Normally your cornea, the clear outer lens or "windshield" of the eye, has a dome shape, like a ball. Sometimes the structure isn’t strong enough to hold its round shape and it bulges outward, like a cone. This is called keratoconus.

Tiny fibers of protein in your eye called collagen help hold your cornea in place. When these fibers get weak, they can’t hold their shape. Your cornea gets more and more cone-like.

It happens when you don’t have enough protective antioxidants in your cornea. Its cells produce harmful byproducts, the same way a car puts out exhaust. Normally, antioxidants get rid of them and protect the collagen fibers. But if levels are low, the collagen weakens and the cornea bulges.