[Baalz]

I don't have a definitive answer, but I had the same question myself and after testing it out it seems to be resisted by head protection. Units with a good helmet are much less likely to loose an eye. Defense & MR don't seem to help at all, nor body protection.

Also, units with more than two eyes will never lose all their eyes (go blind), they just gain the affliction "lost some eyes". Naturally blind units are not affected by eye loss.