Wilfred Benitez has to be one of the worse cases
http://www.boxingranks.com/Articles/Article71.htm
Quote "He can't walk out of the house by himself because he wouldn't know his way back," Wilfred's mother, Clara, told me recently. "He can't talk," she lamented. "He can't say the word 'cup.' His speech and memory are deteriorating fast. He trembles and can hardly move around."