The brain uses 10x more energy than the body and the retina uses 10x more energy than the brain...so eye uses 100x more energy than other parts. Dr. Myhill talks about this.
Eye muscle weakness and resultant focusing problems have been described in chronic fatigue states. When I first took d-ribose, I could feel my eye fluttering and coming back to life. I think it can also have to do with hypoglycemia and hypothyroid.