A rarer cause of astigmatism is keratoconus (KEHR-a-toh-kohn-nus), in which the cornea progressively becomes cone-shaped and thin. Keratoconus usually strikes around puberty, more commonly in women. The cornea is a sheet of transparent collagen, and eyes with keratoconus have abnormal collagen synthesis. Keratoconus may progress to the point where corneal transplantation is necessary. Corneal transplantation for keratoconus has a very high success rate -- rejection is uncommon.