Ashton Kutcher had a rare autoimmune disorder that robbed him his ability to see, hear, or walk

In the next episode of National Geographic's 'Running Wild with Bear

Grylls: The Challenge', the 44-year-old actor explained: "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. It took me a year to build it all back up."

The condition - which involves an inflammation of blood vessels, which hinders the body's blood flow that can lead to organ and tissue damage - gave Ashton a new zest for life.

He told Bear: "You don't really appreciate it until it's gone. 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.'"

After Bear lauded Ashton's "strength through adversity", the 'No Strings Attached' star - who has seven-year-old daughter Wyatt and five-year-old son Dmitri with his wife Mila Kunis, 37 - admitted he was "lucky to be alive" and detailed how his experience gave him a new view on his life, making it more "fun" after he become more open with his "problems".

Ashton told Bear: "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."

Mila - who he met on the sitcom set in the 70s - revealed her husband "ended up in the hospital twice" on the set of 'Jobs', a biopic of the Apple founder, Steve Jobs.

She said: "He was so dumb. He only ate grapes at one point, it was so stupid. We ended up in the hospital twice with pancreatitis!"