According to the National Institute of Health, multiple sclerosis (MS) is an unpredictable disease of the central nervous system. Primary progressive multiple sclerosis (PPMS) is a type of MS in which symptoms get worse over time, rather than the more common pattern of relapsing-remitting multiple sclerosis (RRMS) (relapses are periods where new symptoms can appear, and previous symptoms can worsen). This means that people with PPMS will usually experience slowly worsening symptoms, with less frequent sudden relapses.
Many people who have RRMS later develop another type of MS called secondary progressive multiple sclerosis (SPMS). SPMS occurs after having RRMS when the symptoms progressively get worse but there is a reduced risk of relapses.