I told work once it became reasonably likely I had MS - i.e. I knew for sure I had both brain lesions and spinal cord lesions.
I suppose, if I’d been lucky, it might have stayed at CIS, but it didn’t, and the rest, as they say, is history.
I think one of the reasons I told them early was to scotch any rumours it might be something else. Although I’d not been off sick, I was having quite a lot of hospital appointments, which didn’t go unnoticed, so rather than fuel speculation that: “Tina’s got cancer”, or even: “Tina’s pregnant”, I thought I’d tell them myself what the tests were about.
Whether it was due to the illness itself, or to the understandable mental turmoil I was going through, I was also aware my work wasn’t up to it’s usual standards - the volume I was getting through had fallen, and I was forgetting things, and making minor mistakes. Nothing drastic, luckily, but out-of-character for me. Looking back, I think I was just too preoccupied with the worry. I didn’t forget anything BIG - which suggests my memory worked fine when it had to - but a lot of the things that could be regarded as petty just fell off the radar altogether. You just have a completely different sense of what’s important, and what’s not.
So again, I thought it better to explain I had a major health issue, than let them think I was developing a bad attitude, or even had a drink or drugs problem.
There was absolutely no obligation to tell them about any of it - even after diagnosis. I just thought it was better for me. The only difficulty with telling them before I was diagnosed was my boss kept asking things like: “But when will you know for sure?” (can’t answer that), and: “How long before you’re in the clear?” (Had to advise him I’d never be completely in the clear, because once it’s happened once, there’s always the risk it WILL go on to become MS - which, in my case, it did).