Unfortunately both this and MVD are considerable risks when you decide to own a cavalier. But consider that your dog is far more likely to have health problems from MVD, including early-onset MVD (eg onset while the dog is still relatively young, meaning under 10 for this disease) than SM. Most of our cavaliers will ultimately die from MVD, and the high rate in the breed has wiped about a fourth to a third off the average lifespan. So MVD is sadly the key problem we still all need to watch for.
SM like MVD seems to be fairly common in the breed especially in comparison to other breeds, and also in varying degrees of seriousness. For those dogs that do end up with syrinxes, which appears to mean at least a third, about a third again seem to develop symptoms that are noticeable and of those with symptoms fortunately a smaller number experience serious pain.
None of that makes things any easier but it may ease concerns a bit -- while reieterating how absolutely important it is to make careful choices when buying puppies and to learn what to watch for with this condition.
There is a whole list of things to watch for, and the other things to check for FIRST that are more likely to be causing symptoms,
here.
A dog chewing at its paws and scratching sounds likely to have allergies. It is something to definitely discuss with your vet. Face rubbing after meals or water is totally normal -- both my clear dogs do this all the time. Please read through the link above and also the symptoms list on the site because many possible symptoms are just normal dog behaviour and you need to really have a sense of which things are normal. Face rubbing from SM looks like a response to pain, not a moment of relaxing pleasure which is more likely and is totally NORMAL in all dogs. Personally I do not consider face rubbing to be a major sign myself and my two dogs with SM almost never face rub. Lucy who is clear does it all the time!
Air scratching is more serious as it is connected with almost no other activity. However a dog might kind of air scratch if it begins to scratch then gets distracted. That is very different from dogs that start to bunny hop as they try to air scratch. If a dog is SM air scratching, it will keep appearing and most likely, start to happen more often. If someone is seeing regular air scratching they really must see a vet to discuss the possibility of SM.
Please remember too that plenty of cavaliers will live a full lifespan with SM and mild to moderate symptoms that can be alleviated through a few medications. A good number of cavaliers with SM will die from MVD, not SM -- again, every cavalier on this board is far more likely to die from heart problems, not SM.
Please never ignore symptoms of either MVD or SM if you truly think there's a concern as there's no reason a dog should suffer and remain in discomfort or pain. But also don't waste time worrying about what hasn't happened.
Both my dogs with SM lead perfectly happy lives right now and I see no point in becoming overly upset at what they have. Instead I care for them and focus my energies on trying to ensure that as few cavaliers as possible in the future will suffer from this. That's a better place to direct energies. Donate to the research, write to the clubs expressing concern about these issues, join the UK CKCS club so you have a vote at the next AGMs, require that the breeder you have chosen be an MRI scanning, cardiologist testing breeder. Contact Carol Fowler on this board by PM to ask ways that you can help the breed. There are many ways to help.