Ashton Kutcher went through a terrifying health scare two years ago. The actor is in the most recent episode of National Geographic’s “Running Wild with Bear Grylls: The Challenge,” and in a sneak peek, he revealed he had a rare form of vasculitis, and was unable to see, hear, or walk.

Vasculitis involves inflammation of the blood vessels, which causes the walls of the vessels to thicken and restrict blood flow. You can suffer from organ and tissue damage if it happens, per the Mayo Clinic. There are many types of it and most of them are rare, which is what Kutcher struggled with. “Two years ago, I had this weird, super rare form of vasculitis that knocked out my vision. It knocked out my hearing. It knocked out all my equilibrium,” he said in the clip.

“You don’t really appreciate it until it’s gone,” the “That 70’s Show” star continued. The condition can be short-term or long-lasting, which left Kutcher uncertain of what would happen next. “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.’”

Kutcher and his wife Mila Kunis seemingly kept his health scare quiet, as this is the first time he has opened up about it. “[I’m] lucky to be alive,” he said.

Thankfully Kutcher recovered, and it helped him change his 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?” he continued. “You start surfing on top of your problems instead of living underneath them.”

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