Watson is doing so much better!
First off, in my follow-up visit (armed with questions) with neurologist Dr. Sammut, she said his syrinx was very small about 1 - 1.5mm (I think it was mm's, anyway). She seriously doubted that anything other than his discs were bothering him at this time and suspected we would not have found the syrinx if it weren't for the discs problem. At least not at this time. She said the lasix was a very small dose and strictly a preventive/can't hurt thing. And that she would never recommend surgery in a case like his (size of malformation/syrinx; has never shown SM-type symptoms). As for the future, who knows. As she wisely pointed out, anything at all can happen tomorrow, to any of us, canine or human.
Meanwhile Watson's no longer exhibiting any back pain whatsoever, and hasn't for some time, but is still (much to his chagrin) on rest, brief walks, and no stairs or play. He's also now off Tramadol, and only taking half the Rimadyl he was when exhibiting pain. I suspect we'll take him off that as well soon.
I guess it's good to know the malformation/syrinx is there and what to look for should symptoms ever arise. I'll always be careful not to allow any pressure on his neck, etc.
I thank you all for your support during a difficult time, wish you all the best with your beloved dogs, and hope to see you in the other forums!