Going a bit sideways from the original topic of communicating with the patients doctor about what is really going on ....
I was involved in the dementia/Alzheimers pathway twice, first with my dad and then tracking about ten years later with my father-in-law. The biggest bit of advice I ever received came from my brother, when he passed on to me something that he gleaned from NY Times columnist Russell Baker's "Growing Up" memoir. Baker discusses being with his mother in her advanced dementia, and him deciding to be with her where she was mentally. He's asking her to describe where she is and what's going on, and in her mind he's a little boy with her on a family outing to Coney Island. She talks to him about things going on. Baker decides to just be with her there, and not try to bring her to "reality".
So I did the same with my Dad and FIL. When I saw them I would ask a starter question, such as tell me about when you were in high school. That would get them started, and with both of them they had pretty good recall of that time of their life. Then I would have them continue narrating their life, like "what did you do after high school?" And I would just keep it going until the memory breakup occurred.
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With my father, there was a three-year period, starting a few years before I was born, when memory breakup occurred. Some things were clear, but other significant elements were gone. I'm the fourth of four, but memory of the birth of my nearest sister (#3) was gone, even though that was within the period. About the time I was born, the memory loss was nearly complete, though he did remember when we moved to Minneapolis when I was a one-year old. But that was good, and we could carry on a useful conversation, that he seemed to enjoy.
Since my Dad stuck at about age 38-40, he saw me as some adult male who was not that much older than him. It was clear I was closely to my Mom (his wife), he usually figured that I was a brother of my Mom's that he had never met. I'm meeting him where he is, so I don't bother with trying to correct that; I just go with it on that basis. that led to some very entertaining sessions, which are among my favorite memories of him.
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With my FIL, the breakup period wasn't as distinct. But my FIL had kept a scrapbook of his ETO service during WWII. For the last part of the war, my FIL served as Quartermaster on Patton's command staff, so he was located whereever Patton was at the time. He was all over western Europe, and kept records and momentos from all over. I shared with my MIL my approach, so one time she gave me the scrapbook to review with him. Again, that led to some very enjoyable times. It also gave me an opportunity to ask him about his experiences in the war.
Later I shared some of things he told me with my wife and others in her family, including my MIL. I learned then that he had told me more about his wartime experiences than he had told anyone else in the family.