Most of the focusing power in the eye occurs along the front surface, called the cornea, which is the clear "window" in the front of the eyeball. The next structure involved in focussing is the lens, which sits behind the cornea inside the eye. The ideal cornea has a round, symmetrically curved surface, like a basketball. Astigmatism is caused by an "out of round" cornea or a lens that is not symmetrical. As a result, people with significant astigmatism may have distorted or blurry vision.
Astigmatism is measured in diopters. More than 1 diopter typically requires correction with glasses or soft contact lenses.
Astigmatism can run in families and often occurs in combination with other refractive problems such as nearsightedness or farsightedness. It can also increase over time due to age.
What Causes Astigmatism?
The cause of astigmatism is almost always unknown and not associated with an external or genetic cause.
One in 400 people has a higher level of astigmatism that comes from keratoconus (KEHR-a-toh-kohn-nus), a condition 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 fibers that hold the cornea shape in place much like a building is held up by steel beams. Eyes with keratoconus have weaker collagen fibers. This causes the cornea to bulge out and distort vision. 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 is used when other measures are no longer sufficient to produce high quality, comfortable vision and has a high success rate. Corneal cross linking is a very promising treatment to arrest the progression of keratoconus and help patients avoid a cornea transplant.
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.