My lipizzan has that exact same thing. I asked the vet and she spent a good 10 minutes shining lights into his eye and stuff and told me that she didn't know exactly what it was, but his vision was fine. I wasn't really worried about the rings around his eyes, I figured that was a natural pigmentation thing, I was worried about the pupils being so blue, like a cataract.
I even had myself worked into a tizzy thinking it was something wrong, and then he started skittering around the ring at stuff, but that just turned out to be him misbehaving. So if you do find out that it is something, I would be curious to know what it is.