The normal cornea usually has a spherical curvature. A disease called keratoconus causes the cornea to undergo progressive thinning and thus protrude outward, forming a cone-like curvature, which leads to distortion and decreased vision. The disease is thought to be inherited and begins to manifest itself sometime in the late-teen through early adult years.
In some cases, glasses or contact lenses may improve vision in patients with keratoconus. More severe cases may require removal of the affected cornea and replacement with donor tissue through a cornea transplant operation.