Keratoconus is the degeneration of the structure of the cornea, which is the clear tissue covering the front of the eye. in this condition, the shape of the cornea slowly changes from the normal round shape to a cone shape. most people who develop keratoconus start out nearsighted, which tends to become worse over time. the cause is unknown, but the tendency to develop keratoconus is probably present from birth. keratoconus is thought to involve a defect in collagen, the tissue that makes up most of the cornea. some researchers believe that allergy and eye rubbing may play a role.