Consider this case:
At 50, younger-Meredith (YM), just diagnosed with Alzheimer’s, signs an advance directive (AD) expressing her preference that her doctors do not give her extraordinary treatment to keep her alive if she gets ill after having become demented. Meredith is an intellectual, someone who values the life of the mind, and she feels that to be kept alive when demented would make of her life a cruel joke. At 60, older-Meredith (OM) is demented but seemingly quite content with her life when she contracts pneumonia. When asked if she wants to be kept alive via extraordinary treatment, OM says yes. Whose preferences are morally authoritative, YM’s or OM’s?
Read more reflections on the case here.
USUPhilosophy 
What was her mental state when she signed the AD? What was her mental state when she asked to be treated? If those could be determined, which would be hard to do, (in criminal law they ask about the mental state of the murderer at the time the murder was committed and at the time after the murder was committed in order to find a just sentence)a reasonable decision on how to treat the patient might be possible. This is relying heavily on mental states, but I think this is a good way to approach the question.
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My guess (from the summary) is: healthy mental state when signing the AD, and demented by the time she was asked about extraordinary treatment. Acc to the longer article, though, in reality the preferences expressed in the AD are NOT followed UNLESS someone on the patient’s behalf pursues legal action.
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