I joined the team i'm on at work in March 2007, just before "my episodes" started happening. By the time they were in full force, i decided that i had to tell my manager something; I couldn't keep this from her even though we are a virtual team and she was in Arizona at the time. I told her my "little tale of woe" and she was most understanding and, in fact, it was her encouragement that eventually led me to the discovery of the Multiple Sclerosis Center of Atlanta.
I've spoken to other people who've hidden their medical issues from their managers - I'm not a believer in that. I feel as though, at the minimum, your immediate manager should know what's going on. Now, realistically speaking, not everyone may be as understanding as my manager, but i do think that it's important that they be kept in the loop because u just never know what might happen to you and what u might need from them 1 day.
I am LUCKY! When i joined the team i was working from home 2 days a week, now i work from home permanently. Not having to fight with getting ready to go to an office, fighting traffic (MS or no MS, let's keep it real), having to explain myself to people when things are a little off and a HUGE fear that i have (story for a later posting) are small things that I am grateful for. I am also grateful for an understanding, supportive manager with whom i can be totally honest every step of the way.