What Causes Seeing Of Prisms In Vision?

I ve been having like a prism in my eyes in the center, which doesn t let me read, but it goes opening to the sides and then at least I can read. It usually last about 3 - 4 minutes all together. The first time I blame a cleaner that I used to clean the bathroom, and it happen 15 years ago. The last one was while I was watching tv, and suddenly I couldn t see in the center. I check my blood pressure and my blood sugar and both were normal.