I can totally understand your frustration of your doctor seemingly brushing aside your concerns, and I'm not sure that just because someone is on a drug for a certain number of years means that side effects can't show up. Especially when we're talking about a small child, as he's not the same kid he was 3 years ago, right? I mean I used to be able to drink a lot, sleep three hours, and go to work the next day, maybe a bit groggy, but still pretty chipper. If I tried that today, I'd be completely useless, feel terrible, etc. Maybe that's not the best example, but I think you get my point. Bodies do change over time, and headache and nausea are both reported side effects. Not sure if taking a BP would somehow indicate that the Fleconide, but I do think your doctor shouldn't immediately brush off that concern, without at least considering the possibility and trying to help you figure out what might be the cause otherwise because then you're basically back to square one and on your own with a new condition that needs diagnosis, right?
So I would push your doctor harder to open his mind to this possibility if that's what your suspicion is. Ask if it's really IMPOSSIBLE, absolutely unheard of, for a young child who is going through so many changes to develop some side effects to this drug. I'm not certain, but I really don't think the answer to that question is yes.
That said, of course any number of things can cause headache and nausea, so you've got to keep your mind open as well. Good luck!
Flecainide is a strong medication, and I see no reason to believe side-effects can occur simply because they didn't occur in the beginning... that's just my view. Is headache listed as a possible side-effect?
Has there been a discussion of employing ablation surgery to cure the WPW? I believe I have read on this Community many reports of WPW being cured with ablation surgery.