Epilepsy is one of the faults that affect marriage and prevent some of its aims being fulfilled. If one of the spouses has this problem and conceals it, and the other both does not know of it until after the marriage has been consummated, then he or she has the choice of either continuing with the marriage or annulling it . 

It says in Mataalib Ooli al-Nuha, 5/147: 

Among the faults that confirm the option (of annulling) is insanity, even if he has seizures sometimes, because one cannot feel at ease with someone who has this condition; this also applies in the case of epilepsy. End quote. 

al-Nawawi (may Allah have mercy on him) said in al-Minhaaj, in Baab al-Khiyaar fi’l-Nikaah: if one of the spouses finds that the other has a kind of mental illness… 

al-Haytami said in al-Tuhfah (7/345): the phrase mental illness includes epilepsy. End quote. 

Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If he has lost his mind, even for a short time, i.e., if it is proven that he has been insane even for a short period, that is regarded as a fault, whether it is in the woman or the man, and that includes epilepsy. End quote. 

Al-Sharh al-Mumti‘, 12/215 

The husband should return the mahr to the one who deceived him – if he consummated the marriage with her. 

Al-Hajaawi said in al-Zaad, p. 167: 

If it is before consummation, there is no mahr, but after that she is entitled to what was agreed upon, and the one who was the cause of this deceit is responsible for paying it, if applicable. End quote. 

This applies if it is not possible to treat it, but if it is possible to treat it and it has been treated, then there is no option (of annulment). 

See: al-Sharh al-Mumti‘, 12/218-219 

If the fault becomes apparent after consummation of the marriage and the other partner accepts it or shows signs of accepting it, then he or she does not have the option of annulment either. 

It says in Zaad al-Mustaqni‘, p. 166 

Whoever accepts a fault or gives indications of accepting it when he finds out about it, he does not have the option (of annulment). End quote. 

To sum up: 

If you did not know about the fault until after consummation of the marriage with her, and this fault cannot be treated, then you have the choice: either you can accept it and put up with your wife’s sickness, or you can annul your marriage to her, and the woman is entitled to the mahr because of the intimacy that she allowed. But the one who should be responsible for paying the mahr is the one who deceived you and concealed from you this sickness that she has. 

See al-Sharh al-Mumti‘, 12/229-230 

What we advise you to do is not to be hasty regarding this matter; put up with it and bear it with patience, and try to find treatment for her by all possible means. Putting up with this problem will bring an abundant reward if you seek reward with Allah, especially if she is of good character and religiously committed. Perhaps if you do that for the sake of Allah, Allah will relieve her of this harm and hardship, and you will be rewarded for your patience. 

And Allah knows best. 

See also the answer to question no. 128221.