My father-in-law suffered from this and succumbed to it after a little over 5 years. I have seen the progression. You need to start planning. Medicine for this is primarily experimental or of unknown assistance. Even still, medication at this point only promises to slow down the progression, but even that is open to debate.
Best to you. It is difficult, but it is a blessing that you are there to care, or direct the care, of your parents.
take care of yourself iows...