As a member of partial RBB - I have a class 1 medical. Speaking as a pilot - I was informed that it should not have any significant effect, providing there were no changes to it's presentation. So if it shows up on the ECG and it is assessed within limits they(the doc's) have to keep an eye on it.
I was also told that it was fairly common in fit healthy individuals. I was pretty upset to discover this and decided to stop running and get drunk more often.