SM is the most horrible condition ever. I have the greatest admiration how you are managing with your badly affected young dogs.
Here in the UK we are not put under the same pressure to agree to surgery, in fact it is the less common option, and most Neurologists will work to help owners that choose not to go that route
The problem with medicating for SM is that different drugs or combinations help some dogs, while causing overwhelming drowsiness, or not easing the symptoms in any way, for others. It really is such a case of trial and error and you do have to start again every time your cavalier's condition deteriorates.
I found out Tommy had SM in 2005 when he had a low cost breeders scan, but his symptoms were so mild, an occasional yelp for no reason once every few months and a slight reluctance to have his ears brushed, that he needed no medication.
I did ask my vet to prescribe a small daily dose of frusemide, more as a preventitive than anything else, and that was all he had for years.
Tommy has never really scratched, except for a one-off episode in my vet's waiting room when a frantic bout covered the floor in piles of drifting fur, but gradually the yelping started when he was picked up, and then the constant pawing at my legs demanding attention. We started the rimadyl, metacam, gabapentin juggling act.
Tommy started on 100 mg x 3 and that held things for quite some time. He went on to 200mg x 3 gabapentin when the lower dose stopped working. That again gave some respite but he gradually started being very restless and became badly affected by muscle twitching in all different parts of his body (myclonic jerking is apparently not proven to be associated with SM, but it is a neurological problem often seen in Cavaliers )
Tommy was switched to Lyrica and I have found 75mg x 2 has really kept him comfortable and happy when combined with a daily dose of metacam ( I have always found that Metacam seems to increase the effectiveness of whatever medication Tom is taking ) He still has 10mg x 2 frusemide daily.
The twitching and jerking in his body & head is so much better, it is only his left back leg muscle that seems to still be in perpetual motion now.
Unless they find yet another new miracle drug ( that is what Lyrica has been for Tommy ) then the only next option will be steroids. Not what I would want for him, but I will do whatever is necessary to keep him comfortable and let him go when that proves to be no longer possible.
Tommy is eleven now so his MVD ( bad murmur but symptoms as yet ) may be what eventually forces my hand.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi