There's a lot of information on SM at Rod's site,
www.cavalierheath.org, and I also have info at
www.smcavalier.com; also see
www.cavaliermatters.org and
www.cavaliercampaign.com. Rates of SM appear to run to at least 60% of the breed (I have seen a formal clinical group of asymptomatc dogs that shows it is actually higher -- but the data has not yet been published). That would mean the rate of affectedness is considerably higher as this very large sample were only asymptomatic dogs who on a neurologist's professional exam, showed no symptoms, and ranged in age from puppies to age 10+. So I think estimates will be revised upwards by quite a bit and may well be closer (actually probably higher) to the estimate of 70% or more eventually affected with SM, not just CM, made many years ago by Dr Clare Rusbridge (and much criticised at the time by some).
Of those a good portion will not have symptoms at least until later in life, as the large clinical sample shows. But every study points to MOST of us having a dog with SM, asymptomatic or not. The more serious cases tend to show symptoms between 6 months and four years, with the most severe tending to show symptoms before age 1.
There's a lot more info on the links above. Yes it can be manageable through either surgery or medications but it does tend to be progressive with a considerable number needing to be euthenised for pain eventually. I have two and probably three of my five with some degree of SM -- one is serious, one is mild, (those both scanned and verified cases) and a third dog is also fairly mild but her scratching would suggest she has SM as well (if you own SM dogs you can pretty well iscern the difference between normal scratching and SM scratching). Two of mine scanned clear, one at almost 10, one at 1 and again at nearly 6. If clear at 5 or 6 the dog is considered unlikely to ever have a syrinx or probably only a very tiny one.
Despite the seriousness of these health problems many of us love this breed, will always wish to own at least one cavalier, and wish to see the breed survive -- and for these problems to be minimised. That is why it is really important to only use a breeder who definitely breeds for health -- all will say they do but far fewer actually do the proper heart testing via a cardiologist, follow the MVD protocol (as opposed to saying they do) and also make public results after age 5 unless they are good... and very few in N America are MRI scanning for SM. Cost is a factor but in essence they are breeding blind for a serious health issue and I would not use a breeder who isn't scanning as well. Pet buyers will be the single greatest influence on whether this breed survives and these health problems begin to come under control and dogs lead less painful lives -- because we hold the purse strings. If puppy buyers only work with truly health-focused, scanning and testing breeders, that will become the norm in breeding practice. So use your power to select a breeder carefully.