Strange eyesight distortions like prisms?

Migraine. I am guessing that you have intermittant episodes of odd lights in oval patterns with some greying out of the vision lasting 2 minutes to 2 hours. This is the pattern of occipital migraine. If it clears completly in between, that is almost cerainly what is happening. If something else is happening, or the vision does not return, then see your ophthalmologist.