Hi Barton,
I wish I knew some answers for you. However, postlingal childhood deafness isn't as rare as you might think. There are still quite a few postlingals out there and as I recall they make up at least 5% of childhood deafness cases.
I do know that in about 20% of all childhood deafness cases, hard of hearing kids will eventually become deaf. You might find a cause but you might not. Most disabilties are idoipathic meaning that doctors,researchers and scientists don't know what caused the disabilty.
Has your daughter been tested for Usher's Syndrome? This is a syndrome which causes deaf-blindness. I have heard of some scary syndromes which cause deaf-blindness and many of them are those rare degenerative nereological ones like that little boy had in Lorenzo's Oil. I can totally 100% understand your fears and concerns in this case. I do hope everything will be allright. On the other hand, if your daughter's schoolwork is doing OK, then I wouldn't worry that it's one of those really scary disorders. Most degenerative disorders start out subtly and then go downhill.
Maybe a good idea would be to hook up with some parents who are also dealing with deaf and hard of hearing kids. Try the American Society for Deaf Children's parent-deafhh ********. It is an excellent ********, and there are even some parents of postlingally deaf kids on it, as well as kids with progressive losses.
I know it's in a parent's nature to worry....but your child is still going to be the same kid she always was....and she'll be a regular kid. Having additional complications isn't going to change that. She's just like anyone else except that she wears hearing aids. And I have to tell you, that I grew up in a VERY snobby town, and grew up VERY self-conscieous about my aids. Every single time I run into someone I knew from back then they always say " I never even thought of you as "different" b/c you wore aids. I remember running into my friend a few years ago. I was telling her something about how easy it was to get services for disabled students (I'm in college) and Carla looked at me funny and said " You're disabled?!?!?!?"
Last word of advice.....you may want to look into things like ASL and things like that to enrich your daughter's life and give her an edge. I know the thinking is that postlingally deaf kids don't "need" things like Sign and exposure to deaf culture....big deal...it might give her an edge and it might really really help her. Hope this helped!