While obviously neurodegerative disorders manifest somewhat differently for everyone, I have some anecdotal evidence pointing the opposite direction. I've known people starting to undergo dementia (as a general term, may have been Parkinson's, etc.) that were aware of their decline. I think the most common example of that was the feeling among them that "I used to be able to do this..."

I'm by no means qualified to make any actual statements about this case, but I don't think that the ability to reflect on degradation is perfectly coupled with a lack of it.