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Kutcher discusses his fight with an autoimmune condition two years ago in a sneak peek for the actor's forthcoming appearance on Running Wild with Bear Grylls: The Challenge.

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"I had this weird, super rare form of vasculitis that knocked out my vision"

Ashton Kutcher

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"it knocked out my hearing, it knocked out like all my equilibrium”

Ashton Kutcher

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“It took me about a year to build it all back up.”

Ashton Kutcher

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“You don’t really appreciate it until it’s gone,”

Ashton Kutcher

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“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"

Ashton Kutcher

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"I don’t know if I’m ever going to be able to walk again."

Ashton Kutcher

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“I am lucky to be alive,”

Ashton Kutcher

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In the National Geographic programme's sneak preview, Grylls and Kutcher converse as they hike through a forested region covered in branches.

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Kutcher divulges much more about the way of thinking that enabled him to go through the challenging moment.