It seems from my limited understanding that the Relapsing Remitting phase of MS is followed by the Secondary Progressive phase. I saw one paper which said that moving to the SP phase can range from 3 - 33 years. Whereas RR seems to be driven by inflamamtion, SP is about neuro-degeneration (with inflammation having much less of a role, although some at SP still have relapses). The following article from Japanese researchers suggests how the SP phase is driven on.
PS The more of these papers I see the more confused I get. I understand T cells and B cells but the list of baddies in this paper seems endless!