I've been thinking about his Mangekyo and how it seems to have had no impact on his eye sight and how he could have escaped the effects. After much thought on the topic I think I might have the solution and am wondering if this is a valid theory or not. My theory is you need both Mangekyo to hurt your vision. Since he had only 1 eye this effectively allows him to use part of its power for no effect compared to people like Itachi and Sasuke who do have both and had issues with this. Basically if I didn't make it clear enough with only 1 eye he couldn't get Susanoo or the other great abilities but also didn't have the downside of losing his sight. So do you think this is a valid theory? I know its unprovable either way just trying to see if I'm the only one who thinks this is a possible solution.
it is due to Hashirama's cells, plus I have always been under the believe that any Uchiha and Senju/Uzumaki pairing would negate any of the physical side effects of the sharingan due to being of the both the physical and spiritual sides of the sage.
it could also have to do with the fact that with one eye missing he can not use susanoo which might have been damaging and intense enough to overwhelm the healing abilities of Hashirama's cells.