Unless this is something totally unrelated -- I'll try to explain what might be going on. It sounds like he may have the skull malformation (occipital hypoplasia, which is a little spur of skull growing inward into the area that the brain sits) and possibly herniation of the brain into the spinal cord, but no syrinx. Sometimes the two top vertebrae are affected. It is good that he has no syrinx, which means he doesn't have SM, but if she has prescribed frusimide then it sounds like she is trying to help prevent syrinx formation (I am afraid that not having syrinxes -- eg SM -- is only part of the picture and that syrinxes can develop over time in dogs where there's a restricted flow of the CSF (cerebro-spinal fluid).)
Frusimide is a drug often given to cavaliers when they get congestive heart failure as it helps to get rid of the excess water they produce (it is a diuretic -- that is why it makes them wee a lot, but they gradually adjust. Just be sure to get him out about 60 to 90 minutes after a pill and never give one right before bedtime or when you won't be able to get him outside. They are at their maximum bladder size at about an hour ot 90 minutes after. Leo used to leak a lot at first as he wasn't used to having this extra urine production, but now he is generally happy up to 2 hours).
The frusimide is prescribed because it seems to also lower the pressure at which the CSF fluid circulates around the brain and down the spine. There's no data to support this, only anecdotal evidence. Many neurologists only started widely prescribing this in the past year or so -- up to then really only Geoff Skerritt in Wales used it. He believes that if you don;t use it a dog showing symptoms will almost certainly develop syrinxes and they do develop and worsen in dogs taking it as well, as is the case with my Leo. Recently the frusemide stopped having a helpful effect for him, and while he still takes it for whatever benefits it might provide, he is now on the neurological painkiller neurontin, which has totally stopped the scracthing sessions he was having. Clare believes the scratching is connected to pain caused by neurological damage, in turn caused by the increased pressure of the CSF.
You'll want to aks Clare to explain this more clearly so you know what this actually is, and what to be watching for in future. Did she give you a sheet that lists the terms of what was noted during the MRI?
Some background:
Many neurologists no longer refer to SM as SM but as COMS (Caudal Occipital Malformation Syndrome) as they believe in more approapriate to refer to this condition as a result of the malformation, not of the syrinxes alone. This is because the SM is caused by the malformation and probably some other factors, but doesn;t need to be present for the dog to have the same symtpoms as SM and the dog can often progress to SM -- that is what Clare is trying to help you avoid or to minimise.
Similarly in humans, some have SM and some have what is called Chiari I, or the same type of malformation. Sometimes the Chiari has SM as a complication but Chairi I has many of the same symptoms. It can help understanding of this if you see what it means in human terms. I recommend this website:
http://asap.org/
Where you can see a lik to a document explaining SM, and another explaining Chiari. As with our dogs, these are related and one can become the other.