I was very confused with all this at the start and I’m sure I had at least one relapse which wasn’t logged as I had new symptoms which I just thought was part of getting over the last relapse. Symptoms are the residual problems you are left with after you have had relapses and they are pretty much there all the time. Like my left hand which has some numbness most of the time, mostly in the thumb and first finger. Right now my left arm is numb right to the shoulder and this is a relapse. My toes are numb/tingly most of the time but, again, my right leg is now tingly up to the knee and the calf is stiff and this is due to a relapse which has slowly progressed over the last month.
I now believe a relapse is any new symptom which lasts longer than 24 hours and which isn’t caused by something like a cold or virus which has raised your temperature as this is a psuedo relapse. I had one in October 2009 where I contacted my local nurse due to the severe burning pain in my left arm which left me unable to sleep for several nights. I asked her if it was a relapse and she said it wasn’t as it hadn’t stopped me doing anything (er, except sleeping?!). She did come out at the end of the week and prescribed amitriptyline which took a further 5 days to kick in by which time I was a complete zombie. When I reported this to my neuro he logged it as a relapse. Result - I don’t contact my local nurse any more; I go directly to the nursing team at the hospital as I don’t have much faith in the local nurse. When I’m still in my first year after diagnosis and I need to know what is a relapse and she doesn’t know, I don’t really trust her knowledge any more.
I would say if in doubt contact your nurse/neuro and get the symptoms logged if they have dragged on for a while. At the very least, they will have a more complete picture of your health the next time they see you and if they feel it is a relapse and needs intervention they will soon tell you.
To answer your question about DMDs they do, on average, reduce the rate and severity of relapses by about 1/3 in most patients. That is to say they work better for some than they do for others. Some progression of disability is caused by the lingering symptoms and stiffness left behind after relapses so by reducing the amount and severity of relapses, this could reduce some of the progression of the disability. This is why DMDs do not work for people with PPMS. I have been on Rebif since August 2009 and this is my first relapse since October 2009 so I think I have had a good run and for me it has certainly reduced my relapses. (Of course, I may have been due for a good spell anyway, I will never know but I wasn’t prepared to take the chance after two severe relapses in three months prior to starting on Rebif.)
No doubt, someone else will be able to answer your questions much more thoroughly than I have been able to.