Flare-ups happen when inflammation in your nervous system damages the layer that covers and protects nerve cells. This slows or stops nerve cell signals from getting to the parts of your body where they need to go.
If you have relapsing-remitting MS, you may have flare-ups followed by symptom-free periods called remissions. To be a true relapse, the symptom must start at least 30 days after your last flare-up and should stick around for at least 24 hours.