I don't think we know why he has it. Like you say some dogs are known to have a severe head impact or whiplash type injury which causes the condition, but we cannot think of anything that happened to Blue like this, he is with us 24/7. I think given the severity of his scan results & the fact that he has coped so well, it kind of suggests that it has been developing slowly for some time, and his nervous system has compensated. He had a bad fall which led to the scan being done, but the concensus is that the SM caused the fall, and he was exhibiting scratching & yelping symptoms about a month before the fall.
I halved his frusemide from Wednesday night as suggested by Mr S, but by last night I could see he wasn't quite as bright, he was certainly wobbling again & was catching his right paw. Interestingly his left paw is the bad one, so i can assume that the damage to the nerves supplying his left paw is over & done with. When he deteriorates its his good right leg that starts to weaken, which makes gait real difficult, so the nerves must be ok as long as the pressure is kept down. I put him back on the full tablet last night & his balance has improved, and he seems a bit brighter.
Obviously without the tablets he wouldn't be here now, and its such a fine line between him being happy & the SM becoming a problem. Its a horrid condition, you seem to have a happy spell where you can almost forget there is a problem, then something crops up and bashes you over the head!