Mark Cannon's parents noticed that their French Mastiff, Alfie, had a peculiar habit of always placing himself on Mark's right side whether it was just sitting by him or walking beside him. But this unusual habit made more sense after a trip to the eye doctor revealed that Mark had severe astigmatism in his right eye, which was so bad that if it had gone undiagnosed, he would have lost his vision completely in that eye.
Sharlene and Mark Cannon believe that Alfie instinctively knew of Mark's vision impairment and was guiding and protecting the boy. Animal experts have only begun to scratch the surface regarding the ability of our canine friends to detect disease or disabilities.
What they do know and understand is that dogs have thousands to millions of times more ability than humans when it comes to the sense of smell. Dogs have been trained to detect low insulin in diabetics and the onset of seizures in epileptics so Alfie's ability to sense Mark's vision loss doesn't seem that unbelievable at all.*
http://www.telegraph.co.uk/news/newstopics/howaboutthat/12181685/How-a-familys-13-stone-dog-saved-a-boys-eyesight.html