I had a terrier with this - by the end we had to keep him confined when we weren't home for his own safety, and he developed incontinence, so we let him go at that point. He was always such a sharp little character, it was so hard to watch him, but in the end there was no quality of life left, and quite frankly I don't think he remembered who we were most of the time.
All the best to you as you deal with this