Understanding why dementia patients ask the same question repeatedly may help you handle the situation without frustration. Because, let’s face it—it’s annoying to answer the same question over and over again. This is especially true if you do not understand why it is happening. The elephant in the room is simply that they do not remember they asked the question.
When it comes to storing memories for dementia or Alzheimer’s patient’s the elephant in the room is really a hippo. The hippocampus to be exact, which is about the size and shape of a seahorse and is part of the limbic system in the brain.
Scientists have shown in several recent studies that the hippocampus shrinks in Alzheimer’s patients and may be a highly contributing factor to their ability to store memories from short term to long term memory. The hippocampus is also involved in several other functions: emotional responses, navigation and spatial orientation. Understanding how the hippocampus works is the beginning of understanding why dementia patients ask the same question repeatedly.
Humans create memories in two steps. When we first experience or learn something, it is processed in the hippocampus. The hippocampus registers the event and then sends it to multiple brain regions, such as the auditory cortex or the visual cortex. Imagine how difficult it would be to remember something if it was never properly registered. If it was never allowed to flow to the other brain regions that actually store those memories. Would you remember what you had for breakfast?
The damage that happens to the hippocampus in dementia and Alzheimer’s patients is why your Mom will ask you if it’s Tuesday fifteen times in a day and still be able to tell you every detail of a school play she was in as a child. She may even tell you that story six times in one day because she can remember the story (stored in long term memory), but she cannot remember that she just told you the story five minutes ago. Long term memories do not rely on the hippocampus; only short term memories do.
Psychologists and neuroscientists generally agree that the hippocampus plays an important role in the formation of new memories about experienced events (episodic or autobiographical memory). 
The damage to the hippocampus is why she repeats herself over and over again. Since the hippocampus is not functioning properly she simply does not and cannot register the fact (the memory) that she is has told you this before.
You cannot insist that she has been repeating herself, because (to her) it has never happened. No amount of effort on her part or yours will teach her not to repeat something. The hippocampus cannot process the information so it will never make to a place in the brain where she can remember it.
Understanding why dementia patients ask the same question repeatedly is easy once you get how memories work. For someone with dementia, trying to remember a recent event is like pouring water into a bucket with a hole in it.
Our main findings are: (i) in mild cognitive impairment and Alzheimer’s disease, progression of hippocampal loss was detected over 6 months and accelerated over 1 year, whereas in the normal group hippocampal loss was detected over 1 year with no indication of acceleration
When your hippocampus is functioning properly you easily remember questions and stories you heard minutes ago. A functioning hippocampus also means that dealing with questions and stories you just heard minutes ago is frustrating. So when you think you simply can’t deal with answering that same question from her one more time, pause, take a deep breath and realize it’s just the hippo in the room. Don’t’ talk about it, just recognize it is there. Then calm yourself, smile and say, “Today is Tuesday Mom.”