How did itachi Uchiha get blind?

Itachi overused his mangekyo sharingan. It's a special sharingan acquired by killing your best friend and then complicated other stuff but I'm not gonna get deep into that explenation. Anyway, using the mangekyo sharingan a lot weakened his eyesight. By the way, he never went blind, but his eyesight is so bad I doubt it would make much of a difference if he did go blind. U can see how bad his eyesight is in chapter 389. Or more precisely, the bottom of this page. (warning: this contains spoilers) http://www.onemanga.com/Naruto/389/08/