Keratoconus is a deterioration of the structure of the cornea with gradual bulging from the normal round shape to a cone shape. This condition causes decreased visual acuity. It is frequently discovered during adolescence. Keratoconus is more common in contact lens wearers and people with nearsighted eyes. Some researchers believe that allergy may play a role.

Keratoconus causes the bulge of the cornea seen in this picture.