It's actually a form of dementia and not a synonym. However, it does seem to be used as a catch-all for all forms of demential kind of like Kleenex, Band-Aid and Coke are used to represent tissue, bandages and soft drinks. I lost a parent to dementia and my heart goes out to the Sayers family. It's such a cruel disease. Also related to Mr. Sayers, I remember being so saddened by Brian's Song as a young child that for a few years after, just hearing the theme would make me sad.