At present, the consensus is that a diagnosis must already have been made. With that said, it could be a very early stage of the disease. A person could not request medical assistance in dying based on a fear of eventually having a certain disease.
On the other hand, it is possible to document their preferences in writing, even if no disease has yet been declared. We are already doing that when we complete a protection mandate to facilitate our decisions in the event of incapacity.
A person can use a special clause to tell their designated representative whether or not they want extraordinary measures to be performed in the event of a stroke, for example, if there is no hope of recovery. Could we include medical assistance in dying in those wishes? I don't know.
To answer your question specifically, I will say that we have to wait for a precise diagnosis.