Secondary-progressive MS (SPMS) often occurs as a second phase of the disease, in people who initially experience a relapsing-remitting disease course. The disease gradually changes from the inflammatory process seen in RRMS to a more steadily progressive phase characterized by nerve damage or loss. During the more steadily progressive phase, a person may or may not continue to experience relapses (also called attacks or exacerbations) caused by inflammation.
In progressive-relapsing MS (PRMS), a person experiences both a steady worsening of neurologic function from the onset and occasional relapses. SPMS, which occurs in the majority of people who start with a relapsing-remitting course, is much more common than PRMS, which occurs in only 5% of people with MS.