Hmmmm. I live in the United States, and as such, I do not have to state any medical condition on application for employment.
Yes, a lot of states have at-will statutes, however, it is illegal for an employer in an at-will state to terminate an individual's employment because they seize at work, or it comes out that they have a seizure disorder. (yes, the employer can fire you for other reasons, but it can come down to how it looks. if you have a seizure at work and the next day you are fired for failing to meet corporate expectations, that is a very bad move on the employer's part as that puts a target on their back for a discrimination lawsuit,.)
Here in the states, an employer can be forced into going as far as providing a place for you to go to rest after a seizure, or if you feel like you may have one coming on. The American's With Disabilities Act of 1990 has an entire section on epileptics. The section even states that an epileptic would not be prohibited from working as a police officer simply because of their condition.
I can't rightly make any suggestions on this matter to anyone who does not live in the states, however, I will tell you how I have handled this.
I never tell my employer before I am hired or before the 90 day probationary hire period ends and I am considered a full time permanent employee. And then I have only disclosed the information after I have a reason to. Just because I have a doc appt, isn't a reason. If I have a grand mal and will be in the hospital for 72hrs for monitoring, then I will inform. However, if you are in the states and in a situation like that, you can simply ask a doctor for a note stating you will be gone or need to be excused for X dates, and that should be sufficient.
Employers do have to abide by the Privacy Act of 1974, doctors and other health officials have to follow HIPPA guidelines, and then of course there is the ADA of 1990.