From a pwMS point of view, PPMS and RRMS differ dramatically. The severity of RRMS waxes and wanes, as symptomatic periods alternate with stretches in which the illness appears to fade or even disappear. In contrast, people with PPMS, who account for about 15% of cases, can expect their condition to continually deteriorate.
PPMS and RRMS also diverge in their demographics and response to drugs. When RRMS patients are diagnosed, they are usually in their 20s or 30s, whereas PPMS patients are around 10 years older. Although RRMS is two to three times more common among women than among men, the gender ratio for PPMS is close to 50:50. When researchers have tested anti-inflammatory and immune-modulating drugs that are beneficial for RRMS they’ve detected little effect on PPMS
Despite these disparities, in some ways PPMS and RRMS are indistinguishable. For example, studies have reported that they overlap in characteristics such as the amount of demyelination and remyelination, the number of cortical lesions, and the extent of acute axon damage
Researchers haven’t been able to pinpoint any imaging finding or pathological change that distinguishes PPMS from RRMS. Under the microscope or in MRI, they look the same. The convergence between the two MS forms also extends to susceptibility genes.
The 1.5-tesla and 3-T MRI scanners used in hospitals and most studies can’t reveal as much detail as ultrahigh-field 7-T MRI scanners, where it was concluded that PPMS and RRMS belong to the same disease spectrum.