I think it's possible for a medical+legal panel to resolve issues like that, but the thing that would probably cause me to chicken (should it happen to me down the line) out would be actually doing it sufficiently long after the initial screening (I don't have a statistic but it isn't uncommon for the result to come long before the really nasty symptoms) but before said symptoms set in. In future we can hopefully spot things like this much earlier (I don't know what came of it but I heard a brain scanning hat similar in mapping capability to an MRI - passive only apparently - being discussed in a physics department, apparently past prototype stage)

I think the same argument can be made for terminal illnesses also but I think the issue there is extreme pain rather than loss of mental faculties which is potentially not the same thing in the eyes of a court.