ok, heres what i thought while watching it. (with some background) I had been under the impression that his face was like that because of extended/heavy use of the dark side, possibly used to extend his life. but during the scene when he changed i felt that he was changing because even though he had it under control and was just trying to get Anakin to react as he did, the consequence of letting his own force lightning be 'reflected' back at(into) him cause him to shrivel (guess thats the best word)

but honestly now i dont know

-Darth Simon