I would imagine that they look at all the medical evidence supplied by your GP an MS clinic.
By “looking” I mean reading reports about your condition, I wouldn’t of thought they had the expertise or time to have someone go through your MRI scans and make a decision.
From what I can gather the decision to offer a licence by the DVLA is based on how your GP and MS clinic report back how your condition is effecting you at that time.
As for the a medical licence, it’s pretty much the norm to be put on one after after declaring a medical condition, whether that be MS, diabetes, epilepsy ect ect.
If I recall when I had a letter back from the DVLA confirming my declaration and saying they were in the progress of making a decision, the letter showed the options that were open to them, which was based on medical reports that they would get from your GP and MS clinic, and there weren’t that many options.
I think it was something like :
A medical licence ( 1, 2 or 3 year, depending on medical reports)
No licence at all.
No change in current licence.
I cant recall any other options ?
The way I see it is, as long as your GP/ MS clinic declares you fit to drive it covers the DVLA legally to offer you a licence, and thats all their interested in, covering themselves legally.