wolff-parkinson-white syndrome

my 19 year old daugther was just diagnosis with this, its weird because when she was 7 she had the same symptoms and they could not find anything, and las year to the day she was in the E.R. for the same thing and they found nothing, so this is the first time that they were able to catch it while it was going on, you know how you go to the doctor and when you get there all the symptoms stop... why is that