspar MRId with this and it probably is the cause of some similar symptoms to SM in Jaspar.
If you haven't viewed the videos on my SM site, have a look at those. The one of the older dog will probably be most informative.
