Dilated eyes in a 21 year old cat?

My 21 year old cat's eyes have become completely dilated. She is definitely having trouble focusing and isn't able to follow my fingers but she can sense movement sporadically. I'm sure she is partially blind. Obviously she needs to go to the vet, and I will call in the morning, but that doesn't do much for the apprehension I feel tonight. Please don't post about following up with the vet as I will be doing that, I'm asking if anyone has experienced this with an older cat and what was the cause and the outcome. I realize it is most likely a sign of something more serious, I'm just trying to get an idea of what she is facing. She is otherwise in pretty good health... she still has an appetite, sleeps on a heated cat bed and seems generally content. She gets a B-12 shot about once a month, but the past few nights she has been meowing, not necessarily in pain but definitely she is anxious. If she cries more than once or twice, we go pick her up and hold her for a while and then she goes back to sleep. Anyone willing to share their experience would be most appreciated. She is just the sweetest most gentle cat, and I want to make sure we do the right thing for her. Thanks.