I agree. You wouldn't say "Oh, by the way - I have chronic kidney stones. Just thought I should tell you before they flare up."

I have been in the situation before where I ended up telling my employer because my work was being affected. I was lucky that I had a laid-back boss and we talked about whether there was anything he could do to help (yes, flex-time) and I agreed to keep him updated. It was a situation that lasted for a few months. It can be risky to tell your boss. Always a tough call.