The definition of an MS relapse (or exacerbation) is the sudden onset of new symptoms or worsened symptoms that last for more than 24 hours and is separated from a previous relapse by at least 30 days.

But the presence of worsened symptoms does not always signal a new relapse, as there are many triggers that can lead to a reappearance of old MS symptoms or bring on new symptoms that mimic a relapse.