Reference: Fataawa Ramadhaan – Volume 1, Page 243, Fatwa No.189
Majmoo’ Fataawa Shaykhul-Islaam Ibn Taymiyyah – Voulme 25, Page 217
Question: There is a man who becomes unconscious every time he wishes to fast, thereby striking out with his arms and legs, remaining in this state of unconsciousness for days and as a result he is assumed insane, however, [upon examination] he is not proven to be insane.
Response: All praise is for Allaah [alone]. If fasting inflicts upon him the likes of such an illness, then certainly he should not fast, [rather] he should make up the day[s] he was unable to fast. However, if this [illness] strikes him anytime he fasts then he is [considered] incapable of fasting, so [in this case] he [should] feed a poor and needy person (Muslim) for each and every day [he is unable to fast], and Allaah [alone] has the Complete knowledge [of all affairs].