Sometimes there are weird rumors floating around when characters up and leave without much in the way of a satisfactory explanation (well, other than the actress landing film roles). And sometimes, these rumors fall under the category of "crazypants." Thirteen being blind when she eventually returns would qualify, according to Watch With Kristin:
Erica: Is it true that Thirteen (Olivia Wilde) is going to be blind from brain cancer when she returns to House? All I know for sure is that I recently ran this rumor by a source close to the show, and he/she/it thought that plotline was more than a little crazypants. Thirteen may well be coming back in bad shape, but if that's the case, it's still a ways out, storyline-wise.