I saw your request when first posted. I waited for some clarification before attempting an answer. I'm sure you want answers for your fears as well as information about the disease. How much of this you want to discuss on the forum as opposed to in private messages, I do not know. And I can't say I know what the experience is like internally to the patient. It varies considerably and they cannot tell us much directly.
Have you read on the Internet about Alzheimer's Disease and about dementia diseases in general? Alzheimer's is the very common one that has everyone so scared as it approaches a pandemic with longer life spans, but there are others with slightly different characteristics. I suggest that you read up some and then get back to me when you know more what you want and need in the way of information. I will tell you now that the greatest reaction seen is the fear of the patient as they realize that everything, their mind, is slipping away. This is referred to as agitation and can represent great personality change with considerable display of anger and aggression towards those who formerly were the closest. A lot of paranoia. You will quickly find that you are asking a very broad question if you do look for answers in common references such as the Alzheimer's Association. http://www.alz.org/index.asp
There are many other sources of information, including the government, that will be more up to date on latest research and such than I.
One thing that you will find if you attempt to remain in contact with your grandmother is that she is still in there. You will not see her as she was very often, or perhaps never. But brief and amazing returns are possible. And remember that she, within her higher mind, is still there as ever. It is only her PMR virtual mind, as governed by the PMR rule sets, that is defining her state as an Alzheimer's patient here in PMR.
Let me know what you need when you are more ready.