Poor vision that cannot be corrected fully with glasses may indicate a condition known as conical cornea or keratoconus. A rare condition, keratoconus is a degenerative disorder of the eye that causes structural changes within the cornea. Keratoconus classically begins affecting young adults (ages 10-20) and progressively worsens until about age 40.
With keratoconus, the cornea, the “clear window” at the front of the eye, may become thin and bow outwards causing the cornea to become shaped like a cone. It is this irregular distortion of the cornea that makes vision correction with glasses less than optimal. For this reason other means of correcting vision are often necessary.