Agree on this. And on
www.smcavaliers.com I have many videos of SM dogs with a range of symptoms to different degrees. Lead-scratching is quite particular in the way it looks -- and SM scratching as you can see in my Leo's video tends to be intense and it is hard to distract the dog from scratching. Several of my cavaliers face rub including the two that do/did not have SM and only very mild CM.
You'd really want to see a few suspicious, maintained behaviours over time where a vet can find no other reason/they recur without any clear reason. I'd always however keep a question in my mind of vets come back saying disk disease or allergies -- these can have very similar symptoms but I know way too many diagnoses where the true problem was untreated SM. Vets do not have the professional ability (speciality qualifications, skills and experience) to differentiate between those conditions and CM/SM which means they can be positive a symptom is something it is not, and the dog experiences inadequately treated pain/discomfort often for a very, very long time.
For
recurrent "disk" or "allergy" symptoms in this breed, I'd eventually ask for a neurologist referral.
There's also growing proof that many dogs have significant symptoms from CM alone which can be dismissed by some neurologists less familiar with SM (and many radiologists/neurologists are poor to mediocre at even reading MRIs for CM).
Neurologist Geoff Skerritt actually advises that NO dogs with even mild CM be bred, though clearly this advice is ignored by the many breeders who have him do their scans in the UK, as almost all dogs scan with some degree of CM. I understand not a single cavalier has scanned free of CM on the BVA/KC scanning programme, demonstrating what a challenge this problem is in the breed. It also has to raise serious douvbts about the readings from a couple of US neurologists who have claimed to find several CM-clear cavaliers, which their breeders then broadcast (despite being told how unlikely this would be, and not going for a second opinion reading
... caveat emptor to puppy buyers on any breeder making such claims!). Realistically, breeders have to work with dogs with some degree of CM and focus instead on syrinxes, and scan their older dogs, as the ones clear of syrinxes still after 5-6 seem (according to researchers) to have a protective gene(s) that prevents them getting SM or at least, significant SM.
In looking for a breeder, I'd be asking for whether they scan their older dogs, and the results... that to me is more valuable information almost than a given dog's scan, if it is available (unfortunately it rarely is). Rupert's Fund is of course trying to make this scanning option available for free to as many breeders as possible so they have this information for their breeding programme.