Well maybe it can be that nothing in your eye is damaged so it still works properly That's what i think not sure though.
What Happens To Your Eyes After A Concussion?
I'm not exactly sure, but I have hit my head before at Karate and my teacher always checks my eyes...I think its to see if your pupils are normal...i.e. If they both get smaller when I look at light, and they are both the same size. Hope this helps! :)