People with severe alzheimers have a major problem with short-term memory. So if they are in an environment where everything is the same as it has been for a long time, then can manage a long time, because their long-term memory is fine. But when something changes (like your grandfather going into hospital), it can often completely destabilize them and they often don't seem to get back to their prior level of functioning. I'm sorry your family had to go through that.