I'm pretty sure the words "exacerbation" and "flare" can be used interchangeably. I tend to do that, altho lately I've preferred the word "flare" because it's easier to say when you're kvetching (complaining) to your neuro about any new symptoms. (I've done quite a bit of kvetching to my neuro over the last 3 months...stupid MS is picking on me!)

I googled this since I dont think my MS brain will be able to type out a coherent explanation tonight (like I said, the MS is picking on me). I got the info from About.com's Multiple Sclerosis site

"The Official Definition of a Relapse
The official international definition of a relapse is “a period in which a person with MS experiences an acute worsening of function that lasts for at least 24 hours, usually lasting for several days or weeks, followed by an improvement that lasts for at least one month.” To get even more specific, the symptoms associated with a relapse will develop over a couple of days, remain the same for three to four weeks, then slowly get better over a period of about a month. "

Also from that same page

"You can suspect a relapse if you answer “yes” to the following questions:

* Am I experiencing new symptoms or worsening of existing symptoms?
* Has this worsening happened over the course of 24 hours to a couple of days?
* Have these symptoms lasted more than 24 hours?
* Has it been at least a month since my last relapse? (In other words, had these symptoms been non-existent or stable for at least 30 days before they appeared or got worse?)
* Am I free of fever or infection?"

linkage back to the page if you want to read more http://ms.about.com/od/multiplescler...pse_or_not.htm

and here's a page that explain the pseudo-exacerbation
http://ms.about.com/od/signssymptoms...intoleranc.htm