Joy,
It's hard for me to hear the term "unknown cause". The first time I heard it, I accepted it at face value. When I found out there was a cause, it just was unknown to my neuro at the time, I doubted the term "unknown cause".
This is only the second time I've come across this term and in this case, it wasn't said by a doctor, it's in a medical journal. So, to me, this "unknown cause" is just one that has yet to be determined by the medical community.
I'm curious how many of us hydro sufferers have a low blood platelet count. It's not something my doctor ever mentioned, but now I'm finding out they were low for a long period of time.