Nowadays the term ‘Advanced’ MS is generally used for people who’ve had MS for a number of years (or occasionally not long at all), and are either progressive and quite disabled, or relapsing remitting and quite disabled. It doesn’t mean the same as ‘end stage’ MS.
Advanced seems more appropriate for many people than the more usual labels for MS, me for example. I’ve had MS for 22 years, and was RR for most of that time. While the first 10 or so years were fairly uneventful, and over the next five I was probably just getting gradually a bit more disabled, soon enough, the fact that I’ve been unable to take most disease modifying drugs (not available at first and side effects took care of most of the rest) took its toll. Since a huge relapse in 2012, I’ve been more or less unable to walk. I can manage a couple of metres with FES or an orthotic and a walker, but most of my waking life is spent sat down, either in a wheelchair or otherwise. I was redesignated secondary progressive about 3 years ago and thought that was that, until 2 relapses last year means I am now relapsing progressive. The term Advanced seems very appropriate for someone like me, I’m fed up with all the label changing. I may be able to take a DMD now, but strictly speaking I don’t qualify for them. I’m still waiting for the final decision.
Your question about ‘end stage’ on the other hand is different. For years we were always told ‘MS will not kill you, you won’t die of MS, but with MS’, ie, something else will get you first. And I believe this is still true for most of us. But some people do in fact die of MS. If they didn’t there wouldn’t be a level 10 on the Expanded Disability Status Scale (EDSS), which states ‘Death due to MS’, see https://www.mstrust.org.uk/a-z/expanded-disability-status-scale-edss
It’s a bloody sad fact that some people do die horrible, sad, personally degrading deaths because of MS. As for how many, I don’t know if that figure is known. I personally do not intend to be among their number. I have an Advanced Statement (aka Living Will), so hope that I won’t accidentally end up on any kind of life support. I also plan on some form of assisted death when I get to the point that it seems a better option than living through the ‘end stage’. Hopefully by the time I get there, it will be legal in this country, otherwise it means travelling abroad. This may sound a bit negative, or even depressing. I don’t, I am glad I’ve already got a plan, and have shared that with my family and friends.