Oh, what a shock - that does seem an amazingly quick diagnosis compared to the majority.
A call from a GP does NOT count as an official diagnosis of MS, because (1) They’re not allowed to diagnose, and (2) NICE guidelines say an MS diagnosis should always be given face-to-face (although I appreciate the latter is sometimes shamefully ignored).
If there is talk of treatment, then I assume they must be sure - as it seems premature if not - but your route to diagnosis is still unusual and leaves questions, so I think that is something to clear up when you see the neurologist, i.e. “How can you be sure this is not an isolated incident?” and: “Why don’t I need the usual two confirmed episodes, or two years’ continuous progression?”
I don’t want to give you false hope, but it is possible this is all your GP jumping the gun, and that when you get there, you will be told it’s not quite as clearcut as all that, and that they want to do more tests.
Have you ever had any blood tests in relation to this, for example?
Even if you have confirmed brain lesions, MS is a diagnosis of exclusion, as there are other conditions that can occasionally cause lesions, so your blood should have been tested for a whole host of other suspects before confirming it’s MS - let alone starting you on treatment.
If you’ve never even had a blood test, it seems most unlikely they would proceed straight to the treatment stage without wanting more checks.
Brain lesions are definitely something, so I’m not trying to fob you off with: “Don’t worry, it’s probably nothing”. But your diagnosis does seem irregular, to say the least, so I think you’re quite justified in wondering just how certain they can be. At the appointment, you need to speak openly about your concerns, and clear all this up.
On the positive side, if it is MS, it’s good to spot it early, and to be offered treatment early. Looking back, I now think I had MS at least 6-10 years before diagnosis, and when I really think hard, it could be more like 20 years - my neurologist agrees! That was a factor in my declining treatment, as the whole notion of “catching it early” had flown out the window with me. But in your case, you’ve still got the “catch it early” opportunity - if it really is MS, that is.