They found it initially on a CT after my second fall down the stairs, when I got a pretty bad concussion
. What they told me was, I probably had it for a long time (history of migraines and syncopal episodes off and on since age 4) but that the concussion probably "accelerated" it. I've heard different reports on injury vs. Non injury, and I'm not clear on all of it, but that's what happened to me.