The cause of astigmatism is almost always unknown and not associated with an external or genetic cause.
A rare cause of higher levels of astigmatism is keratoconus (KEHR-a-toh-kohn-nus), in which the cornea progressively becomes cone-shaped and thin. Keratoconus usually appears around puberty or early adulthood. The cornea is made of transparent collagen, and eyes with keratoconus have abnormal collagen synthesis. Keratoconus may progress to the point where corneal rigid contact lenses no longer solve the vision problems and transplantation of the cornea is necessary. Corneal transplantation for keratoconus has a very high success rate -- rejection is uncommon.
Another form of astigmatism is lenticular astigmatism. It is caused by abnormalities in another curved structure inside the eye -- the natural internal crystalline lens that also focuses incoming rays. Blurred vision from lenticular astigmatism can be treated with eyeglasses and contact lenses.
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