- In 85% of cases, the disease initially evolves through relapses that either resolve completely or lead to sequelae in a few weeks or months (pure remitting phase or remitting phase with sequelae)
- However, in approximately 50% of cases, patients who are initially presenting with a remitting course subsequently evolve towards a progressive form (secondary progressive form)
- In 15% of patients, the disease strikes with an immediate progressive setting without relapses (primary progressive form)
In patients presenting regressive attacks, remissions are less complete over time, resulting in functional sequelae, including loss of walking ability occurring on an average 20 years after the beginning of the disease.
Thus, MS can have three evolutive modes:
- Relapsing-remitting form: exacerbations alternating with remissions during which partial or total recovery is observed. The remissions can last months or years. The exacerbations can occur spontaneously or be triggered by certain external factors, such as an infection, post-partum or certain vaccinations.
- Primary progressive form: the disease course is progressive without remissions, and may include occasional plateaus during which progression stops.
- Secondary progressive form: this form follows on to a remitting form and begins with attacks alternating with remissions, followed by a gradual progression of the disease with or without superimposed relapses.
While immunosuppressive or immunomodulatory treatments aiming at inhibiting the inflammatory reaction are effective in decreasing the number or the duration of the active lesions, they have very little effect on long-term disability and little or no effect in the progressive (primary or secondary) forms of the disease.