I thought about this very question long and hard before Tucker had his MRI, and I decided beforehand what I would do if he was diagnosed with SM.
I've known about SM for a very long time - since early 90's - and I think I know a good deal about symtoms. Tucker never had any of the obvious symptoms - sleeps like a rock at night and never moves (right next to me in bed), no light sensitivity or "look in the eyes" or anything obvious. I will say that there are some VERY subtle things that I could attribute to potential SM - occasional yelp or scratch - but those could be other things also such as disk disease esp. at his age.
His neurological exam was normal. His MRI showed CM, not too bad, and slight central canal dilation but no SM. His date of birth is 10/10/2002.
I had decided that if he had SM, I would start a trial course of lyrica - probably go straight for lyrica and skip gabapentin because of the 12 hour versus 8 hour dosage despite the cost difference. Then I would observe for a couple of months to see if there were changes in his demeanor or activity level. Tucker is a gentle, quiet dog with a very soft temperament so I'd watch to see if his activity level changed, etc. For the record, I think that a gentle, quiet demeanor could simply be his temperament and I'm not saying this is always a sign of SM! I might have also added a drug such as Prilosec, but I know that I would not use furosemide. I know that many use this drug, but it is not my personal choice because of what I know (through experience and years of observation on heart and kidney lists) about what the drug can do to kidney function. I personally don't agree with the monitoring recommendations for the drug on the SM protocols where it seems to be considered a benign drug, but that's just my opinion.
If Tucker were to ever develop SM symptoms in the future, I would start a trial of lyrica as above and probably get a second MRI since insurance would help pay for it.
My current second Cavalier (remember that I've had 12) is a spitfire whirlwind of activity with no SM symptoms. She is between 2 and 4. One day I will get an MRI for her, but I'd like to wait until she is older or until I see some symptoms. I would follow the same course as I planned for Tucker.