My Nephew Is 21 And Experiencing Temporary Blindness Is Both

My nephew is 21 and experiencing temporary blindness is both eyes, sometime for upward of 4 hours at a time, he has had an MRI and has seen very good opthamologists and they can't find anything wrong. I believe he is depressed and stressed, I have read about Hysterical blindness, my sister is kind of losing hope and improving his condition. Any answers?