I think it is possible to write an advanced directive that addresses such a situation. It requires an informative structure for the directive, and some thought--and, in some cases, informed consultation.

Nonetheless.

I've known for 40 years that I'm at risk for stroke. I've known for that long that I do not want to live any portion of my life following such an event with no quality of life--as I define it. I insist that that is my right to decide, no-one else's.