Ashton Kutcher lost vision, hearing and ability to walk after rare autoimmune diagnosis

Ashton Kutcher is "lucky to be alive" after dealing with a rare autoimmune disorder two years ago that left him unable to hear, see or walk. The That '70s Show alum opens up about his health battle during an upcoming episode of National Geographic's Running Wild With Bear Grylls: The Challenge.

"Like two years ago, I had this weird, super rare form of vasculitis, that like knocked out my vision, it knocked out my hearing, it knocked out like all my equilibrium," the 44-year-old actor shares in Access Hollywood's sneak peek.

Vasculitis involves inflammation of the blood vessels, according to Mayo Clinic, affecting one or several organs. There are many types of vasculitis, and most of them are rare.

Kutcher told Grylls it took him almost a year to build up his senses again.

"You don't really appreciate it, until it's gone," Kutcher continued. "Until you go, 'I don't know if I’m ever going to be able to see again, I don't know if I'm ever going to be able to hear again, I don't know if I'm ever going to be able to walk again.'"

This is the first time Kutcher has opened up about the diagnosis. "[I'm] lucky to be alive," he said.

"Lucky to be alive," Grylls replied.

Kutcher, who shares two children with wife Mila Kunis, said his health challenges gave him a different outlook on life.

"The minute you start seeing your obstacles as things that are made for you, to give you what you need, then life starts to get fun, right? You start surfing on top of your problems instead of living underneath them," he said.

Running Wild airs Monday at 9 p.m. E.T. on National Geographic.